#419 フロントのテスト追加

Reviewed-on: #418
Co-authored-by: miteruzo <miteruzo@naver.com>

AGENTS.md

@@ -124,7 +124,7 @@ npm run preview
 - For arrays, never put whitespace or a line break immediately before `]`.
 - Keep the first element on the same line as `[` by default.
 - If an array would exceed the line limit, break after `[` and indent
-  elements by 4 spaces.
+  elements 4 spaces deeper than the statement's base indentation.
 
 ### Ruby delimiter and wrapping rules
 
@@ -140,6 +140,32 @@ npm run preview
   99 文字を超えるなら block 形式へ切り替へるか、message 定数化などで縮める。
 - Ruby の method chain や call argument を折り返す際、call-site の `)` を
   block close のやうに独立させない。
+- Ruby では、行末の `\` を用途を問はず一切使用しない。
+- Ruby では、文字列連結、logger message、method call、条件式、SQL 断片、
+  正規表現その他すべての式で、行末バックスラッシュによる継続を禁止する。
+- Ruby の block body は、その基準位置から 2 空白深くする。
+- Ruby の wrapped expression、method argument、array element、hash pair などの
+  continuation indentation は、その statement の基準位置から 4 空白深くする。
+- Ruby の continuation indentation を、行頭からの絶対空白数として扱はない。
+- Ruby では、暗黙的に継続可能な構文を優先し、method call、array、Hash 及び
+  括弧内ではバックスラッシュなしで改行する。
+- Ruby では、行長制限を守るために行末バックスラッシュを導入してはならない。
+- Ruby で行末バックスラッシュが必要に見える場合は、括弧内で自然に改行する、
+  一つの文字列補間へまとめる、中間変数へ分ける、`format` を使ふ、heredoc を
+  使ふ、array 又は Hash を組み立ててから処理する、条件式全体を括弧で囲む、
+  method へ抽出する、のいづれかへ書き換へる。
+- Ruby では、一つの文字列を、改行をまたいだ隣接文字列 literal として記述しない。
+- Ruby では、method argument 内でも、複数の文字列 literal を区切りなしで縦に
+  並べない。
+- RSpec の `describe`、`context`、`it` 等の description が長い場合は、意味を
+  保ったまま一行へ収まる文言へ短縮する。
+- 文字列を短縮できない場合は、用途に応じて `format`、heredoc 又は中間変数を
+  検討する。
+- ただし RSpec description では、原則として簡潔な一行の文字列を使ふ。
+- formatter 又は自動修正にも、Ruby の行末バックスラッシュを生成させない。
+- 新規 code だけでなく、今回触れる Ruby code にも行末バックスラッシュを残さない。
+- 例へば class body 内の array は、class body の 2 空白を基準に、更に 4 空白
+  深くするため、結果として行頭から 6 空白になる。
 
 Bad:
 
@@ -191,6 +217,63 @@ end
 
 Bad:
 
+```rb
+Rails.logger.info(
+    "post_import_metadata_fetch_failure "\
+    "#{ payload.to_json }")
+```
+
+Good:
+
+```rb
+payload = {
+    error: e.class.name,
+    message: e.message }
+
+Rails.logger.info(
+    "post_import_metadata_fetch_failure #{ payload.to_json }")
+```
+
+Bad:
+
+```rb
+message = "first "\
+          "second"
+```
+
+Good:
+
+```rb
+message = format(
+    '%<first>s %<second>s',
+    first: 'first',
+    second: 'second')
+```
+
+Bad:
+
+```rb
+result = first_value + \
+         second_value
+```
+
+Bad:
+
+```rb
+it(
+    'returns 409 when stale changes '
+    'do not conflict'
+) do
+```
+
+Good:
+
+```rb
+it 'returns mergeable 409 for stale non-conflicting changes' do
+```
+
+Bad:
+
 ```rb
 records.each {
   do_work(_1) }
@@ -225,20 +308,37 @@ records.each {
   formatting as the local reference shape: component and callback bodies use
   2-space block indentation, single-line bodies do not gain unnecessary
   braces, wrapped expressions use 4-space continuation indentation, multi-
-  stage ternaries use explicit parentheses, and only complete leading runs
-  of 8 spaces are compressed to tabs.
-- In TypeScript and TSX only, tabs are for leading 8-column compression only.
+  stage ternaries use explicit parentheses, and TypeScript / TSX indentation
+  and alignment whitespace compress every complete run of 8 spaces to tabs.
 - A tab does not represent one indentation level.
-- Do not replace 2-space or 4-space indentation with tabs.
-- First determine visible indentation using 2-space block indentation and
-  4-space continuation indentation, then compress only complete leading runs
-  of 8 spaces into tabs.
-- Tabs are only for leading indentation, never for spaces after non-space
-  text.
-- Keep residual leading 2, 4, or 6 spaces after any tab compression.
+- In TypeScript and TSX, first determine visible indentation using 2-space
+  block indentation and 4-space continuation indentation, then compress every
+  complete run of 8 spaces used for indentation or column alignment to tabs.
+- In TypeScript and TSX, 8-space compression is mandatory, not optional.
+- In TypeScript and TSX, this applies both to leading indentation and to
+  alignment whitespace after non-space text, such as aligned inline type
+  columns.
+- In TypeScript and TSX, keep residual 2, 4, or 6 spaces after each tab
+  compression.
+- In TypeScript and TSX, do not treat a tab as one logical indentation level.
+- In TypeScript and TSX, do not alter string literals, template-literal
+  contents, regular expressions, or user-facing text merely to apply tab
+  compression.
 - Examples: 2 columns = 2 spaces, 4 columns = 4 spaces, 6 columns = 6
   spaces, 8 columns = 1 tab, 10 columns = 1 tab + 2 spaces, 12 columns = 1
-  tab + 4 spaces, 16 columns = 2 tabs.
+  tab + 4 spaces, 14 columns = 1 tab + 6 spaces, 16 columns = 2 tabs.
+- When TypeScript or TSX code already uses column alignment, apply the same
+  8-space compression rule to that alignment whitespace.
+- Example:
+
+```ts
+type Props = {
+    row:	      PostImportRow
+    displayNumber?:   number
+    onEdit?:	  () => void
+    onRetry?:	  () => void
+    onToggleSkip?:    (checked: boolean) => void }
+```
 - TypeScript and TSX imports may stay on one line if they remain within the
   line limit; do not expand short type-only imports mechanically.
 - Keep runtime value imports and type imports in separate declarations.
@@ -474,8 +574,32 @@ and layout reuse, follow `frontend/AGENTS.md`.
   wording and placement before implementing it.
 - Do not invent replacement copy when removing unrequested wording.
 - Do not create, modify, or run tests unless the user explicitly asks for
-  test work. When the user asks for tests, keep working and rerun them until
-  they pass or the remaining failure is clearly blocked.
+  test work. When the user asks for tests, keep working within the permitted
+  test-file scope and rerun them until they pass or the remaining failure is
+  clearly blocked.
+- Test-only work includes adding, updating, deleting, reorganising, or fixing
+  SyntaxError in tests. During test-only work, do not modify production code.
+- During test-only work, do not change production constants, behaviour, API
+  contracts, validation, routes, authentication, permissions, UI, copy,
+  dependencies, limits, thresholds, defaults, migrations, schema, or
+  environment settings to satisfy tests.
+- Do not make production code match failing tests, mock assumptions, fixtures,
+  snapshots, old expectations, or stale setup. This includes changing
+  production constants merely because a test expects a different value.
+- If test work reveals a production bug, spec mismatch, or missing behaviour,
+  stop without modifying production code and report: the failing test or
+  discovered issue, the related production file, the actual behaviour, the
+  expected behaviour, and why a production change appears necessary.
+- Modify production code for test failures only when the user explicitly asks
+  for that production change. Do not expand a test task into a production task
+  on your own authority.
+- If the user explicitly asks for both production implementation and test
+  updates, implement production code to the confirmed specification first,
+  then add or update tests to verify that specification. Never roll production
+  behaviour back to satisfy old tests.
+- If it is unclear whether the test or the production implementation is stale,
+  or a test cannot be corrected without changing production code, ask the user
+  instead of guessing.
 
 ## Backend rules
 
@@ -529,6 +653,11 @@ and layout reuse, follow `frontend/AGENTS.md`.
 - Mobile UI must be checked as a first-class layout. Avoid wide fixed content,
   make dense controls wrap or scroll intentionally, and keep tag/filter
   controls usable without horizontal page overflow.
+- Frontend のスマホ／PC表示境界は原則 `md` とする。
+- button stack、footer action、dialogue action は `md` 未満で縦並び、
+  `md` 以上で横並びとする。
+- 同じ画面内で `sm` と `md` を混在させて中間 layout を作らない。
+- 明確に別の responsive 要件がある component だけを例外とする。
 - For mobile horizontal scrollers, make the scroll direction and item sizing
   explicit, and ensure chip text remains readable in both light and dark modes.
 - In TypeScript and TSX, prefer direct comparison operators such as `===` and
@@ -593,10 +722,10 @@ and layout reuse, follow `frontend/AGENTS.md`.
   beginning of a line.
 - The TSX-specific self-review must confirm JSX closing markers and closing
   parentheses keep the surrounding compact style.
-- The TypeScript/TSX self-review must confirm leading block indentation uses
-  2 spaces per level, wrapped continuations use the repository's 4-space
-  continuation alignment, and complete leading runs of 8 spaces may be
-  compressed to tabs.
+- The TypeScript/TSX self-review must confirm block indentation uses 2 spaces
+  per level, wrapped continuations use the repository's 4-space continuation
+  alignment, and every complete run of 8 spaces used for indentation or
+  alignment has been compressed to tabs.
 - Prefer `const` arrow functions for TypeScript/TSX component and helper declarations.
 - Put two blank lines before and after top-level `const` function
   declarations, unless imports, exports, or file boundaries make that awkward.
@@ -609,9 +738,18 @@ and layout reuse, follow `frontend/AGENTS.md`.
 - Indent the block body 2 spaces deeper than the keyword and opening brace.
 - Put the closing `}` on its own line at the same indentation as the keyword.
 - Do not write `try {`, `catch {`, or `finally {`.
-- In TypeScript and TSX, convert every complete leading run of 8 spaces to a
-  tab character.
-- A leading tab is exactly equivalent to 8 leading spaces.
+- In TypeScript and TSX, convert every complete run of 8 spaces used for
+  indentation or alignment to a tab character.
+- In TypeScript and TSX, a tab is exactly equivalent to 8 columns, whether it
+  appears at the beginning of a line or in alignment whitespace after
+  non-space text.
+- In TSX, JSX nesting uses 2-space block indentation and wrapped JSX
+  attributes use 4-space continuation indentation; after visible columns are
+  determined, compress every complete run of 8 spaces in the resulting
+  indentation or alignment to tabs.
+- In TSX, do not leave JSX subtree indentation at 8, 10, 12, 14, or 16
+  columns as spaces alone; convert each complete run of 8 spaces to tabs and
+  keep only the residual 2, 4, or 6 spaces.
 - In TypeScript and TSX function declarations, including `const` arrow
   function declarations, classify the parameter list before placing the closing
   `)`.
@@ -1254,9 +1392,9 @@ to `.ts` and `.tsx`:
 7. JSX `>` and `/>` stay with the final prop unless nearby code proves
    otherwise.
 8. JSX closing parentheses keep the compact local style.
-9. Leading block indentation uses 2 spaces per level, wrapped continuations
-   use the repository's 4-space continuation alignment, and complete leading
-   runs of 8 spaces may be compressed to tabs.
+9. Block indentation uses 2 spaces per level, wrapped continuations use the
+   repository's 4-space continuation alignment, and every complete run of 8
+   spaces used for indentation or alignment has been compressed to tabs.
 10. No line has trailing whitespace.
 
 Preferred:

backend/app/controllers/application_controller.rb

@@ -85,4 +85,11 @@ class ApplicationController < ActionController::API
                    base_errors: },
            status:
   end
+
+  def normalise_json value
+    return nil if value.nil?
+    return JSON.parse(value) if value.is_a?(String)
+
+    value
+  end
 end

backend/app/controllers/materials_controller.rb

@@ -237,7 +237,7 @@ class MaterialsController < ApplicationController
   end
 
   def resolve_material_tag! tag_name_raw
-    tag_name = TagName.find_undiscard_or_create_by!(name: tag_name_raw)
+    tag_name = TagName.find_or_create_by!(name: tag_name_raw)
     tag = tag_name.tag
     tag || Tag.create!(tag_name:, category: :material)
   end

backend/app/controllers/nico_tags_controller.rb

@@ -15,30 +15,35 @@ class NicoTagsController < ApplicationController
     limit = 1 if limit < 1
 
     post_tag_max_sql =
-        PostTag
-        .select('tag_id, MAX(created_at) AS max_created_at')
-        .group('tag_id')
+        PostExternalTag
+        .select('external_tag_id, MAX(created_at) AS max_created_at')
+        .group('external_tag_id')
         .to_sql
 
-    q = Tag.nico_tags
-           .joins(:tag_name)
-           .joins("LEFT JOIN (#{ post_tag_max_sql }) post_tag_max " \
-                  'ON post_tag_max.tag_id = tags.id')
-           .includes(:tag_name, tag_name: :wiki_page, linked_tags: { tag_name: :wiki_page })
-    q = q.where('tag_names.name LIKE ?', "%#{ name }%") if name
+    q =
+        ExternalTag
+        .joins("LEFT JOIN (#{ post_tag_max_sql }) post_tag_max " \
+               'ON post_tag_max.external_tag_id = external_tags.id')
+        .includes(linked_tags: { tag_name: :wiki_page })
+    if name
+      q = q.where(('external_tags.name LIKE ? ' +
+                   "OR CONCAT(external_tags.platform, ':', external_tags.name) LIKE ?"),
+                  "%#{ name }%", "%#{ name }")
+    end
+
     if linked_tag
       linked_tag_ids =
-        Tag
-        .joins(:tag_name)
-        .where('tag_names.name LIKE ?', "%#{ linked_tag }%")
-        .pluck(:id)
+          Tag
+          .joins(:tag_name)
+          .where('tag_names.name LIKE ?', "%#{ linked_tag }%")
+          .pluck(:id)
       linked_nico_tag_ids = NicoTagRelation.where(tag_id: linked_tag_ids).pluck(:nico_tag_id)
       q = q.where(id: linked_nico_tag_ids)
     end
     if link_status.in?(['linked', 'unlinked'])
       exists_sql =
           'EXISTS (SELECT 1 FROM nico_tag_relations ' \
-          'WHERE nico_tag_relations.nico_tag_id = tags.id)'
+          'WHERE nico_tag_relations.nico_tag_id = external_tags.id)'
       q = link_status == 'linked' ? q.where(exists_sql) : q.where("NOT #{ exists_sql }")
     end
 
@@ -46,21 +51,21 @@ class NicoTagsController < ApplicationController
     sort_sql =
         case order[0]
         when 'name'
-          'tag_names.name'
+          'external_tags.name'
         when 'updated_at'
           'post_tag_max.max_created_at'
         else
-          "tags.#{ order[0] }"
+          "external_tags.#{ order[0] }"
         end
-    tags = q.reselect('tags.*',
+    tags = q.reselect('external_tags.*',
                       Arel.sql('post_tag_max.max_created_at AS recent_post_tag_created_at'))
-            .order(Arel.sql("#{ sort_sql } #{ order[1] }, tags.id #{ order[1] }"))
+            .order(Arel.sql("#{ sort_sql } #{ order[1] }, external_tags.id #{ order[1] }"))
             .limit(limit)
             .offset((page - 1) * limit)
             .to_a
 
     render json: { tags: tags.map { |tag|
-      TagRepr.base(tag).merge(
+      external_tag_json(tag).merge(
           recent_post_tag_created_at: tag.recent_post_tag_created_at,
           linked_tags: tag.linked_tags.map { |lt| TagRepr.base(lt) })
     }, count: }
@@ -72,8 +77,7 @@ class NicoTagsController < ApplicationController
 
     id = params[:id].to_i
 
-    tag = Tag.find(id)
-    return render_bad_request('ニコニコ・タグを指定してください．') unless tag.nico?
+    tag = ExternalTag.find(id)
 
     linked_tag_names = params[:tags].to_s.split
     linked_tags = nil
@@ -81,16 +85,15 @@ class NicoTagsController < ApplicationController
     ApplicationRecord.transaction do
       linked_tags = Tag.normalise_tags!(linked_tag_names, with_tagme: false,
                                                           with_no_deerjikist: false)
-      if linked_tags.any? { |t| t.nico? }
-        raise Tag::NicoTagNormalisationError
-      end
 
       TagVersioning.record_tag_snapshots!(linked_tags, created_by_user: current_user)
 
       tag.linked_tags = linked_tags
       tag.save!
 
-      NicoTagVersionRecorder.record!(tag:, event_type: :update, created_by_user: current_user)
+      NicoTagVersionRecorder.record!(external_tag: tag,
+                                     event_type: :update,
+                                     created_by_user: current_user)
     end
 
     render json: tag.linked_tags.map { |t| TagRepr.base(t) }, status: :ok
@@ -102,6 +105,21 @@ class NicoTagsController < ApplicationController
 
   private
 
+  def external_tag_json tag
+    { id: tag.id,
+      name: "#{ tag.platform }:#{ tag.name }",
+      category: 'nico',
+      post_count: tag.post_count,
+      created_at: tag.created_at,
+      updated_at: tag.created_at,
+      deprecated_at: nil,
+      aliases: [],
+      parents: [],
+      has_wiki: false,
+      material_id: nil,
+      has_deerjikists: false }
+  end
+
   def render_nico_tag_form_record_invalid record
     if record.is_a?(TagName) || record.is_a?(Tag)
       render_validation_error fields: { tags: record.errors.full_messages.map { |message|

backend/app/controllers/post_imports_controller.rb

@@ -1,49 +0,0 @@
-class PostImportsController < ApplicationController
-  before_action :require_member!
-
-  def preview
-    rows = PostImportPreviewer.new.preview_rows(
-        rows: PostImportUrlListParser.parse(params[:source]))
-    render json: { rows: }
-  rescue ArgumentError => e
-    render_bad_request e.message
-  end
-
-  def validate
-    rows = normalised_import_rows allow_warning_fields: true
-    changed_row =
-        if params[:changed_row].to_s == 'all'
-          true
-        else
-          Integer(params[:changed_row], exception: false)
-        end
-    result =
-        PostImportPreviewer.new.preview_rows(rows:,
-                                             fetch_metadata: changed_row,
-                                             metadata_cache: { })
-    render json: { rows: result }
-  rescue ArgumentError => e
-    render_bad_request e.message
-  end
-
-  def create
-    result = PostImportRunner.new(actor: current_user,
-                                  rows: normalised_import_rows).run
-    render json: result, status: result[:created].positive? ? :created : :ok
-  rescue ArgumentError => e
-    render_bad_request e.message
-  end
-
-  private
-
-  def require_member!
-    return head :unauthorized unless current_user
-    return if current_user.gte_member?
-
-    head :forbidden
-  end
-
-  def normalised_import_rows allow_warning_fields: false
-    PostImportRowNormaliser.normalise!(params[:rows], allow_warning_fields:)
-  end
-end

backend/app/controllers/post_versions_controller.rb

@@ -1,7 +1,10 @@
 class PostVersionsController < ApplicationController
   def index
     post_id = params[:post].presence
-    tag_id = params[:tag].presence
+    tag_id = params[:tag].presence&.to_i
+    external_tag_id = params[:external_tag].presence&.to_i
+    return head :bad_request if tag_id && external_tag_id
+
     page = (params[:page].presence || 1).to_i
     limit = (params[:limit].presence || 20).to_i
 
@@ -10,12 +13,6 @@ class PostVersionsController < ApplicationController
 
     offset = (page - 1) * limit
 
-    tag_name =
-        if tag_id
-          TagName.joins(:tag).find_by(tag: { id: tag_id })
-        end
-    return render json: { versions: [], count: 0 } if tag_id && tag_name.blank?
-
     q = PostVersion.joins(<<~SQL.squish)
           LEFT JOIN
             post_versions prev
@@ -23,17 +20,23 @@ class PostVersionsController < ApplicationController
               prev.post_id = post_versions.post_id
               AND prev.version_no = post_versions.version_no - 1
         SQL
-        .select('post_versions.*', 'prev.title AS prev_title', 'prev.url AS prev_url',
-                'prev.thumbnail_base AS prev_thumbnail_base', 'prev.tags AS prev_tags',
+        .select('post_versions.*',
+                'prev.title AS prev_title',
+                'prev.url AS prev_url',
+                'prev.thumbnail_base AS prev_thumbnail_base',
+                'prev.tags_json AS prev_tags_json',
                 'prev.video_ms AS prev_video_ms',
                 'prev.original_created_from AS prev_original_created_from',
                 'prev.original_created_before AS prev_original_created_before')
     q = q.where('post_versions.post_id = ?', post_id) if post_id
-    if tag_name
-      escaped = ActiveRecord::Base.sanitize_sql_like(tag_name.name)
-      q = q.where(("CONCAT(' ', post_versions.tags, ' ') LIKE :kw " +
-                   "OR CONCAT(' ', prev.tags, ' ') LIKE :kw"),
-                  kw: "% #{ escaped } %")
+    if external_tag_id || (tag_id && !(Tag.exists?(id: tag_id)))
+      q = q.where('JSON_CONTAINS(post_versions.tags_json,' +
+                  "JSON_OBJECT('external_tag_id', #{ external_tag_id || tag_id })) " +
+                  'OR JSON_CONTAINS(prev.tags_json,' +
+                  "JSON_OBJECT('external_tag_id', #{ external_tag_id || tag_id }))")
+    elsif tag_id
+      q = q.where("JSON_CONTAINS(post_versions.tags_json, JSON_OBJECT('tag_id', #{ tag_id })) " +
+                  "OR JSON_CONTAINS(prev.tags_json, JSON_OBJECT('tag_id', #{ tag_id }))")
     end
 
     count = q.except(:select, :order, :limit, :offset).count
@@ -48,77 +51,91 @@ class PostVersionsController < ApplicationController
   private
 
   def serialise_versions rows
+    rows = rows.to_a
     user_ids = rows.map(&:created_by_user_id).compact.uniq
     users_by_id = User.where(id: user_ids).pluck(:id, :name).to_h
+    snapshots = rows.flat_map { |row|
+      [normalise_json(row.tags_json),
+       normalise_json(row.attributes['prev_tags_json']) || []]
+    }
+    external_tag_names = external_tag_names_for(snapshots)
 
     rows.map do |row|
-      cur_tags = split_tags(row.tags)
-      prev_tags = split_tags(row.attributes['prev_tags'])
+      cur_tags = snapshot_tag_literals(normalise_json(row.tags_json), external_tag_names)
+      prev_tags = snapshot_tag_literals(
+          normalise_json(row.attributes['prev_tags_json']) || [], external_tag_names)
 
-      {
-        post_id: row.post_id,
+      { post_id: row.post_id,
         version_no: row.version_no,
         event_type: row.event_type,
-        title: {
-          current: row.title,
-          prev: row.attributes['prev_title']
-        },
-        url: {
-          current: row.url,
-          prev: row.attributes['prev_url']
-        },
-        thumbnail: {
-          current: nil,
-          prev: nil
-        },
-        thumbnail_base: {
-          current: row.thumbnail_base,
-          prev: row.attributes['prev_thumbnail_base']
-        },
-        video_ms: {
-          current: row.video_ms,
-          prev: row.attributes['prev_video_ms']
-        },
+        title: { current: row.title, prev: row.attributes['prev_title'] },
+        url: { current: row.url, prev: row.attributes['prev_url'] },
+        thumbnail: { current: nil, prev: nil },
+        thumbnail_base: { current: row.thumbnail_base,
+                          prev: row.attributes['prev_thumbnail_base'] },
+        video_ms: { current: row.video_ms, prev: row.attributes['prev_video_ms'] },
         tags: build_version_tags(cur_tags, prev_tags),
         original_created_from: {
-          current: row.original_created_from&.iso8601,
-          prev: row.attributes['prev_original_created_from']&.iso8601
-        },
+            current: row.original_created_from&.iso8601,
+            prev: row.attributes['prev_original_created_from']&.iso8601 },
         original_created_before: {
-          current: row.original_created_before&.iso8601,
-          prev: row.attributes['prev_original_created_before']&.iso8601
-        },
+            current: row.original_created_before&.iso8601,
+            prev: row.attributes['prev_original_created_before']&.iso8601 },
         created_at: row.created_at.iso8601,
         created_by_user:
-          if row.created_by_user_id
-            {
-              id: row.created_by_user_id,
-              name: users_by_id[row.created_by_user_id]
-            }
-          end
-      }
+            if row.created_by_user_id
+              { id: row.created_by_user_id,
+                name: users_by_id[row.created_by_user_id] }
+            end }
     end
   end
 
-  def build_version_tags(cur_tags, prev_tags)
+  def external_tag_names_for snapshots
+    ids = snapshots.flatten.filter_map { _1['external_tag_id'] }.uniq
+    names = ExternalTag.where(id: ids).pluck(:id, :platform, :name).to_h { |id, platform, name|
+      [id, "#{ platform }:#{ name }"]
+    }
+    missing_ids = ids - names.keys
+
+    NicoTagVersion
+        .where(tag_id: missing_ids)
+        .order(:tag_id, version_no: :desc)
+        .pluck(:tag_id, :name)
+        .each { |id, name| names[id] ||= name }
+
+    names
+  end
+
+  def snapshot_tag_literals snapshots, external_tag_names
+    snapshots
+        .filter_map { |snapshot|
+          if snapshot.key?('tag_id')
+            [tag_category_order(snapshot['category']),
+             Post.tag_snapshot_literal(snapshot)]
+          elsif external_tag_names[snapshot['external_tag_id']]
+            [6, external_tag_names[snapshot['external_tag_id']]]
+          end
+        }
+        .sort_by { |order, name| [order, name.downcase] }
+        .map(&:second)
+  end
+
+  def tag_category_order category
+    ['deerjikist', 'meme', 'character', 'general', 'material', 'meta'].index(category) || 6
+  end
+
+  def build_version_tags cur_tags, prev_tags
     (cur_tags | prev_tags).map do |name|
       type =
-        if cur_tags.include?(name) && prev_tags.include?(name)
-          'context'
-        elsif cur_tags.include?(name)
-          'added'
-        else
-          'removed'
-        end
+          if cur_tags.include?(name) && prev_tags.include?(name)
+            'context'
+          elsif cur_tags.include?(name)
+            'added'
+          else
+            'removed'
+          end
 
-      {
-        name:,
-        type:
-      }
+      { name:, type: }
     end
   end
-
-  def split_tags(tags)
-    tags.to_s.split(/\s+/).reject(&:blank?)
-  end
 end

backend/app/controllers/posts_controller.rb

@@ -1,5 +1,6 @@
 class PostsController < ApplicationController
   Event = Struct.new(:post, :tag, :user, :change_type, :timestamp, keyword_init: true)
+  MAX_BULK_REQUEST_BYTES = 40 * 1024 * 1024
 
   class VideoMsParseError < ArgumentError
     ;
@@ -35,8 +36,8 @@ class PostsController < ApplicationController
     offset = (page - 1) * limit
 
     pt_max_sql =
-        PostTag
-        .select('post_id, MAX(updated_at) AS max_updated_at')
+        PostVersion
+        .select('post_id, MAX(created_at) AS max_updated_at')
         .group('post_id')
         .to_sql
 
@@ -48,10 +49,9 @@ class PostsController < ApplicationController
         filtered_posts
         .joins("LEFT JOIN (#{ pt_max_sql }) pt_max ON pt_max.post_id = posts.id")
         .reselect('posts.*', Arel.sql("#{ updated_at_all_sql } AS updated_at_all"))
-        .preload(:uploaded_user, :parents, :children,
-                 active_post_tags: [:sections,
-                                    { tag: [:deerjikists, :materials,
-                                            { tag_name: :wiki_page }] }])
+        .preload(:external_tags, :uploaded_user, :parents, :children,
+                 post_tags: [:sections, { tag: [:deerjikists, :materials,
+                                                { tag_name: :wiki_page }] }])
         .with_attached_thumbnail
 
     q = q.where('posts.url LIKE ?', "%#{ url }%") if url
@@ -102,25 +102,96 @@ class PostsController < ApplicationController
   end
 
   def random
-    post = filtered_posts.preload(:uploaded_user, :parents, :children,
-                                  active_post_tags: [:sections,
-                                                     { tag: [:deerjikists, :materials,
-                                                             { tag_name: :wiki_page }] }])
-                         .with_attached_thumbnail
-                         .order('RAND()')
-                         .first
+    post =
+        filtered_posts
+        .preload(:uploaded_user, :parents, :children,
+                 post_tags: [:sections, { tag: [:deerjikists, :materials,
+                                                { tag_name: :wiki_page }] }])
+        .with_attached_thumbnail
+        .order('RAND()')
+        .first
     return head :not_found unless post
 
     render json: PostRepr.base(post, current_user)
   end
 
+  def metadata
+    return head :unauthorized unless current_user
+    return head :forbidden unless current_user.gte_member?
+    return render_bad_request('URL は必須です．') if params[:url].blank?
+
+    normal_url = PostUrlNormaliser.normalise(params[:url].to_s)
+    return render_validation_error(fields: { url: ['URL が不正です．'] }) if normal_url.blank?
+
+    Preview::UrlSafety.validate(normal_url)
+    existing_post = Post.with_attached_thumbnail.find_by(url: normal_url)
+    if existing_post.present?
+      return render json: {
+          url: normal_url,
+          title: nil,
+          thumbnail_base: nil,
+          tags: nil,
+          display_tags: [],
+          original_created_from: nil,
+          original_created_before: nil,
+          duration: nil,
+          video_ms: nil,
+          field_warnings: { },
+          base_warnings: [],
+          existing_post_id: existing_post.id,
+          existing_post: compact_post(existing_post.id) }
+    end
+
+    metadata = PostMetadataFetcher.fetch(normal_url)
+    field_warnings = { }
+    field_warnings[:title] = ['タイトルを取得できませんでした．'] if metadata[:title].blank?
+    if metadata[:thumbnail_base].blank?
+      field_warnings[:thumbnail_base] = ['サムネールを取得できませんでした．']
+    end
+
+    render json: {
+        url: normal_url,
+        title: metadata[:title],
+        thumbnail_base: metadata[:thumbnail_base],
+        tags: metadata[:tags],
+        display_tags: metadata[:display_tags],
+        original_created_from: metadata[:original_created_from],
+        original_created_before: metadata[:original_created_before],
+        duration: metadata[:duration],
+        video_ms: metadata[:video_ms],
+        field_warnings: field_warnings,
+        base_warnings: [],
+        existing_post_id: nil,
+        existing_post: nil }
+  rescue ArgumentError => e
+    render_bad_request e.message
+  rescue Preview::UrlSafety::UnsafeUrl => e
+    render_validation_error fields: { url: [e.message] }
+  rescue Preview::HttpFetcher::FetchFailed,
+         Preview::HttpFetcher::FetchTimeout,
+         Preview::HttpFetcher::ResponseTooLarge
+    render json: {
+        url: normal_url,
+        title: nil,
+        thumbnail_base: nil,
+        tags: nil,
+        display_tags: [],
+        original_created_from: nil,
+        original_created_before: nil,
+        duration: nil,
+        video_ms: nil,
+        field_warnings: { url: ['自動取得に失敗しました．'] },
+        base_warnings: [],
+        existing_post_id: nil,
+        existing_post: nil }
+  end
+
   def show
     post =
         Post
         .includes(:uploaded_user, :parents, :children,
-                  active_post_tags: [:sections,
-                                     { tag: [:deerjikists, :materials,
-                                             { tag_name: :wiki_page }] }])
+                  post_tags: [:sections, { tag: [:deerjikists, :materials,
+                                                 { tag_name: :wiki_page }] }])
         .with_attached_thumbnail
         .find_by(id: params[:id])
     return head :not_found unless post
@@ -142,17 +213,41 @@ class PostsController < ApplicationController
     return head :unauthorized unless current_user
     return head :forbidden unless current_user.gte_member?
 
+    preflight = PostCreatePreflight.new(
+        attributes: post_create_attributes,
+        thumbnail: params[:thumbnail],
+        host: request.base_url).run
+    return render json: dry_run_json(preflight) if bool?(:dry)
+    if preflight[:existing_post_id].present?
+      post = Post.new(url: preflight[:url])
+      post.errors.add :url, :taken
+      return render_post_form_record_invalid post
+    end
+
     post = PostCreator.new(actor: current_user,
-                           attributes: { title: params[:title], url: params[:url],
-                                         thumbnail: params[:thumbnail], tags: params[:tags],
-                                         original_created_from: params[:original_created_from],
-                                         original_created_before: params[:original_created_before],
-                                         parent_post_ids: parse_parent_post_ids,
-                                         video_ms: params[:video_ms],
-                                         duration: params[:duration] }).create!
+                           attributes: post_create_attributes.merge(
+                               preflight.slice(
+                                   :url,
+                                   :title,
+                                   :thumbnail_base,
+                                   :tags,
+                                   :parent_post_ids,
+                                   :original_created_from,
+                                   :original_created_before,
+                                   :duration,
+                                   :video_ms,
+                                   :direct_tag_specs,
+                                   :default_tag_specs,
+                                   :snapshot_tag_specs,
+                                   :post_tag_specs,
+                                   :tag_sections,
+                                   :normalised_parent_post_ids).symbolize_keys).merge(
+                               thumbnail: params[:thumbnail])).create!
 
     post.reload
     render json: PostRepr.base(post), status: :created
+  rescue PostCreatePreflight::ValidationFailed => e
+    render_validation_error fields: e.fields, base: e.base_errors
   rescue Tag::NicoTagNormalisationError
     render_validation_error fields: { tags: 'ニコニコ・タグは直接指定できません．' }
   rescue Tag::DeprecatedTagNormalisationError
@@ -161,6 +256,8 @@ class PostsController < ApplicationController
     render_validation_error fields: { tags: ['タグ区間の記法が不正です．'] }
   rescue PostCreator::VideoMsParseError
     render_validation_error fields: { video_ms: ['動画時間の記法が不正です．'] }
+  rescue Post::RemoteThumbnailFetchFailed
+    render_validation_error fields: { thumbnail_base: ['サムネイル画像の取得に失敗しました．'] }
   rescue MiniMagick::Error
     render_validation_error fields: { thumbnail: ['サムネイル画像の変換に失敗しました．'] }
   rescue ArgumentError => e
@@ -169,6 +266,25 @@ class PostsController < ApplicationController
     render_post_form_record_invalid e.record
   end
 
+  def bulk
+    return head :unauthorized unless current_user
+    return head :forbidden unless current_user.gte_member?
+    return head :unsupported_media_type unless request.content_mime_type == Mime[:multipart_form]
+    return head :payload_too_large if request.content_length.to_i > MAX_BULK_REQUEST_BYTES
+    posts = parse_bulk_posts_manifest
+    thumbnails = parse_bulk_thumbnails(posts.length)
+    result = PostBulkCreator.new(
+        actor: current_user,
+        posts:,
+        thumbnails:,
+        host: request.base_url).run
+    render json: result
+  rescue JSON::ParserError
+    render_bad_request 'posts manifest の JSON が不正です．'
+  rescue ArgumentError => e
+    render_validation_error base: [e.message]
+  end
+
   def viewed
     return head :unauthorized unless current_user
 
@@ -267,50 +383,6 @@ class PostsController < ApplicationController
     render_post_form_record_invalid e.record
   end
 
-  def changes
-    id = params[:id].presence
-    tag_id = params[:tag].presence
-    page = (params[:page].presence || 1).to_i
-    limit = (params[:limit].presence || 20).to_i
-
-    page = 1 if page < 1
-    limit = 1 if limit < 1
-
-    offset = (page - 1) * limit
-
-    pts = PostTag.with_discarded
-    pts = pts.where(post_id: id) if id.present?
-    pts = pts.where(tag_id:) if tag_id.present?
-    pts = pts.includes(:post, :created_user, :deleted_user,
-                       tag: [:deerjikists, :materials, { tag_name: :wiki_page }])
-
-    events = []
-    pts.each do |pt|
-      tag = TagRepr.base(pt.tag)
-      post = pt.post
-
-      events << Event.new(
-          post:,
-          tag:,
-          user: pt.created_user && { id: pt.created_user.id, name: pt.created_user.name },
-          change_type: 'add',
-          timestamp: pt.created_at)
-
-      if pt.discarded_at
-        events << Event.new(
-            post:,
-            tag:,
-            user: pt.deleted_user && { id: pt.deleted_user.id, name: pt.deleted_user.name },
-            change_type: 'remove',
-            timestamp: pt.discarded_at)
-      end
-    end
-    events.sort_by!(&:timestamp)
-    events.reverse!
-
-    render json: { changes: (events.slice(offset, limit) || []).as_json, count: events.size }
-  end
-
   private
 
   def filtered_posts
@@ -356,8 +428,20 @@ class PostsController < ApplicationController
     end
   end
 
-  def tagged_post_ids_for(name) =
-      Post.joins(tags: :tag_name).where(tag_names: { name: }).select(:id)
+  def tagged_post_ids_for(name)
+    posts_by_internal_tags =
+        Post
+        .joins(tags: :tag_name)
+        .where(tag_names: { name: })
+        .select(:id)
+
+    posts_by_external_tags =
+        Post
+        .joins(:external_tags)
+        .where("CONCAT(external_tags.platform, ':', external_tags.name) = ?", name)
+
+    (posts_by_internal_tags + posts_by_external_tags).uniq(&:id)
+  end
 
   def sync_post_tags! post, desired_tags, sections
     desired_tags.each do |t|
@@ -385,13 +469,13 @@ class PostsController < ApplicationController
       end
     end
 
-    PostTag.where(post_id: post.id, tag_id: to_remove.to_a).kept.find_each do |pt|
-      pt.discard_by!(current_user)
+    PostTag.where(post_id: post.id, tag_id: to_remove.to_a).find_each do |pt|
+      pt.destroy!
     end
   end
 
   def build_tag_tree_for post
-    post_tags = post.active_post_tags.reject { |post_tag| post_tag.tag.deprecated? }
+    post_tags = post.post_tags.reject { |post_tag| post_tag.tag.deprecated? }
     tags = post_tags.map(&:tag)
     tag_ids = tags.map(&:id)
 
@@ -435,7 +519,10 @@ class PostsController < ApplicationController
       memo[tag_id] = TagRepr.inline(tag).merge(children:, sections:)
     end
 
-    root_ids.filter_map { |id| build_node.call(id, []) }
+    internal_tags = root_ids.filter_map { |id| build_node.call(id, []) }
+    external_tags =
+        post.external_tags.map { ExternalTagRepr.inline(_1).merge(children: [], sections: []) }
+    internal_tags + external_tags
   end
 
   def sibling_posts_by_parent parent_post_ids
@@ -463,6 +550,79 @@ class PostsController < ApplicationController
     }.uniq
   end
 
+  def post_create_attributes
+    { title: params[:title],
+      url: params[:url],
+      thumbnail_base: params[:thumbnail_base],
+      tags: params[:tags],
+      original_created_from: params[:original_created_from],
+      original_created_before: params[:original_created_before],
+      parent_post_ids: parse_parent_post_ids,
+      video_ms: params[:video_ms],
+      duration: params[:duration] }
+  end
+
+  def parse_bulk_posts_manifest
+    manifest = params[:posts]
+    raise ArgumentError, 'posts は必須です．' if manifest.blank?
+    raise ArgumentError, 'posts は JSON 文字列で指定してください．' unless manifest.is_a?(String)
+
+    posts = JSON.parse(manifest)
+    raise ArgumentError, 'posts は配列で指定してください．' unless posts.is_a?(Array)
+    raise ArgumentError, '投稿件数は 1 件以上必要です．' if posts.empty?
+    raise ArgumentError, '投稿件数が多すぎます．' if posts.length > 100
+    raise ArgumentError, 'posts 要素の形式が不正です．' unless posts.all? { _1.is_a?(Hash) }
+
+    posts
+  end
+
+  def parse_bulk_thumbnails post_count
+    thumbnails = { }
+    raw = params[:thumbnails]
+    return thumbnails if raw.blank?
+    raise ArgumentError, 'thumbnail key が不正です．' unless raw.respond_to?(:to_unsafe_h)
+
+    raw.to_unsafe_h.each do |key, value|
+      raise ArgumentError, 'thumbnail key が不正です．' unless key.to_s.match?(/\A\d+\z/)
+
+      index = Integer(key, 10)
+      raise ArgumentError, 'thumbnail index が範囲外です．' if index.negative? || index >= post_count
+      raise ArgumentError, 'thumbnail index が重複しています．' if thumbnails.key?(index)
+      unless value.is_a?(ActionDispatch::Http::UploadedFile)
+        raise ArgumentError, 'thumbnail upload が不正です．'
+      end
+
+      thumbnails[index] = value
+    end
+
+    thumbnails
+  end
+
+  def compact_post post_id
+    return nil if post_id.blank?
+
+    post = Post.with_attached_thumbnail.find_by(id: post_id)
+    PostCompactRepr.base(post, host: request.base_url)
+  end
+
+  def dry_run_json preflight
+    preflight.slice(
+        :url,
+        :title,
+        :thumbnail_base,
+        :tags,
+        :display_tags,
+        :parent_post_ids,
+        :original_created_from,
+        :original_created_before,
+        :duration,
+        :video_ms,
+        :field_warnings,
+        :base_warnings,
+        :existing_post_id,
+        :existing_post)
+  end
+
   def sync_parent_posts! post, parent_post_ids
     if parent_post_ids.include?(post.id)
       post.errors.add :parent_post_ids, '自分自身を親投稿にはできません．'
@@ -509,7 +669,10 @@ class PostsController < ApplicationController
   end
 
   def editable_tag_names_from_version version
-    version.tags.to_s.split.reject { |name| name.downcase.start_with?('nico:') }.sort
+    version.tags_json
+        .select { _1.key?('tag_id') }
+        .map { Post.tag_snapshot_literal(_1) }
+        .sort
   end
 
   def post_snapshot_from_record post
@@ -524,9 +687,7 @@ class PostsController < ApplicationController
   def editable_tag_names_from_post post
     post
         .post_tags
-        .kept
         .joins(tag: :tag_name)
-        .merge(Tag.not_nico)
         .merge(Tag.where(deprecated_at: nil))
         .includes(:sections, tag: :tag_name)
         .order('tag_names.name')
@@ -542,9 +703,9 @@ class PostsController < ApplicationController
 
   def post_incoming_snapshot title:, original_created_from:, original_created_before:,
                              tag_names:, video_ms_param:, duration_param:, parent_post_ids:
+    validate_original_created_values!(original_created_from, original_created_before)
     Tag.normalise_tags!(tag_names, with_tagme: false, deny_deprecated: true,
-                                        with_sections: true) =>
-        { tags:, sections: }
+                                   with_sections: true) => { tags:, sections: }
 
     tags = Tag.expand_parent_tags(tags).reject(&:deprecated?)
     video_ms = normalise_video_ms(tags, video_ms_param:, duration_param:)
@@ -579,6 +740,23 @@ class PostsController < ApplicationController
     value.to_s
   end
 
+  def validate_original_created_values! original_created_from, original_created_before
+    candidate = Post.new(
+        url: 'https://example.invalid/original-created-validation',
+        original_created_from:,
+        original_created_before:)
+    candidate.valid?
+    fields = [:original_created_from, :original_created_before, :original_created_at]
+    relevant_errors = candidate.errors.select { fields.include?(_1.attribute) }
+    return if relevant_errors.empty?
+
+    invalid_post = Post.new
+    relevant_errors.each { |error|
+      invalid_post.errors.add(error.attribute, error.message)
+    }
+    raise ActiveRecord::RecordInvalid, invalid_post
+  end
+
   def section_literal section
     "[#{ Post.ms_to_time(section[0]) }-#{ section[1] ? Post.ms_to_time(section[1]) : '' }]"
   end
@@ -670,15 +848,12 @@ class PostsController < ApplicationController
                  original_created_from: snapshot[:original_created_from],
                  original_created_before: snapshot[:original_created_before])
 
-    Tag.normalise_tags!(snapshot[:tag_names], with_tagme: false,
-                                               deny_deprecated: true,
-                                               with_sections: true) =>
-        { tags: editable_tags, sections: }
-    TagVersioning.record_tag_snapshots!(editable_tags, created_by_user: current_user)
+    Tag.normalise_tags!(snapshot[:tag_names],
+                        with_tagme: false,
+                        deny_deprecated: true,
+                        with_sections: true) => { tags:, sections: }
+    TagVersioning.record_tag_snapshots!(tags, created_by_user: current_user)
 
-    readonly_tags = post.tags.nico.to_a
-
-    tags = readonly_tags + editable_tags
     tags = Tag.expand_parent_tags(tags).reject(&:deprecated?)
 
     post.video_ms = tags.any? { _1.id == Tag.video.id } ? snapshot[:video_ms] : nil

backend/app/controllers/preview_controller.rb

@@ -18,32 +18,9 @@ class PreviewController < ApplicationController
   def thumbnail
     return render_bad_request('URL は必須です．') if params[:url].blank?
 
-    image = MiniMagick::Image.read(Preview::ThumbnailFetcher.fetch(params[:url]))
-    image.auto_orient
-    image.resize '180x180>'
-    image.format 'png'
-    width, height = image.dimensions
-    raise Preview::ThumbnailFetcher::GenerationFailed, 'サムネール画像の変換に失敗しました．' if width > 180 || height > 180
-
-    send_data image.to_blob, type: 'image/png', disposition: 'inline'
-  rescue Preview::UrlSafety::UnsafeUrl => e
-    render_bad_request(e.message)
-  rescue Preview::HttpFetcher::FetchTimeout => e
-    render_preview_error(e.message, :gateway_timeout)
-  rescue Preview::HttpFetcher::ResponseTooLarge => e
-    render_preview_error(e.message, :payload_too_large)
-  rescue Preview::HttpFetcher::FetchFailed => e
-    render_preview_error(e.message, :bad_gateway)
-  rescue Preview::ThumbnailFetcher::GenerationFailed, MiniMagick::Error => e
-    render_unprocessable_entity(e.message)
-  end
-
-  def image
-    return render_bad_request('URL は必須です．') if params[:url].blank?
-
-    attachment = Post.resized_thumbnail_attachment(
-        StringIO.new(Preview::ThumbnailFetcher.fetch_image_response(params[:url]).body))
-    attachment[:io].rewind
+    attachment =
+        Post.resized_thumbnail_attachment(
+            StringIO.new(Preview::ThumbnailFetcher.fetch(params[:url])))
     send_data attachment[:io].read,
               type: attachment[:content_type],
               disposition: 'inline'

backend/app/controllers/tag_children_controller.rb

@@ -10,7 +10,6 @@ class TagChildrenController < ApplicationController
 
     parent = Tag.find(parent_id)
     child = Tag.find(child_id)
-    return render_bad_request('ニコニコ・タグの階層は変更できません．') if parent.nico? || child.nico?
 
     ApplicationRecord.transaction do
       TagVersioning.ensure_snapshot!(child, created_by_user: current_user)
@@ -33,7 +32,6 @@ class TagChildrenController < ApplicationController
 
     parent = Tag.find(parent_id)
     child = Tag.find(child_id)
-    return render_bad_request('ニコニコ・タグの階層は変更できません．') if parent.nico? || child.nico?
 
     ApplicationRecord.transaction do
       TagVersioning.ensure_snapshot!(child, created_by_user: current_user)

backend/app/controllers/tags_controller.rb

@@ -34,49 +34,155 @@ class TagsController < ApplicationController
 
     offset = (page - 1) * limit
 
-    q =
+    tags =
         if post_id.present?
-          Tag.joins(:posts, :tag_name)
+          Tag.joins(:posts, :tag_name).where(posts: { id: post_id })
         else
           Tag.joins(:tag_name)
         end
-        .includes(:tag_name, :materials, tag_name: :wiki_page)
-    q = q.where(posts: { id: post_id }) if post_id.present?
 
-    q = q.where('tag_names.name LIKE ?', "%#{ name }%") if name
-    q = q.where(category:) if category
-    q = q.where('tags.post_count >= ?', post_count_between[0]) if post_count_between[0]
-    q = q.where('tags.post_count <= ?', post_count_between[1]) if post_count_between[1]
-    q = q.where('tags.created_at >= ?', created_between[0]) if created_between[0]
-    q = q.where('tags.created_at <= ?', created_between[1]) if created_between[1]
-    q = q.where('tags.updated_at >= ?', updated_between[0]) if updated_between[0]
-    q = q.where('tags.updated_at <= ?', updated_between[1]) if updated_between[1]
-    if deprecated_given
-      q = deprecated ? q.where.not(deprecated_at: nil) : q.where(deprecated_at: nil)
+    external_tags =
+        if post_id.present?
+          ExternalTag.joins(:posts).where(posts: { id: post_id })
+        else
+          ExternalTag.all
+        end
+
+    if name
+      tags = tags.where('tag_names.name LIKE ?', "%#{ name }%")
+      external_tags =
+          external_tags.where("CONCAT(external_tags.platform, ':', external_tags.name) LIKE ?",
+                              "%#{ name }%")
     end
 
+    if category == 'nico'
+      tags = tags.none
+    elsif category
+      tags = tags.where(category:)
+      external_tags = external_tags.none
+    end
+
+    if post_count_between[0]
+      tags = tags.where('tags.post_count >= ?', post_count_between[0])
+      external_tags = external_tags.where('external_tags.post_count >= ?', post_count_between[0])
+    end
+
+    if post_count_between[1]
+      tags = tags.where('tags.post_count <= ?', post_count_between[1])
+      external_tags = external_tags.where('external_tags.post_count <= ?', post_count_between[1])
+    end
+
+    if created_between[0]
+      tags = tags.where('tags.created_at >= ?', created_between[0])
+      external_tags = external_tags.where('external_tags.created_at >= ?', created_between[0])
+    end
+
+    if created_between[1]
+      tags = tags.where('tags.created_at <= ?', created_between[1])
+      external_tags = external_tags.where('external_tags.created_at <= ?', created_between[1])
+    end
+
+    if updated_between[0]
+      tags = tags.where('tags.updated_at >= ?', updated_between[0])
+      external_tags = external_tags.where('external_tags.created_at >= ?', updated_between[0])
+    end
+
+    if updated_between[1]
+      tags = tags.where('tags.updated_at <= ?', updated_between[1])
+      external_tags = external_tags.where('external_tags.created_at <= ?', updated_between[1])
+    end
+
+    if deprecated_given
+      if deprecated
+        tags = tags.where.not(deprecated_at: nil)
+        external_tags = external_tags.none
+      else
+        tags = tags.where(deprecated_at: nil)
+      end
+    end
+
+    tag_sql = tags.select("'tag' AS source",
+                          'tags.id',
+                          'tag_names.name',
+                          'tags.category',
+                          'tags.post_count',
+                          'tags.created_at',
+                          'tags.updated_at',
+                          'tags.deprecated_at').to_sql
+
+    external_tag_sql = external_tags.select(
+        "'external_tag' AS source",
+        'external_tags.id',
+        "CONCAT(external_tags.platform, ':', external_tags.name) AS name",
+        "'nico' AS category",
+        'external_tags.post_count',
+        'external_tags.created_at',
+        'external_tags.created_at AS updated_at',
+        'NULL AS deprecated_at').to_sql
+
+    union_sql = "#{ tag_sql } UNION ALL #{ external_tag_sql }"
+
     sort_sql =
-        case order[0]
-        when 'name'
-          'tag_names.name'
-        when 'category'
-          'CASE tags.category ' +
+        if order[0] == 'category'
+          'CASE category ' +
           "WHEN 'deerjikist' THEN 0 " +
           "WHEN 'meme' THEN 1 " +
           "WHEN 'character' THEN 2 " +
           "WHEN 'general' THEN 3 " +
           "WHEN 'material' THEN 4 " +
           "WHEN 'meta' THEN 5 " +
-          "WHEN 'nico' THEN 6 END"
+          "WHEN 'nico' THEN 6 " +
+          'END'
         else
-          "tags.#{ order[0] }"
+          order[0]
         end
-    tags = q.order(Arel.sql("#{ sort_sql } #{ order[1] }, tags.id #{ order[1] }"))
-            .limit(limit)
-            .offset(offset)
-            .to_a
 
-    render json: { tags: TagRepr.many(tags), count: q.size }
+    connection = ApplicationRecord.connection
+
+    count = connection.select_value(<<~SQL)
+      SELECT
+        COUNT(0)
+      FROM
+        (#{ union_sql }) legacy_tags
+    SQL
+
+    rows = connection.select_all(<<~SQL).to_a
+      SELECT
+        source
+      , id
+      FROM
+        (#{ union_sql }) legacy_tags
+      ORDER BY
+        #{ sort_sql } #{ order[1] }
+      , id #{ order[1] }
+      , source #{ order[1] }
+      LIMIT
+        #{ limit }
+      OFFSET
+        #{ offset }
+    SQL
+
+    tag_ids = rows.filter { _1['source'] == 'tag' }.map { _1['id'] }
+    external_tag_ids = rows.filter { _1['source'] == 'external_tag' }.map { _1['id'] }
+
+    tags_by_id =
+        Tag
+        .joins(:tag_name)
+        .includes(:tag_name, :materials, tag_name: :wiki_page)
+        .where(id: tag_ids)
+        .index_by(&:id)
+    external_tags_by_id =
+        ExternalTag
+        .where(id: external_tag_ids)
+        .index_by(&:id)
+
+    render json: { tags: rows.map { |row|
+      if row['source'] == 'tag'
+        TagRepr.base(tags_by_id.fetch(row['id']))
+      else
+        external_tag_json(external_tags_by_id.fetch(row['id']))
+      end
+    }, count: }
   end
 
   def with_depth
@@ -100,13 +206,14 @@ class TagsController < ApplicationController
 
   def autocomplete
     q = params[:q].to_s.strip.sub(/\Anot:/i, '')
+    prefix = "#{ ActiveRecord::Base.sanitize_sql_like(q) }%"
 
     with_nico = bool?(:nico, default: true)
     present_only = bool?(:present, default: true)
 
     alias_rows =
         TagName
-        .where('name LIKE ?', "#{ q }%")
+        .where('name LIKE ?', prefix)
         .where.not(canonical_id: nil)
         .pluck(:canonical_id, :name)
 
@@ -118,29 +225,43 @@ class TagsController < ApplicationController
       matched_alias_by_tag_name_id[canonical_id] ||= alias_name
     end
 
-    base = Tag.joins(:tag_name)
-              .includes(:tag_name, :materials, tag_name: :wiki_page)
-              .where(deprecated_at: nil)
+    base =
+        Tag
+        .joins(:tag_name)
+        .includes(:tag_name, :materials, tag_name: :wiki_page)
+        .where(deprecated_at: nil)
     base = base.where('tags.post_count > 0') if present_only
 
-    canonical_hit =
-        base
-        .where(((with_nico ? '(tags.category = ? AND tag_names.name LIKE ?) OR ' : '') +
-                'tag_names.name LIKE ?'),
-               *(with_nico ? ['nico', "nico:#{ q }%"] : []), "#{ q }%")
+    canonical_hit = base.where('tag_names.name LIKE ?', prefix)
 
-    tags =
-        if canonical_ids.present?
-          canonical_hit.or(base.where(tag_name_id: canonical_ids.uniq))
-        else
-          canonical_hit
-        end
+    internal_tags = canonical_hit.or(base.where(tag_name_id: canonical_ids.uniq))
 
-    tags = tags.order(Arel.sql('post_count DESC, tag_names.name')).limit(20).to_a
+    internal_rows =
+        internal_tags
+        .order(Arel.sql('tags.post_count DESC, tag_names.name'))
+        .limit(20)
+        .map { |tag|
+          TagRepr.base(tag).merge(matched_alias: matched_alias_by_tag_name_id[tag.tag_name_id])
+        }
 
-    render json: tags.map { |tag|
-      TagRepr.base(tag).merge(matched_alias: matched_alias_by_tag_name_id[tag.tag_name_id])
-    }
+    return render json: internal_rows unless with_nico
+
+    external_base = ExternalTag.all
+    external_base = external_base.where('post_count > 0') if present_only
+
+    external_rows =
+        external_base
+        .where("CONCAT(platform, ':', name) LIKE ? OR name LIKE ?", prefix, prefix)
+        .order(post_count: :desc, name: :asc)
+        .limit(20)
+        .map { external_tag_json(_1) }
+
+    rows =
+        (internal_rows + external_rows)
+        .sort_by { |row| [-row['post_count'], row['name']] }
+        .first(20)
+
+    render json: rows
   end
 
   def show
@@ -158,14 +279,20 @@ class TagsController < ApplicationController
     name = params[:name].to_s.strip
     return render_bad_request('name は必須です．') if name.blank?
 
-    tag = Tag.joins(:tag_name)
-             .includes(:tag_name, :materials, tag_name: :wiki_page)
-             .find_by(tag_names: { name: })
-    if tag
-      render json: TagRepr.base(tag)
-    else
-      head :not_found
-    end
+    tag =
+        Tag
+        .joins(:tag_name)
+        .includes(:tag_name, :materials, tag_name: :wiki_page)
+        .find_by(tag_names: { name: })
+    return render json: TagRepr.base(tag) if tag
+
+    platform, external_name = name.split(':', 2)
+    return head :not_found unless external_name
+
+    external_tag = ExternalTag.find_by(platform:, name: external_name)
+    return head :not_found unless external_tag
+
+    render json: ExternalTagRepr.base(external_tag)
   end
 
   def deerjikists
@@ -200,14 +327,45 @@ class TagsController < ApplicationController
              .find_by(id: params[:id])
     return head :not_found unless tag
 
+    rows = normalise_deerjikist_rows(tag)
+    return if performed?
+
     ApplicationRecord.transaction do
-      tag.deerjikists = []
-      params[:_json].each.with_index do |item, i|
-        platform = item[:platform]
-        code = normalise_deerjikist_code(platform, item[:code])
-        deerjikist = Deerjikist.find_or_initialize_by(platform:, code:)
-        deerjikist.tag = tag
-        render_deerjikist_form_record_invalid(deerjikist, i) unless deerjikist.save
+      tag.lock!
+
+      requested_keys = rows.map { |row| [row[:platform], row[:code]] }.uniq
+      row_indexes_by_key = rows_by_key(rows)
+      locked_deerjikists = lock_deerjikists_for_tag_update(tag.id, requested_keys)
+      current_deerjikists = locked_deerjikists.filter { |deerjikist|
+        deerjikist.tag_id == tag.id
+      }
+      requested_deerjikists = locked_deerjikists.filter { |deerjikist|
+        row_indexes_by_key.key?([deerjikist.platform, deerjikist.code])
+      }
+
+      render_deerjikist_conflicts(requested_deerjikists, row_indexes_by_key, tag)
+      raise ActiveRecord::Rollback if performed?
+
+      requested_keys_set = requested_keys.to_set
+      current_deerjikists.each do |deerjikist|
+        key = [deerjikist.platform, deerjikist.code]
+        deerjikist.destroy! unless requested_keys_set.include?(key)
+      end
+
+      existing_keys = requested_deerjikists.to_h { |deerjikist|
+        [[deerjikist.platform, deerjikist.code], true]
+      }
+      requested_keys.each do |platform, code|
+        next if existing_keys[[platform, code]]
+
+        deerjikist = Deerjikist.new(platform:, code:, tag:)
+        row_index = row_indexes_by_key[[platform, code]].first
+        begin
+          render_deerjikist_form_record_invalid(deerjikist, row_index) unless deerjikist.save
+        rescue ActiveRecord::RecordNotUnique
+          conflicts = lock_deerjikists_for_tag_update(tag.id, [[platform, code]])
+          render_deerjikist_conflicts(conflicts, row_indexes_by_key, tag)
+        end
         raise ActiveRecord::Rollback if performed?
       end
     end
@@ -251,11 +409,7 @@ class TagsController < ApplicationController
     parent_names = params[:parent_tags].to_s.split.uniq
     deprecated = bool?(:deprecated)
 
-    if tag.nico? && deprecated
-      return render_unprocessable_entity 'ニコタグは廃止できません．', field: :deprecated
-    end
-
-    if tag.nico? || category == 'nico'
+    if category == 'nico'
       return render_unprocessable_entity 'ニコタグは変更できません．', field: :category
     end
 
@@ -303,13 +457,9 @@ class TagsController < ApplicationController
 
     tag = Tag.find(params[:id])
 
-    if tag.nico? && deprecated_given && deprecated
-      return render_unprocessable_entity 'ニコタグは廃止できません．', field: :deprecated
-    end
-
     return unless validate_tag_rename(tag, name)
 
-    if tag.nico? || (category.present? && category == 'nico')
+    if category.present? && category == 'nico'
       return render_unprocessable_entity 'ニコタグは変更できません．', field: :category
     end
 
@@ -516,11 +666,6 @@ class TagsController < ApplicationController
   end
 
   def record_tag_version! tag, event_type:, created_by_user:, name_changed: false, wiki_page: nil
-    if tag.nico?
-      NicoTagVersionRecorder.record!(tag:, event_type:, created_by_user:)
-      return
-    end
-
     TagVersionRecorder.record!(tag:, event_type:, created_by_user:)
 
     return unless name_changed
@@ -542,7 +687,7 @@ class TagsController < ApplicationController
       return false
     end
 
-    target_tag_name = TagName.with_discarded.find_by(name:)
+    target_tag_name = TagName.find_by(name:)
     return true if target_tag_name.nil?
     return true if target_tag_name.canonical_id?
 
@@ -554,17 +699,14 @@ class TagsController < ApplicationController
     return if name == tag.name
 
     current_tag_name = tag.tag_name
-    target_tag_name = TagName.with_discarded.find_by(name:)
+    target_tag_name = TagName.find_by(name:)
 
     if target_tag_name.nil?
       current_tag_name.update!(name:)
       return
     end
 
-    promote_tag_alias!(
-        tag,
-        current_tag_name:,
-        promoted_tag_name: target_tag_name)
+    promote_tag_alias!(tag, current_tag_name:, promoted_tag_name: target_tag_name)
   end
 
   def promote_tag_alias! tag, current_tag_name:, promoted_tag_name:
@@ -574,11 +716,9 @@ class TagsController < ApplicationController
       TagVersioning.ensure_snapshot!(old_owner_tag, created_by_user: current_user)
     end
 
-    promoted_tag_name.undiscard! if promoted_tag_name.discarded?
     promoted_tag_name.update!(canonical: nil)
 
-    TagName.with_discarded
-           .where(canonical_id: current_tag_name.id)
+    TagName.where(canonical_id: current_tag_name.id)
            .where.not(id: promoted_tag_name.id)
            .find_each do |alias_tag_name|
       alias_tag_name.update!(canonical: promoted_tag_name)
@@ -609,7 +749,7 @@ class TagsController < ApplicationController
     end
 
     alias_names.each do |alias_name|
-      alias_tag_name = TagName.find_undiscard_or_create_by!(name: alias_name)
+      alias_tag_name = TagName.find_or_create_by!(name: alias_name)
       affected_tags << alias_tag_name.canonical&.tag
     end
 
@@ -624,7 +764,7 @@ class TagsController < ApplicationController
     end
 
     alias_names.each do |alias_name|
-      alias_tag_name = TagName.find_undiscard_or_create_by!(name: alias_name)
+      alias_tag_name = TagName.find_or_create_by!(name: alias_name)
       alias_tag_name.update!(canonical: tag.tag_name)
     end
 
@@ -635,8 +775,7 @@ class TagsController < ApplicationController
 
   def update_parent_tags! tag, parent_names
     parent_tags = Tag.normalise_tags!(parent_names, with_tagme: false,
-                                                    with_no_deerjikist: false,
-                                                    deny_nico: true)
+                                                    with_no_deerjikist: false)
 
     old_parent_tags = tag.parents.to_a
 
@@ -653,6 +792,7 @@ class TagsController < ApplicationController
   end
 
   def normalise_deerjikist_code platform, code
+    code = code.to_s
     return code if platform != 'youtube' || code[0] != '@'
 
     url = "https://www.youtube.com/#{ code }"
@@ -669,6 +809,82 @@ class TagsController < ApplicationController
     nil
   end
 
+  def normalise_deerjikist_rows tag
+    rows = []
+
+    params[:_json].each.with_index do |item, index|
+      platform = item[:platform]
+      unless Deerjikist.platforms.key?(platform)
+        render_deerjikist_platform_invalid(index)
+        return rows
+      end
+
+      code = normalise_deerjikist_code(platform, item[:code])
+      deerjikist = Deerjikist.new(platform:, code:, tag:)
+      unless deerjikist.valid?
+        render_deerjikist_form_record_invalid(deerjikist, index)
+        return rows
+      end
+
+      rows << { index:, platform:, code: }
+    end
+
+    rows
+  end
+
+  def lock_deerjikists_for_tag_update tag_id, keys
+    clauses = ['tag_id = ?']
+    values = [tag_id]
+
+    keys.each do |platform, code|
+      clauses << '(platform = ? AND code = ?)'
+      values << platform << code
+    end
+
+    Deerjikist
+      .where(clauses.join(' OR '), *values)
+      .order(:platform, :code)
+      .lock
+      .to_a
+  end
+
+  def render_deerjikist_conflicts deerjikists, row_indexes_by_key, tag
+    conflicts = deerjikists.filter { |deerjikist| deerjikist.tag_id != tag.id }
+    return if conflicts.empty?
+
+    tag_names_by_id = Tag
+                      .joins(:tag_name)
+                      .where(id: conflicts.map(&:tag_id).uniq)
+                      .pluck('tags.id', 'tag_names.name')
+                      .to_h
+    fields = { }
+
+    conflicts.each do |deerjikist|
+      message = "この情報は既に「#{ tag_names_by_id[deerjikist.tag_id] }」に紐づいてゐます．"
+      row_indexes_by_key[[deerjikist.platform, deerjikist.code]].each do |index|
+        field = :"deerjikists.#{ index }.code"
+        fields[field] ||= []
+        fields[field] << message
+      end
+    end
+
+    render_validation_error fields:
+  end
+
+  def render_deerjikist_platform_invalid index
+    render_validation_error fields: {
+      :"deerjikists.#{ index }.platform" => ['値が不正です．'],
+    }
+  end
+
+  def rows_by_key rows
+    rows.each_with_object({ }) do |row, result|
+      key = [row[:platform], row[:code]]
+      result[key] ||= []
+      result[key] << row[:index]
+    end
+  end
+
   def render_deerjikist_form_record_invalid deerjikist, index
     fields = { }
 
@@ -687,4 +903,20 @@ class TagsController < ApplicationController
 
     render_validation_error fields:
   end
+
+  def external_tag_json tag
+    { 'id' => tag.id,
+      'name' => "#{ tag.platform }:#{ tag.name }",
+      'category' => 'nico',
+      'deprecated_at' => nil,
+      'created_at' => tag.created_at,
+      'updated_at' => tag.created_at,
+      'post_count' => tag.post_count,
+      'matched_alias' => nil,
+      'aliases' => [],
+      'parents' => [],
+      'has_wiki' => false,
+      'material_id' => nil,
+      'has_deerjikists' => false }
+  end
 end

backend/app/controllers/wiki_pages_controller.rb

@@ -94,7 +94,7 @@ class WikiPagesController < ApplicationController
     return render_unprocessable_entity('タイトルは必須です．', field: :title) if title.blank?
     return render_unprocessable_entity('本文は必須です．', field: :body) if body.blank?
 
-    tag_name = TagName.find_undiscard_or_create_by!(name: title)
+    tag_name = TagName.find_or_create_by!(name: title)
 
     page =
         Wiki::Commit.create_content!(

backend/app/models/external_tag.rb

@@ -0,0 +1,20 @@
+class ExternalTag < ApplicationRecord
+  enum :platform, nico: 'nico'
+  validates :platform, presence: true, inclusion: { in: ExternalTag.platforms.keys }
+
+  has_many :post_external_tags, dependent: :delete_all
+  has_many :posts, through: :post_external_tags
+
+  has_many :nico_tag_versions, foreign_key: :tag_id, inverse_of: :external_tag
+
+  has_many :nico_tag_relations,
+           foreign_key: :nico_tag_id,
+           inverse_of: :nico_tag,
+           dependent: :destroy
+
+  has_many :linked_tags, through: :nico_tag_relations, source: :tag
+
+  def snapshot_linked_tag_names
+    linked_tags.joins(:tag_name).order('tag_names.name').pluck('tag_names.name')
+  end
+end

backend/app/models/nico_tag_relation.rb

@@ -1,24 +1,10 @@
 class NicoTagRelation < ApplicationRecord
-  belongs_to :nico_tag, class_name: 'Tag'
-  belongs_to :tag, class_name: 'Tag'
+  belongs_to :nico_tag,
+             class_name: 'ExternalTag',
+             foreign_key: :nico_tag_id,
+             inverse_of: :nico_tag_relations
+  belongs_to :tag, class_name: 'Tag', foreign_key: :tag_id
 
   validates :nico_tag_id, presence: true
   validates :tag_id, presence: true
-
-  validate :nico_tag_must_be_nico
-  validate :tag_mustnt_be_nico
-
-  private
-
-  def nico_tag_must_be_nico
-    if nico_tag && nico_tag.category != 'nico'
-      errors.add :nico_tag_id, 'タグのカテゴリがニコニコである必要があります．'
-    end
-  end
-
-  def tag_mustnt_be_nico
-    if tag && tag.category == 'nico'
-      errors.add :tag_id, '連携先タグのカテゴリはニコニコであってはなりません．'
-    end
-  end
 end

backend/app/models/nico_tag_version.rb

@@ -1,7 +1,7 @@
 class NicoTagVersion < ApplicationRecord
   include VersionRecord
 
-  belongs_to :tag
+  belongs_to :external_tag, foreign_key: :tag_id, inverse_of: :nico_tag_versions
 
   validates :name, presence: true
 end

backend/app/models/post.rb

@@ -1,7 +1,9 @@
 class Post < ApplicationRecord
   require 'date'
   require 'mini_magick'
+  require 'nokogiri'
   require 'stringio'
+  require 'timeout'
 
   class RemoteThumbnailFetchFailed < StandardError; end
 
@@ -11,44 +13,69 @@ class Post < ApplicationRecord
   ORIGINAL_CREATED_ORDER_MESSAGE = 'オリジナルの作成日時の順番がをかしぃです．'.freeze
   ORIGINAL_CREATED_MINIMUM_RANGE_MESSAGE =
       'オリジナルの作成日時の範囲は1分以上必要です．'.freeze
-
-  def self.resized_thumbnail_attachment(upload)
+  REMOTE_SVG_CONTENT_TYPE = 'image/svg+xml'.freeze
+  MAX_SVG_DIMENSION = 4_096
+  MAX_SVG_PIXELS = 16_777_216
+  THUMBNAIL_PROCESS_TIMEOUT = 5.seconds
+  def self.resized_thumbnail_attachment(upload, content_type: nil)
     upload.rewind
-    image = MiniMagick::Image.read(upload.read)
-    image.resize '180x180^'
-    image.gravity 'Center'
-    image.extent '180x180'
-    image.format 'jpg'
+    bytes = upload.read
+    blob = Timeout.timeout(THUMBNAIL_PROCESS_TIMEOUT) do
+      image = image_for_thumbnail_upload(bytes, content_type:)
+      image.auto_orient
+      image.resize '180x180'
+      image.format 'jpg'
+      image.to_blob
+    end
 
-    { io: StringIO.new(image.to_blob),
+    { io: StringIO.new(blob),
       filename: 'resized_thumbnail.jpg',
       content_type: 'image/jpeg' }
+  rescue Timeout::Error
+    raise MiniMagick::Error, 'サムネイル画像の変換に失敗しました．'
   ensure
     upload.rewind
   end
 
+  def self.remote_thumbnail_attachment(raw_url)
+    response = Preview::ThumbnailFetcher.fetch_image_response(raw_url)
+    resized_thumbnail_attachment(
+        StringIO.new(response.body),
+        content_type: response.content_type)
+  rescue Preview::UrlSafety::UnsafeUrl,
+         Preview::ThumbnailFetcher::GenerationFailed,
+         Preview::HttpFetcher::FetchFailed,
+         Preview::HttpFetcher::FetchTimeout,
+         Preview::HttpFetcher::ResponseTooLarge,
+         Timeout::Error,
+         MiniMagick::Error => e
+    raise RemoteThumbnailFetchFailed, e.message
+  end
+
   belongs_to :uploaded_user, class_name: 'User', optional: true
 
   has_many :post_tags, dependent: :destroy, inverse_of: :post
-  has_many :active_post_tags, -> { kept }, class_name: 'PostTag', inverse_of: :post
-  has_many :post_tags_with_discarded, -> { with_discarded }, class_name: 'PostTag'
-  has_many :tags, through: :active_post_tags
+  has_many :tags, through: :post_tags
   has_many :active_tags, -> { where(tags: { deprecated_at: nil }) },
-           through: :active_post_tags, source: :tag
+           through: :post_tags,
+           source: :tag
 
   has_many :user_post_views, dependent: :delete_all
   has_many :post_similarities, dependent: :delete_all
   has_many :post_versions
+
   has_many :gekanator_guessed_games,
            class_name: 'GekanatorGame',
            foreign_key: :guessed_post_id,
            dependent: :delete_all,
            inverse_of: :guessed_post
+
   has_many :gekanator_correct_games,
            class_name: 'GekanatorGame',
            foreign_key: :correct_post_id,
            dependent: :delete_all,
            inverse_of: :correct_post
+
   has_many :gekanator_question_examples, dependent: :delete_all
 
   has_many :parent_post_implications,
@@ -65,6 +92,9 @@ class Post < ApplicationRecord
            inverse_of: :parent_post
   has_many :children, through: :child_post_implications, source: :post
 
+  has_many :post_external_tags, dependent: :destroy
+  has_many :external_tags, through: :post_external_tags
+
   has_one_attached :thumbnail
 
   attribute :version_no, :integer, default: 1
@@ -98,7 +128,6 @@ class Post < ApplicationRecord
 
   def snapshot_tag_names
     post_tags
-        .kept
         .joins(tag: :tag_name)
         .includes(:sections, tag: :tag_name)
         .order('tag_names.name')
@@ -112,8 +141,46 @@ class Post < ApplicationRecord
     end
   end
 
+  def self.tag_snapshot_literal tag
+    sections = tag.fetch('sections', []).map do |sec|
+      begin_ms = sec.fetch('begin_ms')
+      end_ms = sec['end_ms']
+
+      "[#{ Post.ms_to_time(begin_ms) }-#{ end_ms ? Post.ms_to_time(end_ms) : '' }]"
+    end
+
+    "#{ tag.fetch('name') }#{ sections.join }"
+  end
+
+  def snapshot_tags_json
+    tag_snapshots =
+        post_tags
+            .joins(tag: :tag_name)
+            .includes(:sections, tag: :tag_name)
+            .order('tags.id')
+            .map { |pt|
+          { 'tag_id' => pt.tag.id,
+            'version_no' => pt.tag.version_no,
+            'name' => pt.tag.name,
+            'category' => pt.tag.category,
+            'sections' => pt.sections.sort_by(&:begin_ms).map {
+                            { 'begin_ms' => _1.begin_ms, 'end_ms' => _1.end_ms }
+                          } }
+        }
+
+    external_tag_snapshots =
+        post_external_tags.order(:external_tag_id).map {
+          { 'external_tag_id' => _1.external_tag_id }
+        }
+
+    tag_snapshots + external_tag_snapshots
+  end
+
   def self.section_literal section
-    "[#{ Post.ms_to_time(section.begin_ms) }-#{ section.end_ms ? Post.ms_to_time(section.end_ms) : '' }]"
+    end_ms =
+        section.end_ms ? Post.ms_to_time(section.end_ms) : ''
+
+    "[#{ Post.ms_to_time(section.begin_ms) }-#{ end_ms }]"
   end
 
   def self.ms_to_time ms
@@ -157,17 +224,7 @@ class Post < ApplicationRecord
   end
 
   def attach_thumbnail_from_url! raw_url
-    response = Preview::ThumbnailFetcher.fetch_image_response(raw_url)
-    thumbnail.attach(
-        self.class.resized_thumbnail_attachment(
-            StringIO.new(response.body)))
-  rescue Preview::UrlSafety::UnsafeUrl,
-         Preview::ThumbnailFetcher::GenerationFailed,
-         Preview::HttpFetcher::FetchFailed,
-         Preview::HttpFetcher::FetchTimeout,
-         Preview::HttpFetcher::ResponseTooLarge,
-         MiniMagick::Error => e
-    raise RemoteThumbnailFetchFailed, e.message
+    thumbnail.attach(self.class.remote_thumbnail_attachment(raw_url))
   end
 
   private
@@ -209,6 +266,179 @@ class Post < ApplicationRecord
     self.url = PostUrlNormaliser.normalise(url) || url.strip
   end
 
+  def self.image_for_thumbnail_upload(bytes, content_type: nil)
+    if svg_content_type?(content_type) || svg_document_bytes?(bytes)
+      return decode_svg_thumbnail(bytes)
+    end
+
+    raise MiniMagick::Error, 'サムネイル画像の形式が不正です．' unless raster_thumbnail_bytes?(bytes)
+
+    decode_raster_thumbnail(bytes)
+  end
+
+  def self.raster_thumbnail_bytes?(bytes)
+    raster_thumbnail_format(bytes).present?
+  end
+
+  def self.remote_thumbnail_image_bytes?(bytes, content_type: nil)
+    return true if svg_content_type?(content_type) || svg_document_bytes?(bytes)
+
+    raster_thumbnail_bytes?(bytes)
+  end
+
+  def self.svg_content_type?(content_type)
+    content_type.to_s.split(';', 2).first.to_s.downcase.strip == REMOTE_SVG_CONTENT_TYPE
+  end
+
+  def self.svg_document_bytes?(bytes)
+    document = Nokogiri::XML(
+        bytes,
+        nil,
+        nil,
+        Nokogiri::XML::ParseOptions::STRICT |
+        Nokogiri::XML::ParseOptions::NONET)
+    document.root&.name == 'svg'
+  rescue Nokogiri::XML::SyntaxError
+    false
+  end
+
+  def self.decode_raster_thumbnail(bytes)
+    MiniMagick::Image.read(bytes)
+  end
+
+  def self.decode_svg_thumbnail(bytes)
+    MiniMagick::Image.read(sanitised_svg_bytes(bytes))
+  end
+
+  def self.raster_thumbnail_format(bytes)
+    binary = bytes.to_s.b
+    return 'jpeg' if binary.start_with?("\xFF\xD8\xFF".b)
+    return 'png' if binary.start_with?("\x89PNG\r\n\x1A\n".b)
+    return 'gif' if binary.start_with?('GIF87a'.b) || binary.start_with?('GIF89a'.b)
+    return 'webp' if binary.bytesize >= 12 &&
+                     binary.start_with?('RIFF'.b) &&
+                     binary.byteslice(8, 4) == 'WEBP'
+
+    nil
+  end
+
+  def self.sanitised_svg_bytes(bytes)
+    parse_options =
+        Nokogiri::XML::ParseOptions::STRICT |
+        Nokogiri::XML::ParseOptions::NONET
+    document = Nokogiri::XML(
+        bytes,
+        nil,
+        nil,
+        parse_options)
+    root = document.root
+    raise MiniMagick::Error, 'SVG が不正です．' if root == nil || root.name != 'svg'
+    raise MiniMagick::Error, 'SVG が不正です．' if document.internal_subset != nil
+    raise MiniMagick::Error, 'SVG が不正です．' if svg_uses_disallowed_features?(document)
+
+    width, height = svg_dimensions(root)
+    raise MiniMagick::Error, 'SVG が大きすぎます．' if width == nil || height == nil
+    if width > MAX_SVG_DIMENSION || height > MAX_SVG_DIMENSION || width * height > MAX_SVG_PIXELS
+      raise MiniMagick::Error, 'SVG が大きすぎます．'
+    end
+
+    document.to_xml
+  rescue Nokogiri::XML::SyntaxError
+    raise MiniMagick::Error, 'SVG が不正です．'
+  end
+
+  def self.svg_uses_disallowed_features?(document)
+    document.traverse.any? do |node|
+      next false unless node.element?
+
+      name = node.name.to_s.downcase
+      next true if name == 'script' || name == 'foreignobject'
+      next style_contains_disallowed_urls?(node.text.to_s) if name == 'style'
+
+      node.attribute_nodes.any? do |attribute|
+        attribute_name = attribute.name.to_s.downcase
+        attribute_value = attribute.value.to_s
+        attribute_name.start_with?('on') ||
+            external_svg_reference?(attribute_name, attribute_value)
+      end
+    end
+  end
+
+  def self.external_svg_reference?(attribute_name, attribute_value)
+    if ['href', 'xlink:href', 'src'].include?(attribute_name)
+      return external_svg_url?(attribute_value)
+    end
+    return style_contains_disallowed_urls?(attribute_value) if attribute_name == 'style'
+    return svg_url_function_disallowed?(attribute_value) if attribute_value.match?(/url\s*\(/i)
+
+    false
+  end
+
+  def self.external_svg_url?(value)
+    stripped = value.to_s.strip
+    return false if stripped.blank? || stripped.start_with?('#')
+
+    true
+  end
+
+  def self.style_contains_disallowed_urls?(value)
+    text = value.to_s
+    text.match?(/@import/i) || svg_url_function_disallowed?(text)
+  end
+
+  def self.svg_url_function_disallowed?(value)
+    value.to_s.scan(/url\s*\(([^)]*)\)/i).flatten.any? do |entry|
+      reference =
+          entry.to_s.strip
+               .delete_prefix("'")
+               .delete_prefix('"')
+               .delete_suffix("'")
+               .delete_suffix('"')
+      reference.present? && !(reference.start_with?('#'))
+    end
+  end
+
+  def self.svg_dimensions(root)
+    width = svg_length_to_pixels(root['width'])
+    height = svg_length_to_pixels(root['height'])
+    return [width, height] if width && height
+
+    view_box = root['viewBox'].to_s.strip.split(/\s+/).map { Float(_1) rescue nil }
+    return [nil, nil] if view_box.length != 4 || view_box.any?(&:nil?)
+    return [nil, nil] unless view_box[2].finite? && view_box[2].positive?
+    return [nil, nil] unless view_box[3].finite? && view_box[3].positive?
+
+    [view_box[2], view_box[3]]
+  end
+
+  def self.svg_length_to_pixels(value)
+    return nil if value.blank?
+
+    matched = /\A([0-9]+(?:\.[0-9]+)?)(px)?\z/i.match(value.to_s.strip)
+    return nil if matched == nil
+
+    pixels = Float(matched[1])
+    return nil unless pixels.finite? && pixels.positive?
+
+    pixels
+  rescue ArgumentError
+    nil
+  end
+
+  private_class_method :image_for_thumbnail_upload,
+                       :svg_content_type?,
+                       :decode_raster_thumbnail,
+                       :decode_svg_thumbnail,
+                       :raster_thumbnail_format,
+                       :sanitised_svg_bytes,
+                       :svg_uses_disallowed_features?,
+                       :external_svg_reference?,
+                       :external_svg_url?,
+                       :style_contains_disallowed_urls?,
+                       :svg_url_function_disallowed?,
+                       :svg_dimensions,
+                       :svg_length_to_pixels
+
   def parse_original_created_value field
     raw_value = public_send("#{ field }_before_type_cast")
     value = public_send(field)
@@ -252,7 +482,12 @@ class Post < ApplicationRecord
 
     match =
         value.match(
-            /\A(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2})(?::(\d{2})(?:\.(\d+))?)?(Z|[+-]\d{2}:?\d{2})?\z/)
+            /
+              \A(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2})
+              (?::(\d{2})(?:\.(\d+))?)?
+              (Z|[+-]\d{2}:?\d{2})?
+              \z
+            /x)
     return nil if match.nil?
 
     year = match[1].to_i

backend/app/models/post_external_tag.rb

@@ -0,0 +1,4 @@
+class PostExternalTag < ApplicationRecord
+  belongs_to :post
+  belongs_to :external_tag, counter_cache: :post_count
+end

backend/app/models/post_tag.rb

@@ -1,14 +1,7 @@
 class PostTag < ApplicationRecord
-  include Discard::Model
-
-  before_destroy do
-    raise ActiveRecord::ReadOnlyRecord, '消さないでください．'
-  end
-
   belongs_to :post
   belongs_to :tag, counter_cache: :post_count
   belongs_to :created_user, class_name: 'User', optional: true
-  belongs_to :deleted_user, class_name: 'User', optional: true
 
   has_many :sections, -> { order(:begin_ms) }, class_name: 'PostTagSection',
                                                foreign_key: [:post_id, :tag_id],
@@ -18,18 +11,5 @@ class PostTag < ApplicationRecord
 
   validates :post_id, presence: true
   validates :tag_id, presence: true
-  validates :post_id, uniqueness: {
-      scope: :tag_id,
-      conditions: -> { where(discarded_at: nil) } }
-
-  def discard_by! deleted_user
-    return self if discarded?
-
-    transaction do
-      update!(discarded_at: Time.current, deleted_user:)
-      Tag.where(id: tag_id).update_all('post_count = GREATEST(post_count - 1, 0)')
-    end
-
-    self
-  end
+  validates :post_id, uniqueness: { scope: :tag_id }
 end

backend/app/models/post_tag_section.rb

@@ -4,10 +4,10 @@ class PostTagSection < ApplicationRecord
   belongs_to :post
   belongs_to :tag
 
-  belongs_to :post_tag, -> { kept }, foreign_key: [:post_id, :tag_id],
-                                     primary_key: [:post_id, :tag_id],
-                                     inverse_of: :sections,
-                                     optional: true
+  belongs_to :post_tag, foreign_key: [:post_id, :tag_id],
+                        primary_key: [:post_id, :tag_id],
+                        inverse_of: :sections,
+                        optional: true
 
   validates :post_id, presence: true
   validates :tag_id, presence: true

backend/app/models/tag.rb

@@ -2,8 +2,6 @@ require 'set'
 
 
 class Tag < ApplicationRecord
-  include MyDiscard
-
   class NicoTagNormalisationError < ArgumentError
     ;
   end
@@ -28,15 +26,12 @@ class Tag < ApplicationRecord
   end
 
   has_many :post_tags, inverse_of: :tag
-  has_many :active_post_tags, -> { kept }, class_name: 'PostTag', inverse_of: :tag
-  has_many :post_tags_with_discarded, -> { with_discarded }, class_name: 'PostTag'
-  has_many :posts, through: :active_post_tags
-
-  has_many :nico_tag_relations, foreign_key: :nico_tag_id, dependent: :destroy
-  has_many :linked_tags, through: :nico_tag_relations, source: :tag
+  has_many :posts, through: :post_tags
 
   has_many :reversed_nico_tag_relations,
-           class_name: 'NicoTagRelation', foreign_key: :tag_id, dependent: :destroy
+           class_name: 'NicoTagRelation',
+           foreign_key: :tag_id,
+           dependent: :destroy
   has_many :linked_nico_tags, through: :reversed_nico_tag_relations, source: :nico_tag
 
   has_many :tag_implications, foreign_key: :parent_tag_id, dependent: :destroy
@@ -54,7 +49,6 @@ class Tag < ApplicationRecord
   has_many :materials
 
   has_many :tag_versions
-  has_many :nico_tag_versions
 
   belongs_to :tag_name
   delegate :wiki_page, to: :tag_name
@@ -69,17 +63,13 @@ class Tag < ApplicationRecord
                   character:   'character',
                   general:     'general',
                   material:    'material',
-                  nico:        'nico',
                   meta:        'meta'
 
   validates :category, presence: true, inclusion: { in: Tag.categories.keys }
 
-  validate :nico_tag_name_must_start_with_nico
+  validate :tag_name_mustnt_start_with_nico
   validate :tag_name_must_be_canonical
   validate :category_must_be_deerjikist_with_deerjikists
-  validate :nico_tags_cannot_be_deprecated
-
-  scope :nico_tags, -> { nico }
 
   CATEGORY_PREFIXES = {
       'general:'    => :general,
@@ -114,10 +104,9 @@ class Tag < ApplicationRecord
 
   def self.normalise_tags! tag_names, with_tagme: true,
                                       with_no_deerjikist: true,
-                                      deny_nico: true,
                                       deny_deprecated: false,
                                       with_sections: false
-    if deny_nico && tag_names.any? { |n| n.downcase.start_with?('nico:') }
+    if tag_names.any? { |n| n.downcase.start_with?('nico:') }
       raise NicoTagNormalisationError
     end
 
@@ -234,10 +223,10 @@ class Tag < ApplicationRecord
   end
 
   def self.find_or_create_by_tag_name! name, category:
-    tn = TagName.find_undiscard_or_create_by!(name: name.to_s.strip)
+    tn = TagName.find_or_create_by!(name: name.to_s.strip)
     tn = tn.canonical if tn.canonical_id?
 
-    Tag.find_undiscard_or_create_by!(tag_name_id: tn.id) do |t|
+    Tag.find_or_create_by!(tag_name_id: tn.id) do |t|
       t.category = category
     end
   rescue ActiveRecord::RecordNotUnique
@@ -259,11 +248,11 @@ class Tag < ApplicationRecord
 
         TagVersioning.ensure_snapshot!(source_tag, created_by_user:)
 
-        source_tag.post_tags.kept.find_each do |source_pt|
+        source_tag.post_tags.find_each do |source_pt|
           post_id = source_pt.post_id
           affected_post_ids << post_id
-          source_pt.discard_by!(created_by_user)
-          unless PostTag.kept.exists?(post_id:, tag: target_tag)
+          source_pt.destroy!
+          unless PostTag.exists?(post_id:, tag: target_tag)
             PostTag.create!(post_id:, tag: target_tag)
           end
         end
@@ -275,14 +264,10 @@ class Tag < ApplicationRecord
         end
 
         TagVersioning.record!(source_tag, event_type: :discard, created_by_user:)
-        source_tag.discard!
+        source_tag.destroy!
 
-        if source_tag.nico?
-          source_tag_name.discard!
-        else
-          source_tag_name.update_columns(canonical_id: target_tag.tag_name_id,
-                                         updated_at: Time.current)
-        end
+        source_tag_name.update_columns(canonical_id: target_tag.tag_name_id,
+                                       updated_at: Time.current)
 
         TagVersioning.record!(target_tag, event_type: :update, created_by_user:)
       end
@@ -293,13 +278,13 @@ class Tag < ApplicationRecord
       end
 
       # 投稿件数を再集計
-      target_tag.update_columns(post_count: PostTag.kept.where(tag: target_tag).count)
+      target_tag.update_columns(post_count: PostTag.where(tag: target_tag).count)
     end
 
     target_tag.reload
   end
 
-  def snapshot_aliases = tag_name.aliases.kept.order(:name).pluck(:name)
+  def snapshot_aliases = tag_name.aliases.order(:name).pluck(:name)
 
   def snapshot_parent_tag_ids = parents.order(:id).pluck(:id)
 
@@ -309,11 +294,9 @@ class Tag < ApplicationRecord
 
   private
 
-  def nico_tag_name_must_start_with_nico
-    n = name.to_s
-    if ((nico? && !(n.downcase.start_with?('nico:'))) || 
-        (!(nico?) && n.downcase.start_with?('nico:')))
-      errors.add :name, 'ニコニコ・タグの命名規則に反してゐます．'
+  def tag_name_mustnt_start_with_nico
+    if name.to_s.downcase.start_with?('nico:')
+      errors.add :name, 'タグの命名規則に反してゐます．'
     end
   end
 
@@ -354,10 +337,4 @@ class Tag < ApplicationRecord
 
     total_s * 1_000 + match[:ms].to_s.ljust(3, '0')[0, 3].to_i
   end
-
-  def nico_tags_cannot_be_deprecated
-    if nico? && deprecated_at.present?
-      errors.add :deprecated_at, 'ニコタグは廃止できません．'
-    end
-  end
 end

backend/app/models/tag_name.rb

@@ -1,6 +1,4 @@
 class TagName < ApplicationRecord
-  include MyDiscard
-
   has_one :tag
   has_one :wiki_page
 

backend/app/models/tag_name_sanitisation_rule.rb

@@ -32,7 +32,7 @@ class TagNameSanitisationRule < ApplicationRecord
             elsif source_tag
               source_tag.update_columns(tag_name_id: existing_tn.id, updated_at: Time.current)
             end
-            tn.discard!
+            tn.destroy!
 
             next
           end

backend/app/representations/external_tag_repr.rb

@@ -0,0 +1,20 @@
+module ExternalTagRepr
+  module_function
+
+  def base tag
+    { 'id' => tag.id,
+      'name' => "#{ tag.platform }:#{ tag.name }",
+      'category' => 'nico',
+      'post_count' => tag.post_count,
+      'created_at' => tag.created_at,
+      'updated_at' => tag.created_at,
+      'deprecated_at' => nil,
+      'aliases' => [],
+      'parents' => [],
+      'has_wiki' => false,
+      'material_id' => nil,
+      'has_deerjikists' => false }
+  end
+
+  def inline(tag) = base(tag)
+end

backend/app/representations/post_compact_repr.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+
+module PostCompactRepr
+  module_function
+
+  def base post, host: nil
+    return nil if post.nil?
+
+    PostRepr
+        .common(post, host:)
+        .slice(
+            'id',
+            'title',
+            'url',
+            'thumbnail',
+            'thumbnail_base')
+  end
+end

backend/app/representations/post_repr.rb

@@ -17,8 +17,13 @@ module PostRepr
 
   module_function
 
-  def base post, current_user = nil
-    json = common(post)
+  def base post, current_user = nil, host: nil
+    json =
+        if host.present?
+          common(post, host:)
+        else
+          common(post)
+        end
     json['tags'] = tag_json(post)
     json['uploaded_user'] = post.uploaded_user && UserRepr.base(post.uploaded_user)
     json['viewed'] = current_user ? current_user.viewed?(post) : false
@@ -26,50 +31,98 @@ module PostRepr
   end
 
   def detail post, current_user = nil, parent_posts: [], child_posts: [],
-             sibling_posts: { }, related: []
-    base(post, current_user).merge(
-        'parent_posts' => cards(parent_posts),
-        'child_posts' => cards(child_posts),
-        'sibling_posts' => sibling_posts.transform_keys(&:to_s).transform_values { |posts|
-          cards(posts)
-        },
-        'related' => cards(related))
+             sibling_posts: { }, related: [], host: nil
+    if host.present?
+      base(post, current_user, host:).merge(
+          'parent_posts' => cards(parent_posts, host:),
+          'child_posts' => cards(child_posts, host:),
+          'sibling_posts' => sibling_posts.transform_keys(&:to_s).transform_values { |posts|
+            cards(posts, host:)
+          },
+          'related' => cards(related, host:))
+    else
+      base(post, current_user).merge(
+          'parent_posts' => cards(parent_posts),
+          'child_posts' => cards(child_posts),
+          'sibling_posts' => sibling_posts.transform_keys(&:to_s).transform_values { |posts|
+            cards(posts)
+          },
+          'related' => cards(related))
+    end
   end
 
-  def card post
-    common(post).merge('parent_posts' => [], 'child_posts' => [])
+  def card post, host: nil
+    if host.present?
+      common(post, host:).merge('parent_posts' => [], 'child_posts' => [])
+    else
+      common(post).merge('parent_posts' => [], 'child_posts' => [])
+    end
   end
 
-  def cards posts
-    posts.map { |post| card(post) }
+  def cards posts, host: nil
+    if host.present?
+      posts.map { |post| card(post, host:) }
+    else
+      posts.map { |post| card(post) }
+    end
   end
 
-  def many posts, current_user = nil
-    posts.map { |p| base(p, current_user) }
+  def many posts, current_user = nil, host: nil
+    if host.present?
+      posts.map { |p| base(p, current_user, host:) }
+    else
+      posts.map { |p| base(p, current_user) }
+    end
   end
 
-  def common post
+  def common post, host: nil
     BASE_FIELDS.to_h { |field| [field.to_s, post.public_send(field)] }
-               .merge('thumbnail' => thumbnail_url(post))
+               .merge(
+                   'thumbnail' =>
+                       if host.present?
+                         thumbnail_url(post, host:)
+                       else
+                         thumbnail_url(post)
+                       end)
   end
 
   def tag_json post
-    post
-        .active_post_tags
+    internal_tags =
+        post
+        .post_tags
         .reject { _1.tag.deprecated? }
         .sort_by { _1.tag.name }
         .map { |post_tag|
-      TagRepr.inline(post_tag.tag).merge(
-          'children' => [],
-          'sections' => post_tag.sections.as_json(only: [:begin_ms, :end_ms]))
-    }
+          TagRepr.inline(post_tag.tag).merge(
+              'children' => [],
+              'sections' => post_tag.sections.as_json(only: [:begin_ms, :end_ms]))
+        }
+
+    external_tags =
+        post
+        .external_tags
+        .sort_by { _1.name }
+        .map { ExternalTagRepr.base(_1).merge('children' => [], 'sections' => []) }
+
+    internal_tags + external_tags
   end
 
-  def thumbnail_url post
+  def thumbnail_url post, host: nil
     return nil unless post.thumbnail.attached?
 
-    Rails.application.routes.url_helpers.rails_blob_url(post.thumbnail, only_path: false)
-  rescue
+    options = { only_path: false }
+    options[:host] = host if host.present?
+
+    Rails.application.routes.url_helpers.rails_storage_proxy_url(post.thumbnail, **options)
+  rescue ActionController::UrlGenerationError, ArgumentError, URI::InvalidURIError => e
+    payload = {
+        post_id: post.id,
+        attachment_id: post.thumbnail.attachment&.id,
+        blob_id: post.thumbnail.blob&.id,
+        error_class: e.class,
+        message: e.message }
+
+    Rails.logger.warn("PostRepr.thumbnail_url failed #{ payload.to_json }")
     nil
   end
 end

backend/app/services/nico_tag_version_recorder.rb

@@ -1,19 +1,22 @@
 class NicoTagVersionRecorder < VersionRecorder
-  def self.record! tag:, event_type:, created_by_user:
-    new(tag:, event_type:, created_by_user:).record!
+  def self.record! external_tag:, event_type:, created_by_user:
+    new(external_tag:, event_type:, created_by_user:).record!
   end
 
-  def initialize tag:, event_type:, created_by_user:
-    super(record: tag, event_type:, created_by_user:)
+  def initialize external_tag:, event_type:, created_by_user:
+    super(record: external_tag, event_type:, created_by_user:)
   end
 
   private
 
   def version_class = NicoTagVersion
   def version_association = :nico_tag_versions
-  def record_key = :tag
+  def record_key = :external_tag
 
   def snapshot_attributes
-    { name: @record.name, linked_tags: @record.snapshot_linked_tag_names.join(' ') }
+    { name: "#{ @record.platform }:#{ @record.name }",
+      linked_tags: @record.snapshot_linked_tag_names.join(' ') }
   end
+
+  def tracks_version_no_on_record? = false
 end

backend/app/services/post_bulk_creator.rb

@@ -0,0 +1,202 @@
+class PostBulkCreator
+  def initialize actor:, posts:, thumbnails:, host: nil
+    @actor_id = actor.id
+    @posts = posts
+    @thumbnails = thumbnails
+    @host = host
+  end
+
+  def run
+    results = Array.new(@posts.length)
+    mutex = Mutex.new
+    next_index = 0
+
+    workers = Array.new(2) do
+      Thread.new do
+        Rails.application.executor.wrap do
+          ActiveRecord::Base.connection_pool.with_connection do
+            actor = User.find(@actor_id)
+            loop do
+              index = nil
+              begin
+                index = mutex.synchronize do
+                  current = next_index
+                  next_index += 1
+                  current
+                end
+                break if index >= @posts.length
+
+                attributes = @posts[index]
+                results[index] = create_row(actor, attributes, index)
+              rescue StandardError => e
+                Rails.logger.error(
+                    "post_bulk_creator_worker_failure #{ { error: e.class.name,
+                                                          message: e.message,
+                                                          index: }.to_json }")
+                results[index] = {
+                    status: 'failed',
+                    recoverable: false,
+                    errors: { base: ['登録中にエラーが発生しました．'] },
+                    base_errors: [] }
+              end
+            end
+          end
+        end
+      end
+    end
+    workers.each(&:join)
+
+    results.each_index do |index|
+      next if results[index].present?
+
+      results[index] = {
+          status: 'failed',
+          recoverable: false,
+          errors: { base: ['登録中にエラーが発生しました．'] },
+          base_errors: [] }
+    end
+
+    { results: }
+  end
+
+  private
+
+  def create_row actor, attributes, index
+    preflight =
+        PostCreatePreflight.new(
+            attributes: attributes,
+            thumbnail: thumbnail_for(index, attributes),
+            host: @host).run
+    if preflight[:existing_post_id].present?
+      return {
+          status: 'skipped',
+          existing_post_id: preflight[:existing_post_id],
+          existing_post: preflight[:existing_post] }
+    end
+
+    post = PostCreator.new(
+        actor: actor,
+        attributes: normalised_attributes(attributes, preflight, index)).create!
+    result = {
+        status: 'created',
+        post: { id: post.id } }
+    result[:field_warnings] = preflight[:field_warnings] if preflight[:field_warnings].present?
+    result[:base_warnings] = preflight[:base_warnings] if preflight[:base_warnings].present?
+    result
+  rescue PostCreatePreflight::ValidationFailed => e
+    {
+        status: 'failed',
+        recoverable: true,
+        errors: e.fields,
+        base_errors: e.base_errors }
+  rescue ActiveRecord::RecordInvalid => e
+    existing_post = existing_post_for_race(attributes, e.record)
+    if existing_post.present?
+      return {
+          status: 'skipped',
+          existing_post_id: existing_post[:id],
+          existing_post: existing_post }
+    end
+
+    {
+        status: 'failed',
+        recoverable: true,
+        errors: e.record.errors.to_hash,
+        base_errors: e.record.errors[:base] }
+  rescue ActiveRecord::RecordNotUnique => e
+    if e.message.include?('index_posts_on_url')
+      existing_post = existing_post_for_race(attributes)
+      return {
+          status: 'skipped',
+          existing_post_id: existing_post[:id],
+          existing_post: existing_post } if existing_post.present?
+    end
+
+    Rails.logger.error(
+        "post_bulk_creator_record_not_unique #{ { error: e.class.name,
+                                                 message: e.message }.to_json }")
+    {
+        status: 'failed',
+        recoverable: false,
+        errors: { base: ['登録中にエラーが発生しました．'] },
+        base_errors: [] }
+  rescue Tag::NicoTagNormalisationError
+    {
+        status: 'failed',
+        recoverable: true,
+        errors: { tags: ['ニコニコ・タグは直接指定できません．'] },
+        base_errors: [] }
+  rescue Tag::DeprecatedTagNormalisationError
+    {
+        status: 'failed',
+        recoverable: true,
+        errors: { tags: ['廃止済みタグは付与できません．'] },
+        base_errors: [] }
+  rescue PostCreator::VideoMsParseError
+    {
+        status: 'failed',
+        recoverable: true,
+        errors: { video_ms: ['動画時間の記法が不正です．'] },
+        base_errors: [] }
+  rescue Post::RemoteThumbnailFetchFailed
+    {
+        status: 'failed',
+        recoverable: true,
+        errors: { thumbnail_base: ['サムネイル画像の取得に失敗しました．'] },
+        base_errors: [] }
+  rescue ArgumentError => e
+    {
+        status: 'failed',
+        recoverable: true,
+        errors: { base: [e.message] },
+        base_errors: [] }
+  rescue StandardError => e
+    Rails.logger.error(
+        "post_bulk_creator_failure #{ { error: e.class.name,
+                                        message: e.message }.to_json }")
+    {
+        status: 'failed',
+        recoverable: false,
+        errors: { base: ['登録中にエラーが発生しました．'] },
+        base_errors: [] }
+  end
+
+  def normalised_attributes attributes, preflight, index
+    {
+        url: preflight[:url],
+        title: preflight[:title],
+        thumbnail_base: preflight[:thumbnail_base],
+        thumbnail: thumbnail_for(index, attributes),
+        tags: preflight[:tags],
+        parent_post_ids: preflight[:parent_post_ids],
+        original_created_from: preflight[:original_created_from],
+        original_created_before: preflight[:original_created_before],
+        duration: preflight[:duration],
+        video_ms: preflight[:video_ms],
+        direct_tag_specs: preflight[:direct_tag_specs],
+        default_tag_specs: preflight[:default_tag_specs],
+        snapshot_tag_specs: preflight[:snapshot_tag_specs],
+        post_tag_specs: preflight[:post_tag_specs],
+        tag_sections: preflight[:tag_sections],
+        normalised_parent_post_ids: preflight[:normalised_parent_post_ids] }
+  end
+
+  def thumbnail_for index, attributes
+    return nil if attributes['thumbnail_base'].present? || attributes[:thumbnail_base].present?
+
+    @thumbnails[index]
+  end
+
+  def existing_post_for_race attributes, record = nil
+    return nil if record.present? && !(record.errors.of_kind?(:url, :taken))
+
+    normal_url = PostUrlNormaliser.normalise(attributes['url'] || attributes[:url])
+    return nil if normal_url.blank?
+
+    compact_existing_post(Post.with_attached_thumbnail.find_by(url: normal_url))
+  end
+
+  def compact_existing_post post
+    PostCompactRepr.base(post, host: @host)
+  end
+end

backend/app/services/post_create_plan.rb

@@ -0,0 +1,269 @@
+class PostCreatePlan
+  VIDEO_TAG_NAME = '動画'.freeze
+  TAGME_TAG_NAME = 'タグ希望'.freeze
+  NO_DEERJIKIST_TAG_NAME = 'ニジラー情報不詳'.freeze
+
+  def initialize attributes:
+    @attributes = attributes.symbolize_keys
+    @existing_tags_by_name = nil
+  end
+
+  def build!
+    direct_tag_specs, tag_sections = parse_direct_tag_specs
+    default_tag_specs = build_default_tag_specs(direct_tag_specs)
+    snapshot_tag_specs = merge_tag_specs(direct_tag_specs + default_tag_specs)
+    preload_existing_tags_by_name!(snapshot_tag_specs.map { _1[:name] })
+    validate_new_tag_specs!(snapshot_tag_specs)
+    post_tag_specs = expand_parent_tag_specs(snapshot_tag_specs)
+    video_ms = normalise_video_ms(snapshot_tag_specs)
+    validate_video_sections!(video_ms, tag_sections)
+    parent_post_ids = normalise_parent_post_ids
+    validate_parent_post_ids!(parent_post_ids)
+
+    { url: @attributes[:url],
+      title: @attributes[:title].to_s,
+      thumbnail_base: @attributes[:thumbnail_base].presence,
+      original_created_from: @attributes[:original_created_from].presence,
+      original_created_before: @attributes[:original_created_before].presence,
+      tags: serialised_tags(direct_tag_specs, tag_sections),
+      display_tags: display_tags(direct_tag_specs, tag_sections),
+      duration: @attributes[:duration].to_s,
+      video_ms: video_ms,
+      parent_post_ids: parent_post_ids.join(' '),
+      direct_tag_specs: direct_tag_specs,
+      default_tag_specs: default_tag_specs,
+      snapshot_tag_specs: snapshot_tag_specs,
+      post_tag_specs: post_tag_specs,
+      tag_sections: tag_sections,
+      normalised_parent_post_ids: parent_post_ids }
+  end
+
+  private
+
+  def tag_names = @attributes[:tags].to_s.split
+
+  def parse_direct_tag_specs
+    tag_sections = { }
+    direct_tag_specs = []
+
+    tag_names.each do |raw_name|
+      tag_name, category, sections = parse_raw_tag_name(raw_name)
+      existing_tag = existing_tags_by_name[tag_name]
+      raise Tag::DeprecatedTagNormalisationError, [existing_tag.name] if existing_tag&.deprecated?
+
+      direct_tag_specs << {
+          name: tag_name,
+          category: (category || existing_tag&.category || 'general').to_sym }
+      if sections.present?
+        tag_sections[tag_name] ||= []
+        tag_sections[tag_name].concat(sections)
+        tag_sections[tag_name] = Tag.merge_section_ranges(tag_sections[tag_name])
+        tag_sections.delete(tag_name) if tag_sections[tag_name] == [[0, nil]]
+      end
+    end
+
+    [merge_tag_specs(direct_tag_specs), tag_sections]
+  end
+
+  def parse_raw_tag_name raw_name
+    name = raw_name.to_s
+    prefix, category =
+        Tag::CATEGORY_PREFIXES.find {
+          name.downcase.start_with?(_1[0])
+        } || ['', nil]
+    name = name.sub(/\A#{ prefix }/i, '')
+
+    sections = []
+    while (match = name.match(/\A(\S*?)\[([^\[\]\s]*)-([^\[\]\s]*)\](\S*)\z/))
+      name = "#{ match[1] }#{ match[4] }"
+      next if match[2].empty? && match[3].empty?
+
+      sections << Tag.normalise_section_range!(
+          begin_raw: match[2],
+          end_raw: match[3],
+          tag_name: name)
+    end
+    if name.include?('[') || name.include?(']')
+      raise Tag::SectionLiteralParseError.new(raw_name, raw_name)
+    end
+
+    [resolved_tag_name(name), category&.to_sym, sections]
+  end
+
+  def build_default_tag_specs direct_tag_specs
+    default_tag_specs = []
+    if direct_tag_specs.length < 10 && direct_tag_specs.none? { _1[:name] == TAGME_TAG_NAME }
+      default_tag_specs << {
+          name: TAGME_TAG_NAME,
+          category: :meta }
+    end
+    if direct_tag_specs.none? { deerjikist_tag_spec?(_1) }
+      default_tag_specs << {
+          name: NO_DEERJIKIST_TAG_NAME,
+          category: :meta }
+    end
+
+    default_tag_specs
+  end
+
+  def validate_new_tag_specs! specs
+    Array(specs).each do |spec|
+      next if existing_tags_by_name.key?(spec[:name])
+
+      validate_new_tag_spec!(spec)
+    end
+  end
+
+  def validate_new_tag_spec! spec
+    tag_name = TagName.new(name: spec[:name])
+    tag = Tag.new(category: spec[:category], tag_name:)
+    return if tag_name.valid? && tag.valid?
+
+    post = Post.new
+    tag_name.errors[:name].each do |message|
+      post.errors.add :tags, "#{ spec[:name] }: #{ message }"
+    end
+    tag.errors.each do |error|
+      next if error.attribute == :tag_name
+
+      post.errors.add :tags, "#{ spec[:name] }: #{ error.message }"
+    end
+    raise ActiveRecord::RecordInvalid, post
+  end
+
+  def expand_parent_tag_specs snapshot_tag_specs
+    existing_snapshot_tags = snapshot_tag_specs.filter_map { existing_tags_by_name[_1[:name]] }
+    expanded_parent_specs =
+        Tag.expand_parent_tags(existing_snapshot_tags)
+           .reject(&:deprecated?)
+           .map { |tag|
+             { name: tag.name,
+               category: tag.category.to_sym }
+        }
+    merge_tag_specs(snapshot_tag_specs + expanded_parent_specs)
+  end
+
+  def merge_tag_specs specs
+    specs.each_with_object({ }) { |spec, merged|
+      merged[spec[:name]] =
+          if merged.key?(spec[:name]) && merged[spec[:name]][:category] != :general
+            merged[spec[:name]]
+          else
+            {
+                name: spec[:name],
+                category: spec[:category] }
+          end
+    }.values.sort_by { _1[:name] }
+  end
+
+  def existing_tags_by_name
+    @existing_tags_by_name ||= begin
+      names = tag_names.map { canonical_tag_name_without_sections(_1) }.uniq
+      Tag.joins(:tag_name).where(tag_names: { name: names }).index_by(&:name)
+    end
+  end
+
+  def preload_existing_tags_by_name! names
+    wanted_names = Array(names).map { _1.to_s }.reject(&:blank?).uniq
+    missing_names = wanted_names - existing_tags_by_name.keys
+    return if missing_names.empty?
+
+    existing_tags_by_name.merge!(
+        Tag.joins(:tag_name)
+           .where(tag_names: { name: missing_names })
+           .index_by(&:name))
+  end
+
+  def canonical_tag_name_without_sections raw_name
+    name, = parse_raw_tag_name(raw_name)
+    name
+  end
+
+  def deerjikist_tag_spec? spec
+    return true if spec[:category] == :deerjikist
+
+    existing_tags_by_name[spec[:name]]&.deerjikist?
+  end
+
+  def normalise_parent_post_ids
+    Array(@attributes[:parent_post_ids]).flat_map { _1.to_s.split }.map { |token|
+      id = Integer(token, exception: false)
+      raise ArgumentError, "親投稿 Id. が不正です: #{ token }" if id.nil? || id <= 0
+
+      id
+    }.uniq.sort
+  end
+
+  def validate_parent_post_ids! ids
+    missing = ids - Post.where(id: ids).pluck(:id)
+    raise ArgumentError, "存在しない親投稿 Id. があります: #{ missing.join(' ') }" if missing.present?
+  end
+
+  def serialised_tags direct_tag_specs, tag_sections
+    direct_tag_specs.map { |spec|
+      "#{ spec[:name] }#{ tag_sections[spec[:name]].to_a.map { section_literal(_1) }.join }"
+    }.sort.join(' ')
+  end
+
+  def display_tags direct_tag_specs, tag_sections
+    direct_tag_specs.map { |spec|
+      {
+          name: spec[:name],
+          category: spec[:category].to_s,
+          section_literals: tag_sections[spec[:name]].to_a.map { section_literal(_1) } }
+    }.sort_by { _1[:name] }
+  end
+
+  def section_literal range
+    begin_ms, end_ms = range
+    "[#{ Post.ms_to_time(begin_ms) }-#{ end_ms ? Post.ms_to_time(end_ms) : '' }]"
+  end
+
+  def normalise_video_ms snapshot_tag_specs
+    return nil unless snapshot_tag_specs.any? { _1[:name] == VIDEO_TAG_NAME }
+
+    video_ms = @attributes[:video_ms]
+    if video_ms.present?
+      value = Integer(video_ms, exception: false)
+      raise PostCreator::VideoMsParseError unless value&.positive?
+
+      return value
+    end
+
+    duration = @attributes[:duration]
+    return nil if duration.blank?
+
+    value = Tag.time_to_ms!(duration.to_s, tag_name: '動画時間')
+    raise PostCreator::VideoMsParseError unless value.positive?
+
+    value
+  rescue Tag::SectionLiteralParseError
+    raise PostCreator::VideoMsParseError
+  end
+
+  def validate_video_sections! video_ms, tag_sections
+    return unless video_ms
+
+    tag_sections.each_value do |ranges|
+      ranges.each do |begin_ms, end_ms|
+        if begin_ms >= video_ms
+          post = Post.new
+          post.errors.add :video_ms, 'タグ区間の開始が動画時間以上です．'
+          raise ActiveRecord::RecordInvalid, post
+        end
+        if end_ms && end_ms > video_ms
+          post = Post.new
+          post.errors.add :video_ms, 'タグ区間の終端が動画時間を超えてゐます．'
+          raise ActiveRecord::RecordInvalid, post
+        end
+      end
+    end
+  end
+
+  def resolved_tag_name name
+    tag_name = TagName.includes(:canonical).find_by(name:)
+    return name if tag_name.nil?
+
+    (tag_name.canonical || tag_name).name
+  end
+end

backend/app/services/post_create_preflight.rb

@@ -0,0 +1,141 @@
+class PostCreatePreflight
+  class ValidationFailed < StandardError
+    attr_reader :fields, :base_errors
+
+    def initialize fields: { }, base_errors: []
+      super('入力内容を確認してください．')
+      @fields = fields
+      @base_errors = base_errors
+    end
+  end
+
+  def initialize attributes:, thumbnail: nil, host: nil
+    @attributes = attributes.symbolize_keys
+    @thumbnail = thumbnail
+    @host = host
+  end
+
+  def run
+    preview = PostImportPreviewer.new.preview_rows(
+        rows: [preview_row],
+        fetch_metadata: false).first
+    if preview[:existing_post_id].present?
+      return {
+          url: preview[:url],
+          title: preview[:attributes]['title'],
+          thumbnail_base: preview[:attributes]['thumbnail_base'],
+          tags: preview[:attributes]['tags'],
+          parent_post_ids: preview[:attributes]['parent_post_ids'],
+          original_created_from: preview[:attributes]['original_created_from'],
+          original_created_before: preview[:attributes]['original_created_before'],
+          duration: preview[:attributes]['duration'],
+          video_ms: preview[:attributes]['video_ms'],
+          display_tags: preview[:display_tags] || [],
+          field_warnings: final_field_warnings(preview[:field_warnings] || { }),
+          base_warnings: preview[:base_warnings],
+          existing_post_id: preview[:existing_post_id],
+          existing_post: existing_post_compact(preview[:existing_post_id]) }
+    end
+
+    if preview[:validation_errors].present?
+      raise ValidationFailed.new(fields: preview[:validation_errors])
+    end
+
+    validate_thumbnail_upload!
+
+    plan = PostCreatePlan.new(
+        attributes: {
+            url: preview[:url],
+            title: preview[:attributes]['title'],
+            thumbnail_base: preview[:attributes]['thumbnail_base'],
+            tags: preview[:attributes]['tags'],
+            parent_post_ids: preview[:attributes]['parent_post_ids'],
+            original_created_from: preview[:attributes]['original_created_from'],
+            original_created_before: preview[:attributes]['original_created_before'],
+            duration: preview[:attributes]['duration'],
+            video_ms: preview[:attributes]['video_ms'] }).build!
+
+    {
+        url: plan[:url],
+        title: plan[:title],
+        thumbnail_base: plan[:thumbnail_base],
+        tags: plan[:tags],
+        parent_post_ids: plan[:parent_post_ids],
+        original_created_from: plan[:original_created_from],
+        original_created_before: plan[:original_created_before],
+        duration: plan[:duration],
+        video_ms: plan[:video_ms],
+        display_tags: plan[:display_tags],
+        direct_tag_specs: plan[:direct_tag_specs],
+        default_tag_specs: plan[:default_tag_specs],
+        snapshot_tag_specs: plan[:snapshot_tag_specs],
+        post_tag_specs: plan[:post_tag_specs],
+        tag_sections: plan[:tag_sections],
+        normalised_parent_post_ids: plan[:normalised_parent_post_ids],
+        field_warnings: final_field_warnings(preview[:field_warnings] || { }),
+        base_warnings: preview[:base_warnings],
+        existing_post_id: preview[:existing_post_id],
+        existing_post: existing_post_compact(preview[:existing_post_id]) }
+  end
+
+  private
+
+  def preview_row
+    {
+        source_row: 1,
+        url: @attributes[:url].to_s,
+        attributes: {
+            'title' => @attributes[:title].to_s,
+            'thumbnail_base' => @attributes[:thumbnail_base].to_s,
+            'original_created_from' => @attributes[:original_created_from].to_s,
+            'original_created_before' => @attributes[:original_created_before].to_s,
+            'duration' => @attributes[:duration].to_s,
+            'video_ms' => @attributes[:video_ms],
+            'tags' => @attributes[:tags].to_s,
+            'parent_post_ids' => parent_post_ids_text },
+        provenance: {
+            'url' => 'manual',
+            'title' => 'manual',
+            'thumbnail_base' => 'manual',
+            'original_created_from' => 'manual',
+            'original_created_before' => 'manual',
+            'duration' => 'manual',
+            'video_ms' => 'manual',
+            'tags' => 'manual',
+            'parent_post_ids' => 'manual' },
+        tag_sources: {
+            'automatic' => '',
+            'manual' => @attributes[:tags].to_s } }
+  end
+
+  def parent_post_ids_text
+    Array(@attributes[:parent_post_ids]).flat_map { _1.to_s.split }.join(' ')
+  end
+
+  def validate_thumbnail_upload!
+    return if @attributes[:thumbnail_base].present?
+    return if @thumbnail.blank?
+
+    PostThumbnailUploadValidator.validate!(@thumbnail)
+  rescue PostThumbnailUploadValidator::InvalidUpload => e
+    raise ValidationFailed.new(fields: { thumbnail: [e.message] })
+  end
+
+  def existing_post_compact post_id
+    return nil if post_id.blank?
+
+    post = Post.with_attached_thumbnail.find_by(id: post_id)
+    PostCompactRepr.base(post, host: @host)
+  end
+
+  def final_field_warnings field_warnings
+    thumbnail_warnings = (field_warnings['thumbnail_base'] || []).reject { _1 == 'サムネールなし' }
+    if @attributes[:thumbnail_base].blank? && @thumbnail.blank?
+      thumbnail_warnings = (thumbnail_warnings + ['サムネールなし']).uniq
+    end
+
+    next_warnings = field_warnings.except('thumbnail_base')
+    next_warnings['thumbnail_base'] = thumbnail_warnings if thumbnail_warnings.present?
+    next_warnings
+  end
+end

backend/app/services/post_creator.rb

@@ -10,103 +10,122 @@ class PostCreator
   end
 
   def create!
+    thumbnail_attachment = prepare_thumbnail_attachment
     post = Post.new(title: @attributes[:title].presence,
                     url: @attributes[:url],
                     thumbnail_base: @attributes[:thumbnail_base].presence,
                     uploaded_user: @actor,
                     original_created_from: @attributes[:original_created_from].presence,
                     original_created_before: @attributes[:original_created_before].presence)
-    attach_thumbnail!(post)
 
     ApplicationRecord.transaction do
       post.save!
-      Tag.normalise_tags!(tag_names, deny_deprecated: true, with_sections: true) =>
-          { tags:, sections: }
-      TagVersioning.record_tag_snapshots!(tags, created_by_user: @actor)
-      tags = Tag.expand_parent_tags(tags).reject(&:deprecated?)
-      post.video_ms = normalise_video_ms(tags)
-      validate_video_sections!(post.video_ms, sections)
+      post.thumbnail.attach(thumbnail_attachment) if thumbnail_attachment.present?
+      snapshot_tags = planned_snapshot_tags
+      post_tags = planned_post_tags
+      sections = planned_sections
+      TagVersioning.record_tag_snapshots!(snapshot_tags, created_by_user: @actor)
+      post.video_ms = planned_video_ms
       post.save!
-      sync_post_tags!(post, tags, sections)
-      sync_parent_posts!(post, parent_post_ids)
+      sync_post_tags!(post, post_tags, sections)
+      sync_parent_posts!(post, planned_parent_post_ids)
       PostVersionRecorder.record!(post:, event_type: :create, created_by_user: @actor)
     end
     post
+  rescue StandardError
+    post&.thumbnail&.purge if post&.thumbnail&.attached?
+    raise
   end
 
   private
 
-  def attach_thumbnail! post
-    thumbnail = @attributes[:thumbnail]
-    if thumbnail.present?
-      post.thumbnail.attach(Post.resized_thumbnail_attachment(thumbnail))
-      return
-    end
-
-    thumbnail_base = post.thumbnail_base
-    return if thumbnail_base.blank?
-
-    post.attach_thumbnail_from_url!(thumbnail_base)
-  rescue Post::RemoteThumbnailFetchFailed => e
-    @field_warnings[:thumbnail_base] = [e.message]
+  def prepare_thumbnail_attachment
+    PostThumbnailAttachmentBuilder.build(
+        thumbnail: @attributes[:thumbnail],
+        thumbnail_base: @attributes[:thumbnail_base].presence)
   end
 
-  def tag_names = @attributes[:tags].to_s.split
+  def planned_snapshot_tags = planned_create_attributes[:snapshot_tags]
 
-  def parent_post_ids
-    Array(@attributes[:parent_post_ids]).flat_map { _1.to_s.split }.map { |token|
-      id = Integer(token, exception: false)
-      raise ArgumentError, "親投稿 Id. が不正です: #{ token }" if id.nil? || id <= 0
+  def planned_post_tags = planned_create_attributes[:post_tags]
 
-      id
-    }.uniq
+  def planned_sections = planned_create_attributes[:tag_sections]
+
+  def planned_parent_post_ids = planned_create_attributes[:normalised_parent_post_ids]
+
+  def planned_video_ms
+    planned_create_attributes[:video_ms]
   end
 
-  def normalise_video_ms tags
-    return nil unless tags.any? { _1.id == Tag.video.id }
-
-    video_ms = @attributes[:video_ms]
-    if video_ms.present?
-      value = Integer(video_ms, exception: false)
-      raise VideoMsParseError unless value&.positive?
-
-      return value
-    end
-    duration = @attributes[:duration]
-    return nil if duration.blank?
-
-    value = Tag.time_to_ms!(duration.to_s, tag_name: '動画時間')
-    raise VideoMsParseError unless value.positive?
-
-    value
-  rescue Tag::SectionLiteralParseError
-    raise VideoMsParseError
-  end
-
-  def validate_video_sections! video_ms, sections
-    return unless video_ms
-
-    sections.each_value do |ranges|
-      ranges.each do |begin_ms, end_ms|
-        post = Post.new
-        if begin_ms >= video_ms
-          post.errors.add :video_ms, 'タグ区間の開始が動画時間以上です．'
-          raise ActiveRecord::RecordInvalid, post
-        end
-        if end_ms && end_ms > video_ms
-          post.errors.add :video_ms, 'タグ区間の終端が動画時間を超えてゐます．'
-          raise ActiveRecord::RecordInvalid, post
-        end
+  def planned_create_attributes
+    @planned_create_attributes ||= begin
+      if @attributes.key?(:snapshot_tag_specs)
+        snapshot_tags = materialise_tags(@attributes[:snapshot_tag_specs] || [])
+        post_tags = materialise_tags(@attributes[:post_tag_specs] || [])
+        {
+            snapshot_tags: snapshot_tags,
+            post_tags: post_tags,
+            tag_sections: materialise_sections(
+                @attributes[:tag_sections] || { },
+                snapshot_tags,
+                post_tags),
+            normalised_parent_post_ids: @attributes[:normalised_parent_post_ids] || [],
+            video_ms: @attributes[:video_ms] }
+      else
+        build_materialised_plan
       end
     end
   end
 
+  def build_materialised_plan
+    plan = PostCreatePlan.new(attributes: @attributes).build!
+    snapshot_tags = materialise_tags(plan[:snapshot_tag_specs] || [])
+    post_tags = materialise_tags(plan[:post_tag_specs] || [])
+    {
+        snapshot_tags: snapshot_tags,
+        post_tags: post_tags,
+        tag_sections: materialise_sections(
+            plan[:tag_sections] || { },
+            snapshot_tags,
+            post_tags),
+        normalised_parent_post_ids: plan[:normalised_parent_post_ids] || [],
+        video_ms: plan[:video_ms] }
+  end
+
+  def materialise_tags specs
+    Array(specs).each_with_object({ }) do |spec, tags|
+      name = spec[:name] || spec['name']
+      category = spec[:category] || spec['category']
+      next if name.blank? || category.blank?
+
+      tag = Tag.find_or_create_by_tag_name!(name, category:)
+      tag.update!(category:) if tag.category.to_sym != category.to_sym
+      tags[name] ||= tag
+    end.values
+  end
+
+  def materialise_sections sections_by_name, snapshot_tags, post_tags
+    tags_by_name = post_tags.index_by(&:name)
+    snapshot_tags.each do |tag|
+      tags_by_name[tag.name] ||= tag
+    end
+
+    sections_by_name.each_with_object({ }) do |(tag_name, ranges), sections|
+      tag = tags_by_name[tag_name.to_s]
+      next if tag.nil?
+
+      sections[tag.id] = Array(ranges).map { |range| [range[0], range[1]] }
+    end
+  end
+
   def sync_post_tags! post, desired_tags, sections
     desired_ids = desired_tags.map(&:id).to_set
     current_ids = post.tags.pluck(:id).to_set
+
     Tag.where(id: desired_ids - current_ids).find_each do |tag|
       PostTag.create_or_find_by!(post:, tag:, created_user: @actor)
     end
+
     PostTagSection.where(post_id: post.id).destroy_all
     sections.each do |tag_id, ranges|
       ranges.each do |begin_ms, end_ms|
@@ -116,10 +135,9 @@ class PostCreator
                                end_ms:)
       end
     end
+
     PostTag.where(post_id: post.id,
-                  tag_id: (current_ids - desired_ids).to_a).kept.find_each do |post_tag|
-      post_tag.discard_by!(@actor)
-    end
+                  tag_id: (current_ids - desired_ids).to_a).destroy_all
   end
 
   def sync_parent_posts! post, ids

backend/app/services/post_import_previewer.rb

@@ -3,13 +3,14 @@ require 'timeout'
 
 class PostImportPreviewer
   FIELDS = [
-    'title',
-    'thumbnail_base',
-    'original_created_from',
-    'original_created_before',
-    'duration',
-    'tags',
-    'parent_post_ids'].freeze
+      'title',
+      'thumbnail_base',
+      'original_created_from',
+      'original_created_before',
+      'video_ms',
+      'duration',
+      'tags',
+      'parent_post_ids'].freeze
   FETCH_WARNING_FIELDS = ['url', 'title', 'thumbnail_base'].freeze
   TITLE_FETCH_WARNING = 'タイトルを取得できませんでした．'.freeze
   THUMBNAIL_FETCH_WARNING = 'サムネールを取得できませんでした．'.freeze
@@ -125,7 +126,6 @@ class PostImportPreviewer
     validate_basic_data(attributes, validation_errors)
     validate_preview_tags(merged_tags(tag_sources, provenance['tags']),
                           validation_errors,
-                          field_warnings,
                           known_tags)
     validate_parents(attributes['parent_post_ids'], validation_errors, existing_parent_ids)
     attributes.delete('url')
@@ -162,6 +162,8 @@ class PostImportPreviewer
       normalised =
           if field == 'duration'
             normalise_duration_attribute(value)
+          elsif field == 'video_ms'
+            value.nil? ? '' : value.to_s
           else
             value.to_s
           end
@@ -194,7 +196,7 @@ class PostImportPreviewer
 
   def clear_automatic_values! attributes, provenance, tag_sources
     ['title', 'thumbnail_base', 'original_created_from',
-     'original_created_before', 'duration'].each do |field|
+     'original_created_before', 'video_ms', 'duration'].each do |field|
       attributes[field] = '' if provenance[field] == 'automatic'
     end
     tag_sources['automatic'] = ''
@@ -223,15 +225,15 @@ class PostImportPreviewer
     end
     { data:, warnings:, validation_errors: { } }
   rescue Preview::UrlSafety::UnsafeUrl => e
+    payload = { error: e.class.name, message: e.message }
     Rails.logger.info(
-        "post_import_metadata_fetch_unsafe_url "\
-        "#{ { error: e.class.name, message: e.message }.to_json }")
+        "post_import_metadata_fetch_unsafe_url #{ payload.to_json }")
     { data: { }, warnings: { }, validation_errors: { url: [e.message] } }
   rescue Preview::HttpFetcher::FetchFailed,
          Preview::HttpFetcher::ResponseTooLarge => e
+    payload = { error: e.class.name, message: e.message }
     Rails.logger.info(
-        "post_import_metadata_fetch_failure "\
-        "#{ { error: e.class.name, message: e.message }.to_json }")
+        "post_import_metadata_fetch_failure #{ payload.to_json }")
     { data: { }, warnings: { 'url' => [METADATA_FETCH_WARNING] }, validation_errors: { } }
   end
 
@@ -252,7 +254,15 @@ class PostImportPreviewer
   def sanitise_metadata_url value
     return nil unless value.is_a?(String)
 
-    PostUrlNormaliser.normalise(value)
+    stripped = value.strip
+    return nil if stripped.blank?
+
+    uri = URI.parse(stripped)
+    return nil unless uri.is_a?(URI::HTTP) && uri.host.present?
+
+    stripped
+  rescue URI::InvalidURIError
+    nil
   end
 
   def sanitise_metadata_time value
@@ -325,14 +335,14 @@ class PostImportPreviewer
   def safe_fetch_metadata url
     fetch_metadata(url)
   rescue Preview::UrlSafety::UnsafeUrl => e
+    payload = { error: e.class.name, message: e.message }
     Rails.logger.info(
-        "post_import_metadata_fetch_unsafe_url "\
-        "#{ { error: e.class.name, message: e.message }.to_json }")
+        "post_import_metadata_fetch_unsafe_url #{ payload.to_json }")
     { data: { }, warnings: { }, validation_errors: { url: [e.message] } }
   rescue StandardError => e
+    payload = { error: e.class.name, message: e.message }
     Rails.logger.error(
-        "post_import_metadata_fetch_unexpected_failure "\
-        "#{ { error: e.class.name, message: e.message }.to_json }")
+        "post_import_metadata_fetch_unexpected_failure #{ payload.to_json }")
     { data: { }, warnings: { 'url' => [METADATA_FETCH_WARNING] }, validation_errors: { } }
   end
 
@@ -439,7 +449,7 @@ class PostImportPreviewer
     []
   end
 
-  def validate_preview_tags raw, errors, field_warnings, known_tags
+  def validate_preview_tags raw, errors, known_tags
     names = raw.to_s.split
     return if names.empty?
     if names.any? { _1.downcase.start_with?('nico:') }
@@ -451,11 +461,6 @@ class PostImportPreviewer
     existing = parsed.filter_map { known_tags[_1] }
     deprecated = existing.select(&:deprecated?).map(&:name)
     errors[:tags] = ["廃止済みタグがあります: #{ deprecated.join(' ') }"] if deprecated.present?
-    known = existing.reject(&:deprecated?).map(&:name)
-    new_tags = parsed.uniq - known
-    if new_tags.present?
-      add_field_warning!(field_warnings, 'tags', "新規タグを作成します: #{ new_tags.join(' ') }")
-    end
   rescue Tag::SectionLiteralParseError
     errors[:tags] = ['タグ区間の記法が不正です．']
   end
@@ -466,7 +471,7 @@ class PostImportPreviewer
                     thumbnail_base: attributes['thumbnail_base'].presence,
                     original_created_from: attributes['original_created_from'].presence,
                     original_created_before: attributes['original_created_before'].presence,
-                    video_ms: parse_duration(attributes['duration'], errors))
+                    video_ms: parse_video_ms(attributes, errors))
     post.valid?
     post.errors.each do |error|
       next if error.attribute == :url && error.type == :taken
@@ -475,7 +480,21 @@ class PostImportPreviewer
     end
   end
 
-  def parse_duration value, errors
+  def parse_video_ms attributes, errors
+    return nil unless attributes['tags'].to_s.split.include?('動画')
+
+    video_ms = attributes['video_ms']
+    if video_ms.present?
+      value = Integer(video_ms, exception: false)
+      if value&.positive?
+        return value
+      end
+
+      errors[:video_ms] = ['動画時間の記法が不正です．']
+      return nil
+    end
+
+    value = attributes['duration']
     return nil if value.blank?
 
     Tag.time_to_ms!(value.to_s, tag_name: '動画時間')

backend/app/services/post_import_row_normaliser.rb

@@ -1,190 +0,0 @@
-class PostImportRowNormaliser
-  ORIGINS = ['automatic', 'manual'].freeze
-  STRING_FIELDS = [
-    'title',
-    'thumbnail_base',
-    'original_created_from',
-    'original_created_before',
-    'duration',
-    'tags',
-    'parent_post_ids'].freeze
-  FLEXIBLE_FIELDS = ['video_ms'].freeze
-  ATTRIBUTE_FIELDS = (STRING_FIELDS + FLEXIBLE_FIELDS).freeze
-
-  def self.normalise! rows, allow_warning_fields: false
-    raise ArgumentError, '取込行の形式が不正です．' unless rows.is_a?(Array)
-    raise ArgumentError, '取込件数が多すぎます．' if rows.length > PostImportUrlListParser::MAX_ROWS
-
-    normalised_rows = rows.map { normalise_row!(_1, allow_warning_fields:) }
-    source_rows = normalised_rows.map { _1['source_row'] }
-    raise ArgumentError, '元行番号が重複しています．' if source_rows.uniq.length != source_rows.length
-    if normalised_rows.sum { row_bytesize(_1) } > PostImportUrlListParser::MAX_BYTES
-      raise ArgumentError, '取込データが大きすぎます．'
-    end
-
-    normalised_rows
-  end
-
-  def self.normalise_row! row, allow_warning_fields:
-    unless row.is_a?(Hash) || row.is_a?(ActionController::Parameters)
-      raise ArgumentError, '取込行の形式が不正です．'
-    end
-
-    parameters =
-        row.is_a?(ActionController::Parameters) ? row : ActionController::Parameters.new(row)
-    permitted = parameters.permit(*permitted_keys(allow_warning_fields))
-    normalised = permitted.to_h.deep_transform_keys { _1.to_s.underscore }
-
-    normalised['source_row'] = normalise_source_row!(normalised['source_row'])
-    normalise_url!(normalised['url'])
-    normalise_metadata_url!(normalised['metadata_url'])
-    normalise_attributes!(normalised.fetch('attributes', { }))
-    normalise_provenance!(normalised.fetch('provenance', { }))
-    normalise_tag_sources!(normalised['tag_sources'])
-    normalise_warning_values!(normalised, allow_warning_fields:)
-    if row_bytesize(normalised) > PostImportUrlListParser::MAX_BYTES
-      raise ArgumentError, '取込行が大きすぎます．'
-    end
-
-    normalised
-  end
-
-  def self.permitted_keys allow_warning_fields
-    keys = [
-      :source_row,
-      :sourceRow,
-      :url,
-      :metadata_url,
-      :metadataUrl,
-      { attributes: {} },
-      { provenance: {} },
-      { tag_sources: {} },
-      { tagSources: {} }]
-    return keys unless allow_warning_fields
-
-    keys + [
-      { field_warnings: {} },
-      { fieldWarnings: {} },
-      { base_warnings: [] },
-      { baseWarnings: [] }]
-  end
-  private_class_method :permitted_keys
-
-  def self.normalise_source_row! value
-    source_row = Integer(value, exception: false)
-    raise ArgumentError, '元行番号が不正です．' if source_row.nil? || source_row <= 0
-
-    source_row
-  end
-  private_class_method :normalise_source_row!
-
-  def self.normalise_url! value
-    unless value.is_a?(String)
-      raise ArgumentError, 'URL の形式が不正です．'
-    end
-    if value.bytesize > PostImportUrlListParser::MAX_URL_BYTES
-      raise ArgumentError, 'URL が長すぎます．'
-    end
-  end
-  private_class_method :normalise_url!
-
-  def self.normalise_metadata_url! value
-    raise ArgumentError, 'metadata_url の形式が不正です．' unless value.nil? || value.is_a?(String)
-    if value.to_s.bytesize > PostImportUrlListParser::MAX_URL_BYTES
-      raise ArgumentError, 'metadata_url が長すぎます．'
-    end
-  end
-  private_class_method :normalise_metadata_url!
-
-  def self.normalise_attributes! attributes
-    raise ArgumentError, 'attributes の形式が不正です．' unless attributes.is_a?(Hash)
-    raise ArgumentError, '取込項目が不正です．' unless (attributes.keys - ATTRIBUTE_FIELDS).empty?
-
-    attributes.each do |key, value|
-      case key
-      when *STRING_FIELDS
-        raise ArgumentError, '取込項目の型が不正です．' unless value.nil? || value.is_a?(String)
-      when *FLEXIBLE_FIELDS
-        unless value.nil? || value.is_a?(String) || value.is_a?(Numeric)
-          raise ArgumentError, '取込項目の型が不正です．'
-        end
-      end
-      if value.to_s.bytesize > PostImportUrlListParser::MAX_URL_BYTES
-        raise ArgumentError, '取込項目が大きすぎます．'
-      end
-    end
-  end
-  private_class_method :normalise_attributes!
-
-  def self.normalise_provenance! provenance
-    unless provenance.is_a?(Hash)
-      raise ArgumentError, 'provenance の形式が不正です．'
-    end
-
-    allowed = ATTRIBUTE_FIELDS + ['url']
-    unless (provenance.keys - allowed).empty?
-      raise ArgumentError, '値の由来が不正です．'
-    end
-    unless provenance.values.all? { ORIGINS.include?(_1) }
-      raise ArgumentError, '値の由来が不正です．'
-    end
-  end
-  private_class_method :normalise_provenance!
-
-  def self.normalise_tag_sources! tag_sources
-    return if tag_sources.nil?
-
-    unless tag_sources.is_a?(Hash)
-      raise ArgumentError, 'タグ由来の形式が不正です．'
-    end
-    unless (tag_sources.keys - ORIGINS).empty?
-      raise ArgumentError, 'タグ由来の形式が不正です．'
-    end
-    unless tag_sources.values.all? { _1.is_a?(String) }
-      raise ArgumentError, 'タグ由来の形式が不正です．'
-    end
-    if tag_sources.values.any? { _1.bytesize > PostImportUrlListParser::MAX_URL_BYTES }
-      raise ArgumentError, 'タグ由来が大きすぎます．'
-    end
-    if tag_sources.values.sum(&:bytesize) > PostImportUrlListParser::MAX_URL_BYTES
-      raise ArgumentError, 'タグ由来が大きすぎます．'
-    end
-  end
-  private_class_method :normalise_tag_sources!
-
-  def self.normalise_warning_values! normalised, allow_warning_fields:
-    return unless allow_warning_fields
-
-    field_warnings = normalised['field_warnings']
-    unless field_warnings.nil? || field_warnings.is_a?(Hash)
-      raise ArgumentError, '警告の形式が不正です．'
-    end
-    field_warnings&.each do |key, values|
-      unless ATTRIBUTE_FIELDS.include?(key) || key == 'url'
-        raise ArgumentError, '警告の形式が不正です．'
-      end
-      unless values.is_a?(Array) && values.all? { _1.is_a?(String) }
-        raise ArgumentError, '警告の形式が不正です．'
-      end
-      if values.any? { _1.bytesize > PostImportUrlListParser::MAX_URL_BYTES }
-        raise ArgumentError, '警告が大きすぎます．'
-      end
-    end
-
-    base_warnings = normalised['base_warnings']
-    return if base_warnings.nil?
-
-    unless base_warnings.is_a?(Array) && base_warnings.all? { _1.is_a?(String) }
-      raise ArgumentError, '警告の形式が不正です．'
-    end
-    if base_warnings.any? { _1.bytesize > PostImportUrlListParser::MAX_URL_BYTES }
-      raise ArgumentError, '警告が大きすぎます．'
-    end
-  end
-  private_class_method :normalise_warning_values!
-
-  def self.row_bytesize row
-    row.to_json.bytesize
-  end
-  private_class_method :row_bytesize
-end

backend/app/services/post_import_runner.rb

@@ -1,109 +0,0 @@
-class PostImportRunner
-  def initialize actor:, rows:
-    @actor = actor
-    @rows = rows
-  end
-
-  def run
-    normalised_rows = PostImportRowNormaliser.normalise!(@rows)
-    previews = PostImportPreviewer.new.preview_rows(rows: normalised_rows,
-                                                    fetch_metadata: false)
-    preview_map = previews.index_by { _1[:source_row] }
-
-    results = normalised_rows.map do |row|
-      run_row(row, preview_map.fetch(row['source_row']))
-    end
-
-    { created: results.count { _1[:status] == 'created' },
-      skipped: results.count { _1[:status] == 'skipped' },
-      failed: results.count { _1[:status] == 'failed' },
-      rows: results }
-  end
-
-  private
-
-  def run_row row, preview
-    attributes = row.fetch('attributes', { }).transform_keys { _1.to_s.underscore }
-    return { source_row: row['source_row'],
-             status: 'failed',
-             errors: preview[:validation_errors],
-             recoverable: true } if preview[:validation_errors].present?
-    if preview[:skip_reason] == 'existing'
-      return { source_row: row['source_row'],
-               status: 'skipped',
-               existing_post_id: preview[:existing_post_id] }
-    end
-
-    attributes['tags'] = preview[:attributes]['tags']
-    attributes['url'] = row['url']
-    creator = PostCreator.new(actor: @actor, attributes:)
-    post = creator.create!
-    result = { source_row: row['source_row'], status: 'created', post: PostRepr.base(post) }
-    if creator.field_warnings.present?
-      result[:field_warnings] = creator.field_warnings
-    end
-    result
-  rescue ActiveRecord::RecordInvalid => e
-    existing_post = existing_post_for_race(row, e.record)
-    if existing_post
-      return { source_row: row['source_row'],
-               status: 'skipped',
-               existing_post_id: existing_post.id }
-    end
-
-    { source_row: row['source_row'],
-      status: 'failed',
-      errors: e.record.errors.to_hash,
-      recoverable: true }
-  rescue ActiveRecord::RecordNotUnique => e
-    raise unless url_record_not_unique?(e)
-
-    existing_post = existing_post_for_race(row)
-    raise unless existing_post
-
-    { source_row: row['source_row'],
-      status: 'skipped',
-      existing_post_id: existing_post.id }
-  rescue Tag::NicoTagNormalisationError
-    { source_row: row['source_row'],
-      status: 'failed',
-      errors: { tags: ['ニコニコ・タグは直接指定できません．'] },
-      recoverable: true }
-  rescue Tag::DeprecatedTagNormalisationError
-    { source_row: row['source_row'],
-      status: 'failed',
-      errors: { tags: ['廃止済みタグは付与できません．'] },
-      recoverable: true }
-  rescue PostCreator::VideoMsParseError
-    { source_row: row['source_row'],
-      status: 'failed',
-      errors: { duration: ['動画時間の記法が不正です．'] },
-      recoverable: true }
-  rescue ArgumentError
-    { source_row: row['source_row'],
-      status: 'failed',
-      errors: { base: ['入力値が不正です．'] },
-      recoverable: true }
-  rescue StandardError => e
-    Rails.logger.error("post_import_runner_failure #{ { error: e.class.name,
-                                                        message: e.message }.to_json }")
-    { source_row: row['source_row'],
-      status: 'failed',
-      errors: { base: ['登録中にエラーが発生しました．'] } }
-  end
-
-  def existing_post_for_race row, record = nil
-    if record && !(record.errors.of_kind?(:url, :taken))
-      return nil
-    end
-
-    normal_url = PostUrlNormaliser.normalise(row['url'])
-    return nil if normal_url.blank?
-
-    Post.find_by(url: normal_url)
-  end
-
-  def url_record_not_unique? error
-    error.message.include?('index_posts_on_url')
-  end
-end

backend/app/services/post_import_url_list_parser.rb

@@ -1,25 +0,0 @@
-class PostImportUrlListParser
-  MAX_ROWS = 100
-  MAX_BYTES = 1.megabyte
-  MAX_URL_BYTES = 20.kilobytes
-
-  def self.parse source
-    raw = source.to_s
-    raise ArgumentError, '入力が大きすぎます．' if raw.bytesize > MAX_BYTES
-
-    rows = raw.split(/\r\n|\n|\r/).each_with_index.filter_map { |line, index|
-      url = line.strip
-      next if url.blank?
-
-      if url.bytesize > MAX_URL_BYTES
-        raise ArgumentError, "#{ index + 1 } 行目: URL が長すぎます．"
-      end
-
-      { source_row: index + 1, url: }
-    }
-    raise ArgumentError, 'URL を入力してください．' if rows.empty?
-    raise ArgumentError, "取込件数は #{ MAX_ROWS } 件までです．" if rows.length > MAX_ROWS
-
-    rows
-  end
-end

backend/app/services/post_metadata_fetcher.rb

@@ -3,10 +3,9 @@ require 'date'
 
 class PostMetadataFetcher
   TIMESTAMP_PATTERN =
-      Regexp.new(
-          '\A(\d{4})-(\d{2})-(\d{2})T(\d{2})' \
-          '(?::(\d{2})(?::(\d{2})(?:\.(\d+))?)?)?' \
-          '(Z|[+-]\d{2}:?\d{2})?\z')
+      /\A(\d{4})-(\d{2})-(\d{2})T(\d{2})
+        (?::(\d{2})(?::(\d{2})(?:\.(\d+))?)?)?
+        (Z|[+-]\d{2}:?\d{2})?\z/x
 
   def self.fetch raw_url
     uri, = Preview::UrlSafety.validate(raw_url)
@@ -32,6 +31,7 @@ class PostMetadataFetcher
       original_created_from: serialise_time(created_range&.first),
       original_created_before: serialise_time(created_range&.last),
       duration: serialise_duration(duration),
+      display_tags: display_tags(platform_tags),
       tags: platform_tags.join(' ') }
   end
 
@@ -42,6 +42,24 @@ class PostMetadataFetcher
     []
   end
 
+  def self.display_tags names
+    return [] if names.blank?
+
+    existing_tags =
+        Tag
+        .joins(:tag_name)
+        .where(tag_names: { name: names })
+        .index_by(&:name)
+
+    names.map { |name|
+      tag = existing_tags[name]
+      {
+          name: name,
+          category: (tag&.category || 'meta'),
+          section_literals: [] }
+    }
+  end
+
   def self.original_created_range value
     return nil if value.blank?
 
@@ -168,6 +186,7 @@ class PostMetadataFetcher
   end
 
   private_class_method :platform_tags,
+                       :display_tags,
                        :original_created_range,
                        :parse_timestamp_range,
                        :parse_nanoseconds,

backend/app/services/post_thumbnail_attachment_builder.rb

@@ -0,0 +1,11 @@
+class PostThumbnailAttachmentBuilder
+  def self.build thumbnail:, thumbnail_base:
+    if thumbnail_base.present?
+      return Post.remote_thumbnail_attachment(thumbnail_base)
+    end
+
+    return nil if thumbnail.blank?
+
+    Post.resized_thumbnail_attachment(thumbnail)
+  end
+end

backend/app/services/post_thumbnail_upload_validator.rb

@@ -0,0 +1,34 @@
+class PostThumbnailUploadValidator
+  MAX_THUMBNAIL_BYTES = 20 * 1024 * 1024
+  ALLOWED_CONTENT_TYPES = Preview::ThumbnailFetcher::RASTER_IMAGE_CONTENT_TYPES.freeze
+
+  class InvalidUpload < StandardError; end
+
+  def self.validate! thumbnail
+    return if thumbnail.blank?
+    unless thumbnail.is_a?(ActionDispatch::Http::UploadedFile)
+      raise InvalidUpload, 'thumbnail upload が不正です．'
+    end
+    raise InvalidUpload, 'thumbnail file size が大きすぎます．' if thumbnail.size > MAX_THUMBNAIL_BYTES
+    raise InvalidUpload, 'サムネイル画像の形式が不正です．' unless allowed_content_type?(thumbnail.content_type)
+    bytes = thumbnail.read
+    raise InvalidUpload, 'サムネイル画像の形式が不正です．' if Post.svg_document_bytes?(bytes)
+    unless Post.raster_thumbnail_bytes?(bytes)
+      raise InvalidUpload, 'サムネイル画像の形式が不正です．'
+    end
+
+    attachment = Post.resized_thumbnail_attachment(
+        StringIO.new(bytes),
+        content_type: thumbnail.content_type)
+    attachment[:io].close if attachment[:io].respond_to?(:close)
+  rescue MiniMagick::Error, Timeout::Error
+    raise InvalidUpload, 'サムネイル画像の変換に失敗しました．'
+  ensure
+    thumbnail&.rewind if thumbnail.respond_to?(:rewind)
+  end
+
+  def self.allowed_content_type? content_type
+    mime_type = content_type.to_s.split(';', 2).first.to_s.downcase.strip
+    ALLOWED_CONTENT_TYPES.include?(mime_type)
+  end
+end

backend/app/services/post_version_recorder.rb

@@ -25,6 +25,7 @@ class PostVersionRecorder < VersionRecorder
       thumbnail_base: @record.thumbnail_base,
       video_ms: @record.video_ms,
       tags: @record.snapshot_tag_names.join(' '),
+      tags_json: @record.snapshot_tags_json,
       parent_post_ids: @record.snapshot_parent_post_ids.join(' '),
       original_created_from: @record.original_created_from,
       original_created_before: @record.original_created_before }

backend/app/services/preview/thumbnail_fetcher.rb

@@ -1,7 +1,7 @@
 module Preview
   class ThumbnailFetcher
     class GenerationFailed < StandardError; end
-    ALLOWED_IMAGE_CONTENT_TYPES = [
+    RASTER_IMAGE_CONTENT_TYPES = [
       'image/jpeg', 'image/png', 'image/gif', 'image/webp'
     ].freeze
     HTML_MAX_BYTES = 1.megabyte
@@ -28,7 +28,9 @@ module Preview
     def self.fetch_image_response(raw_url)
       uri, = UrlSafety.validate(raw_url)
       response = HttpFetcher.fetch(uri.to_s)
-      unless allowed_image_content_type?(response.content_type)
+      unless Post.remote_thumbnail_image_bytes?(
+                 response.body,
+                 content_type: response.content_type)
         raise GenerationFailed, 'サムネール画像が見つかりませんでした．'
       end
 
@@ -51,7 +53,9 @@ module Preview
       return nil if url.blank?
 
       response = HttpFetcher.fetch(url)
-      return nil unless allowed_image_content_type?(response.content_type)
+      return nil unless Post.remote_thumbnail_image_bytes?(
+                            response.body,
+                            content_type: response.content_type)
 
       response.body
     rescue HttpFetcher::FetchTimeout
@@ -92,13 +96,7 @@ module Preview
       nil
     end
 
-    def self.allowed_image_content_type?(content_type)
-      mime_type = content_type.to_s.split(';', 2).first.downcase.strip
-      ALLOWED_IMAGE_CONTENT_TYPES.include?(mime_type)
-    end
-
     private_class_method :fetch_image_or_nil, :fetch_image!,
-                         :niconico_thumbnail_url,
-                         :allowed_image_content_type?
+                         :niconico_thumbnail_url
   end
 end

backend/app/services/tag_versioning.rb

@@ -1,32 +1,16 @@
 class TagVersioning
   def self.record! tag, event_type:, created_by_user:
-    if tag.nico?
-      NicoTagVersionRecorder.record!(tag:, event_type:, created_by_user:)
-    else
-      TagVersionRecorder.record!(tag:, event_type:, created_by_user:)
-    end
+    TagVersionRecorder.record!(tag:, event_type:, created_by_user:)
   end
 
   def self.ensure_snapshot! tag, created_by_user:
-    if tag.nico?
-      return if tag.nico_tag_versions.exists?
+    return if tag.tag_versions.exists?
 
-      NicoTagVersionRecorder.record!(tag:, event_type: :create, created_by_user:)
-    else
-      return if tag.tag_versions.exists?
-
-      TagVersionRecorder.record!(tag:, event_type: :create, created_by_user:)
-    end
+    TagVersionRecorder.record!(tag:, event_type: :create, created_by_user:)
   end
 
   def self.record_tag_snapshot! tag, created_by_user:
-    event_type =
-        if tag.nico?
-          tag.nico_tag_versions.exists? ? :update : :create
-        else
-          tag.tag_versions.exists? ? :update : :create
-        end
-
+    event_type = tag.tag_versions.exists? ? :update : :create
     record!(tag, event_type:, created_by_user:)
   end
 

backend/app/services/version_recorder.rb

@@ -47,10 +47,14 @@ class VersionRecorder
   end
 
   def update_record_version_no! version_no
+    return unless tracks_version_no_on_record?
+
     @record.update_columns(version_no:)
     @record.version_no = version_no
   end
 
+  def tracks_version_no_on_record? = true
+
   def validate_version_sequence! latest
     if !(latest) && @event_type != 'create'
       raise "#{ version_class.name } first event must be create"
@@ -60,7 +64,7 @@ class VersionRecorder
       raise "#{ version_class.name } create event already exists"
     end
 
-    return unless latest
+    return if !(latest) || !(tracks_version_no_on_record?)
 
     if @record.version_no != latest.version_no
       raise ("#{ record_class.name }##{ @record.id } version_no is #{ @record.version_no }, " +

backend/app/services/youtube/sync.rb

@@ -103,7 +103,7 @@ module Youtube
     end
 
     def sync_post_tags! post, desired_tag_ids, current_tag_ids: nil
-      current_tag_ids ||= PostTag.kept.where(post_id: post.id).pluck(:tag_id).to_set
+      current_tag_ids ||= PostTag.where(post_id: post.id).pluck(:tag_id).to_set
       desired_tag_ids = desired_tag_ids.compact.to_set
 
       to_add = desired_tag_ids - current_tag_ids
@@ -117,8 +117,8 @@ module Youtube
         end
       end
 
-      PostTag.where(post_id: post.id, tag_id: to_remove.to_a).kept.find_each do |pt|
-        pt.discard_by!(nil)
+      PostTag.where(post_id: post.id, tag_id: to_remove.to_a).find_each do |pt|
+        pt.destroy!
       end
     end
 

backend/config/environments/development.rb

@@ -1,4 +1,4 @@
-require "active_support/core_ext/integer/time"
+require 'active_support/core_ext/integer/time'
 
 Rails.application.configure do
   # Settings specified here will take precedence over those in config/application.rb.
@@ -17,8 +17,8 @@ Rails.application.configure do
 
   # Enable/disable Action Controller caching. By default Action Controller caching is disabled.
   # Run rails dev:cache to toggle Action Controller caching.
-  if Rails.root.join("tmp/caching-dev.txt").exist?
-    config.public_file_server.headers = { "cache-control" => "public, max-age=#{2.days.to_i}" }
+  if Rails.root.join('tmp/caching-dev.txt').exist?
+    config.public_file_server.headers = { 'cache-control' => "public, max-age=#{2.days.to_i}" }
   else
     config.action_controller.perform_caching = false
   end
@@ -36,7 +36,11 @@ Rails.application.configure do
   config.action_mailer.perform_caching = false
 
   # Set localhost to be used by links generated in mailer templates.
-  config.action_mailer.default_url_options = { host: "localhost", port: 3000 }
+  config.action_mailer.default_url_options = { host: 'localhost', port: 3000 }
+  Rails.application.routes.default_url_options.merge!(
+      host: 'localhost',
+      port: 3002,
+      protocol: 'http')
 
   # Print deprecation notices to the Rails logger.
   config.active_support.deprecation = :log

backend/config/routes.rb

@@ -31,13 +31,6 @@ Rails.application.routes.draw do
   scope :preview, controller: :preview do
     get :title
     get :thumbnail
-    get :image
-  end
-
-  scope 'posts/import', controller: :post_imports do
-    post :preview
-    post :validate
-    post '', action: :create
   end
 
   resources :wiki_pages, path: 'wiki', only: [:index, :show, :create, :update] do
@@ -59,8 +52,9 @@ Rails.application.routes.draw do
 
   resources :posts, only: [:index, :show, :create, :update] do
     collection do
+      get :metadata
+      post :bulk
       get :random
-      get :changes
       get :versions, to: 'post_versions#index'
     end
 

backend/db/migrate/20260727123600_add_tags_json_to_post_versions.rb

@@ -0,0 +1,299 @@
+class AddTagsJsonToPostVersions < ActiveRecord::Migration[8.0]
+  RESOLUTION_GRACE = 1.second
+  SECTION_LITERAL_PATTERN = /\[[^\[\]\s]*-[^\[\]\s]*\]\z/
+
+  class MigrationPostVersion < ActiveRecord::Base
+    self.table_name = 'post_versions'
+  end
+
+  class MigrationTag < ActiveRecord::Base
+    self.table_name = 'tags'
+  end
+
+  class MigrationTagName < ActiveRecord::Base
+    self.table_name = 'tag_names'
+  end
+
+  class MigrationTagVersion < ActiveRecord::Base
+    self.table_name = 'tag_versions'
+  end
+
+  class MigrationNicoTagVersion < ActiveRecord::Base
+    self.table_name = 'nico_tag_versions'
+  end
+
+  def up
+    add_column :post_versions, :tags_json, :json, after: :tags
+    MigrationPostVersion.reset_column_information
+
+    backfill_missing_initial_tag_versions!
+    intervals_by_name = build_intervals_by_name
+
+    say_with_time 'Backfilling post_versions.tags_json' do
+      MigrationPostVersion.where(tags_json: nil).find_each(batch_size: 500) do |version|
+        version.update_columns(tags_json: build_tags_json(version, intervals_by_name))
+      end
+    end
+
+    change_column_null :post_versions, :tags_json, false
+
+    schema = connection.quote(JSON.generate({
+        type: 'array',
+        items: { type: 'object',
+                 properties: { id: { type: 'integer', minimum: 1 },
+                               version_no: { type: 'integer', minimum: 1 },
+                               name: { type: 'string', minLength: 1 },
+                               category: { type: 'string', enum: ['deerjikist',
+                                                                  'meme',
+                                                                  'character',
+                                                                  'general',
+                                                                  'material',
+                                                                  'meta',
+                                                                  'nico'] },
+                               sections: { type: 'array',
+                                           items: { type: 'object',
+                                                    properties: { begin_ms: { type: 'integer',
+                                                                              minimum: 0 },
+                                                                  end_ms: { type: ['integer',
+                                                                                   'null'],
+                                                                            minimum: 0 } },
+                                                    required: ['begin_ms', 'end_ms'],
+                                                    additionalProperties: false } } },
+                 required: ['id', 'version_no', 'name', 'category', 'sections'],
+                 additionalProperties: false } }))
+
+    add_check_constraint :post_versions,
+                         "JSON_SCHEMA_VALID(#{ schema }, tags_json)",
+                         name: 'chk_post_versions_tags_json_schema'
+  end
+
+  def down
+    remove_check_constraint :post_versions, name: 'chk_post_versions_tags_json_schema'
+
+    remove_column :post_versions, :tags_json
+  end
+
+  private
+
+  def backfill_missing_initial_tag_versions!
+    say_with_time 'Backfilling missing initial tag versions' do
+      tag_rows = missing_initial_version_rows(MigrationTagVersion, nico: false)
+      nico_rows = missing_initial_version_rows(MigrationNicoTagVersion, nico: true)
+
+      MigrationTagVersion.insert_all!(tag_rows) if tag_rows.any?
+      MigrationNicoTagVersion.insert_all!(nico_rows) if nico_rows.any?
+
+      tag_rows.length + nico_rows.length
+    end
+  end
+
+  def missing_initial_version_rows version_class, nico:
+    first_versions =
+        version_class
+        .order(:tag_id, :version_no)
+        .to_a
+        .group_by(&:tag_id)
+        .transform_values(&:first)
+    rows = []
+
+    MigrationTag.find_each do |tag|
+      next if (tag.category == 'nico') != nico
+
+      first_version = first_versions[tag.id]
+      next unless first_version
+      next if valid_initial_version?(first_version)
+
+      assert_inferable_initial_version!(tag, first_version)
+      rows << initial_version_row(tag, first_version, nico:)
+    end
+
+    rows
+  end
+
+  def valid_initial_version? version
+    version.version_no == 1 && version.event_type == 'create'
+  end
+
+  def assert_inferable_initial_version! tag, version
+    inferable =
+        version.version_no == 2 &&
+        version.event_type == 'discard' &&
+        tag.created_at < version.created_at
+    return if inferable
+
+    details = [
+        "tag_id=#{ tag.id }",
+        "version_no=#{ version.version_no }",
+        "event_type=#{ version.event_type.inspect }",
+        "tag_created_at=#{ tag.created_at.iso8601(6) }",
+        "version_created_at=#{ version.created_at.iso8601(6) }"]
+
+    raise "Cannot infer initial tag version: #{ details.join(', ') }"
+  end
+
+  def initial_version_row tag, discard_version, nico:
+    row = {
+        tag_id: tag.id,
+        version_no: 1,
+        event_type: 'create',
+        name: discard_version.name,
+        created_at: tag.created_at,
+        created_by_user_id: nil }
+
+    if nico
+      return row.merge(linked_tags: discard_version.linked_tags)
+    end
+
+    row.merge(
+        category: discard_version.category,
+        aliases: discard_version.aliases,
+        parent_tag_ids: discard_version.parent_tag_ids,
+        deprecated_at: discard_version.deprecated_at)
+  end
+
+  def build_intervals_by_name
+    intervals_by_name = Hash.new { |hash, name| hash[name] = [] }
+    versions_by_kind = {
+        tag: versions_by_tag_id(MigrationTagVersion),
+        nico: versions_by_tag_id(MigrationNicoTagVersion) }
+    current_names = current_names_by_tag_id
+
+    MigrationTag.find_each do |tag|
+      nico = tag.category == 'nico'
+      kind = nico ? :nico : :tag
+      versions = versions_by_kind.fetch(kind).fetch(tag.id, [])
+
+      intervals_for(
+          tag,
+          versions,
+          current_name: current_names.fetch(tag.id),
+          nico:).each do |interval|
+        name = interval.delete(:name)
+        intervals_by_name[name] << interval
+      end
+    end
+
+    intervals_by_name
+  end
+
+  def versions_by_tag_id version_class
+    version_class
+        .order(:tag_id, :version_no)
+        .to_a
+        .group_by(&:tag_id)
+  end
+
+  def current_names_by_tag_id
+    MigrationTagName
+        .joins('INNER JOIN tags ON tags.tag_name_id = tag_names.id')
+        .pluck('tags.id', 'tag_names.name')
+        .to_h
+  end
+
+  def intervals_for tag, versions, current_name:, nico:
+    if versions.empty?
+      return [{
+          name: current_name,
+          tag_id: tag.id,
+          version_no: tag.version_no,
+          category: nico ? 'nico' : tag.category,
+          from: tag.created_at,
+          to: tag.discarded_at }]
+    end
+
+    versions.each_with_index.filter_map do |version, index|
+      next if version.event_type == 'discard'
+
+      {
+          name: version.name,
+          tag_id: tag.id,
+          version_no: version.version_no,
+          category: nico ? 'nico' : version.category,
+          from: version.created_at,
+          to: versions[index + 1]&.created_at || tag.discarded_at }
+    end
+  end
+
+  def build_tags_json version, intervals_by_name
+    entries = version.tags.to_s.split.map do |literal|
+      name = tag_name_from_literal(literal)
+      interval = resolve_tag!(intervals_by_name.fetch(name, []), name:, version:)
+
+      { 'id' => interval.fetch(:tag_id),
+        'version_no' => interval.fetch(:version_no),
+        'name' => name,
+        'category' => interval.fetch(:category),
+        'sections' => [] }
+    end
+
+    assert_unique_tag_ids!(version, entries)
+
+    entries.sort_by { |entry| entry.fetch('id') }
+  end
+
+  def tag_name_from_literal literal
+    name = literal.dup
+    name.sub!(SECTION_LITERAL_PATTERN, '') while name.match?(
+        SECTION_LITERAL_PATTERN)
+
+    if name.empty? || name.include?('[') || name.include?(']')
+      raise "Invalid legacy tag literal: #{ literal.inspect }"
+    end
+
+    name
+  end
+
+  def resolve_tag! intervals, name:, version:
+    time = version.created_at
+    candidates = intervals.select do |interval|
+      interval.fetch(:from) <= time &&
+        (interval[:to].nil? || time < interval.fetch(:to))
+    end
+
+    candidates = future_candidates(intervals, time) if candidates.empty?
+
+    return candidates.first if candidates.one?
+
+    candidate_versions = candidates.map do |candidate|
+      [candidate.fetch(:tag_id), candidate.fetch(:version_no)]
+    end
+    details = [
+        "post_version_id=#{ version.id }",
+        "post_id=#{ version.post_id }",
+        "name=#{ name.inspect }",
+        "created_at=#{ time.iso8601(6) }",
+        "candidates=#{ candidate_versions.inspect }"].join(', ')
+
+    raise "Could not resolve tag snapshot: #{ details }"
+  end
+
+  def future_candidates intervals, time
+    candidates = intervals.select do |interval|
+      interval.fetch(:from) > time &&
+        interval.fetch(:from) <= time + RESOLUTION_GRACE
+    end
+    return [] if candidates.empty?
+
+    nearest_from = candidates.map { |interval| interval.fetch(:from) }.min
+
+    candidates.select do |interval|
+      interval.fetch(:from) == nearest_from
+    end
+  end
+
+  def assert_unique_tag_ids! version, entries
+    duplicate_tag_ids =
+        entries
+        .map { |entry| entry.fetch('id') }
+        .tally
+        .select { |_tag_id, count| count > 1 }
+        .keys
+    return if duplicate_tag_ids.empty?
+
+    details = [
+        "post_version_id=#{ version.id }",
+        "duplicate_tag_ids=#{ duplicate_tag_ids.inspect }"].join(', ')
+
+    raise "Duplicate tag IDs: #{ details }"
+  end
+end

backend/db/migrate/20260921000000_delete_inactive_records_from_post_tags.rb

@@ -0,0 +1,53 @@
+class DeleteInactiveRecordsFromPostTags < ActiveRecord::Migration[8.0]
+  def up
+    execute <<~SQL
+      DELETE
+      FROM
+        post_tags
+      WHERE
+        discarded_at IS NOT NULL
+    SQL
+
+    remove_index :post_tags, [:tag_id, :discarded_at]
+    remove_index :post_tags, [:post_id, :discarded_at]
+    remove_index :post_tags, name: 'idx_post_tags_active_unique'
+    remove_index :post_tags, :discarded_at
+
+    remove_foreign_key :post_tags, column: :deleted_user_id
+    remove_index :post_tags, :deleted_user_id
+
+    remove_column :post_tags, :active_unique_key
+    remove_column :post_tags, :is_active
+    remove_column :post_tags, :discarded_at
+    remove_column :post_tags, :deleted_user_id
+    remove_column :post_tags, :updated_at
+
+    execute <<~SQL
+      ALTER TABLE
+        post_tags
+      MODIFY COLUMN
+        id BIGINT NOT NULL
+    SQL
+
+    execute <<~SQL
+      ALTER TABLE
+        post_tags
+      DROP PRIMARY KEY
+    SQL
+
+    remove_column :post_tags, :id
+
+    execute <<~SQL
+      ALTER TABLE
+        post_tags
+      ADD PRIMARY KEY
+        (post_id, tag_id)
+    SQL
+
+    remove_index :post_tags, :post_id
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration, '戻せません．'
+  end
+end

backend/db/migrate/20260921010000_add_foreign_key_on_post_id_and_tag_id_in_post_tag_sections.rb

@@ -0,0 +1,11 @@
+class AddForeignKeyOnPostIdAndTagIdInPostTagSections < ActiveRecord::Migration[8.0]
+  def change
+    remove_foreign_key :post_tag_sections, :posts, column: :post_id
+    remove_foreign_key :post_tag_sections, :tags, column: :tag_id
+
+    add_foreign_key :post_tag_sections, :post_tags,
+                    column: [:post_id, :tag_id],
+                    primary_key: [:post_id, :tag_id],
+                    on_delete: :cascade
+  end
+end

backend/db/migrate/20260921020000_delete_discarded_records_from_tags.rb

@@ -0,0 +1,46 @@
+class DeleteDiscardedRecordsFromTags < ActiveRecord::Migration[8.0]
+  def up
+    remove_foreign_key :tag_versions, :tags, column: :tag_id
+    remove_foreign_key :nico_tag_versions, :tags, column: :tag_id
+    remove_foreign_key :material_versions, :tags, column: :tag_id
+
+    execute <<~SQL
+      DELETE
+        ntr
+      FROM
+        nico_tag_relations ntr
+        INNER JOIN
+          tags t
+          ON
+            t.discarded_at IS NOT NULL
+            AND t.id IN (ntr.tag_id, ntr.nico_tag_id)
+    SQL
+
+    execute <<~SQL
+      DELETE
+        ti
+      FROM
+        tag_implications ti
+        INNER JOIN
+          tags t
+          ON
+            t.discarded_at IS NOT NULL
+            AND t.id IN (ti.tag_id, ti.parent_tag_id)
+    SQL
+
+    execute <<~SQL
+      DELETE
+      FROM
+        tags
+      WHERE
+        discarded_at IS NOT NULL
+    SQL
+
+    remove_index :tags, :discarded_at
+    remove_column :tags, :discarded_at
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration, '戻せません．'
+  end
+end

backend/db/migrate/20260921030000_delete_discarded_records_from_tag_names.rb

@@ -0,0 +1,18 @@
+class DeleteDiscardedRecordsFromTagNames < ActiveRecord::Migration[8.0]
+  def up
+    execute <<~SQL
+      DELETE
+      FROM
+        tag_names
+      WHERE
+        discarded_at IS NOT NULL
+    SQL
+
+    remove_index :tag_names, :discarded_at
+    remove_column :tag_names, :discarded_at
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration, '戻せません．'
+  end
+end

backend/db/migrate/20260921040000_create_external_tags.rb

@@ -0,0 +1,54 @@
+class CreateExternalTags < ActiveRecord::Migration[8.0]
+  def up
+    create_table :external_tags do |t|
+      t.string :platform, limit: 16, null: false
+      t.string :name, limit: 255, null: false
+      t.integer :post_count, null: false, default: 0
+      t.datetime :created_at, null: false
+
+      t.index [:platform, :name], unique: true
+    end
+
+    execute <<~SQL
+      INSERT INTO
+        external_tags(id, platform, name, post_count, created_at)
+      SELECT
+        t.id
+      , 'nico' AS platform
+      , SUBSTR(tn.name, 6) AS name
+      , t.post_count
+      , t.created_at
+      FROM
+        tags t
+        INNER JOIN
+          tag_names tn
+          ON
+            tn.id = t.tag_name_id
+            AND t.category = 'nico'
+    SQL
+
+    execute <<~SQL
+      INSERT INTO
+        external_tags(id, platform, name, post_count, created_at)
+      SELECT
+        ntv.tag_id
+      , 'nico' AS platform
+      , SUBSTR(ntv.name, 6) AS name
+      , 0 AS post_count
+      , ntv.created_at
+      FROM
+        nico_tag_versions ntv
+        LEFT JOIN
+          external_tags et
+          ON
+            et.id = ntv.tag_id
+      WHERE
+        ntv.version_no = 1
+        AND et.id IS NULL
+    SQL
+  end
+
+  def down
+    drop_table :external_tags
+  end
+end

backend/db/migrate/20260921050000_create_post_external_tags.rb

@@ -0,0 +1,29 @@
+class CreatePostExternalTags < ActiveRecord::Migration[8.0]
+  def up
+    create_table :post_external_tags, primary_key: [:post_id, :external_tag_id] do |t|
+      t.references :post, null: false, index: false, foreign_key: true
+      t.references :external_tag, null: false, foreign_key: true
+      t.datetime :created_at, null: false
+    end
+
+    execute <<~SQL
+      INSERT INTO
+        post_external_tags(post_id, external_tag_id, created_at)
+      SELECT
+        pt.post_id
+      , pt.tag_id AS external_tag_id
+      , pt.created_at
+      FROM
+        post_tags pt
+        INNER JOIN
+          tags t
+          ON
+            pt.tag_id = t.id
+            AND t.category = 'nico'
+    SQL
+  end
+
+  def down
+    drop_table :post_external_tags
+  end
+end

backend/db/migrate/20260921060000_change_foreign_key_on_nico_tag_relations.rb

@@ -0,0 +1,6 @@
+class ChangeForeignKeyOnNicoTagRelations < ActiveRecord::Migration[8.0]
+  def change
+    remove_foreign_key :nico_tag_relations, :tags, column: :nico_tag_id
+    add_foreign_key :nico_tag_relations, :external_tags, column: :nico_tag_id
+  end
+end

backend/db/migrate/20260921230000_migrate_external_tags.rb

@@ -0,0 +1,199 @@
+class MigrateExternalTags < ActiveRecord::Migration[8.0]
+  class MigrationPostVersion < ActiveRecord::Base
+    self.table_name = 'post_versions'
+  end
+
+  def up
+    x = connection.select_value(<<~SQL)
+      SELECT
+        COUNT(0)
+      FROM
+        post_tag_sections pts
+      INNER JOIN
+        tags t
+        ON
+          t.id = pts.tag_id
+      WHERE
+        t.category = 'nico'
+    SQL
+    if x > 0
+      raise "post_tag_sections に #{ x } 件のチンカスがあります!"
+    end
+
+    x = connection.select_value(<<~SQL)
+      SELECT
+        COUNT(0)
+      FROM
+        materials m
+        INNER JOIN
+          tags t
+          ON
+            t.id = m.tag_id
+      WHERE
+        t.category = 'nico'
+    SQL
+    if x > 0
+      raise "materials に #{ x } 件のチンカスがあります!"
+    end
+
+    x = connection.select_value(<<~SQL)
+      SELECT
+        COUNT(0)
+      FROM
+        tag_implications ti
+        INNER JOIN
+          tags t
+          ON
+            t.id = ti.tag_id
+            OR t.id = ti.parent_tag_id
+      WHERE
+        t.category = 'nico'
+    SQL
+    if x > 0
+      raise "tag_implications に #{ x } 件のチンカスがあります!"
+    end
+
+    execute <<~SQL
+      DELETE
+        ts
+      FROM
+        tag_similarities ts
+        INNER JOIN
+          tags t
+          ON
+            t.category = 'nico'
+            AND (t.id = ts.tag_id
+                 OR t.id = ts.target_tag_id)
+    SQL
+
+    execute <<~SQL
+      DELETE
+        tset
+      FROM
+        theatre_skip_event_tags tset
+        INNER JOIN
+          tags t
+          ON
+            t.category = 'nico'
+            AND t.id = tset.tag_id
+    SQL
+
+    remove_check_constraint :post_versions, name: 'chk_post_versions_tags_json_schema'
+
+    say_with_time 'Migrate post_versions.tags_json' do
+      count = 0
+
+      MigrationPostVersion.find_each(batch_size: 500) do |version|
+        tags = version.tags_json.map do |tag|
+          if tag.fetch('category') == 'nico'
+            { 'external_tag_id' => tag.fetch('id') }
+          else
+            { 'tag_id' => tag.fetch('id'),
+              'version_no' => tag.fetch('version_no'),
+              'name' => tag.fetch('name'),
+              'category' => tag.fetch('category'),
+              'sections' => tag.fetch('sections') }
+          end
+        end
+
+        version.update_columns(tags_json: tags)
+        count += 1
+      end
+
+      count
+    end
+
+    add_tags_json_constraint!
+
+    tag_name_ids = connection.select_values(<<~SQL)
+      SELECT
+        tag_name_id
+      FROM
+        tags
+      WHERE
+        category = 'nico'
+    SQL
+
+    connection.transaction do
+      execute <<~SQL
+        DELETE
+          pt
+        FROM
+          post_tags pt
+          INNER JOIN
+            tags t
+            ON
+              t.category = 'nico'
+              AND t.id = pt.tag_id
+      SQL
+
+      execute <<~SQL
+        DELETE
+        FROM
+          tags
+        WHERE
+          category = 'nico'
+      SQL
+
+      unless tag_name_ids.empty?
+        execute <<~SQL
+          DELETE
+          FROM
+            tag_names
+          WHERE
+            id IN (#{ tag_name_ids.join(', ') })
+        SQL
+      end
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration, '戻せません．'
+  end
+
+  private
+
+  def add_tags_json_constraint!
+    schema = { type: 'array',
+               items: { oneOf: [internal_tag_schema, external_tag_schema] } }
+
+    quoted_schema = connection.quote(JSON.generate(schema))
+
+    add_check_constraint :post_versions,
+                         "JSON_SCHEMA_VALID(#{ quoted_schema }, tags_json)",
+                         name: 'chk_post_versions_tags_json_schema'
+  end
+
+  def internal_tag_schema
+    { type: 'object',
+      properties: { tag_id: { type: 'integer', minimum: 1 },
+                    version_no: { type: 'integer', minimum: 1 },
+                    name: { type: 'string', minLength: 1 },
+                    category: { type: 'string',
+                                enum: ['deerjikist',
+                                       'meme',
+                                       'character',
+                                       'general',
+                                       'material',
+                                       'meta'] },
+                    sections: sections_schema },
+      required: ['tag_id', 'version_no', 'sections'],
+      additionalProperties: false }
+  end
+
+  def external_tag_schema
+    { type: 'object',
+      properties: { external_tag_id: { type: 'integer', minimum: 1 } },
+      required: ['external_tag_id'],
+      additionalProperties: false }
+  end
+
+  def sections_schema
+    { type: 'array',
+      items: { type: 'object',
+               properties: { begin_ms: { type: 'integer', minimum: 0 },
+                             end_ms: { type: ['integer', 'null'], minimum: 0 } },
+               required: ['begin_ms', 'end_ms'],
+               additionalProperties: false } }
+  end
+end

backend/db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2026_07_13_000000) do
+ActiveRecord::Schema[8.0].define(version: 2026_09_21_230000) do
   create_table "active_storage_attachments", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "name", null: false
     t.string "record_type", null: false
@@ -48,6 +48,14 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_13_000000) do
     t.index ["tag_id"], name: "index_deerjikists_on_tag_id"
   end
 
+  create_table "external_tags", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.string "platform", limit: 16, null: false
+    t.string "name", null: false
+    t.integer "post_count", default: 0, null: false
+    t.datetime "created_at", null: false
+    t.index ["platform", "name"], name: "index_external_tags_on_platform_and_name", unique: true
+  end
+
   create_table "gekanator_ai_runs", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "model", null: false
     t.integer "input_tokens", default: 0, null: false
@@ -281,6 +289,13 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_13_000000) do
     t.check_constraint "`version_no` > 0", name: "nico_tag_versions_version_no_positive"
   end
 
+  create_table "post_external_tags", primary_key: ["post_id", "external_tag_id"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "post_id", null: false
+    t.bigint "external_tag_id", null: false
+    t.datetime "created_at", null: false
+    t.index ["external_tag_id"], name: "index_post_external_tags_on_external_tag_id"
+  end
+
   create_table "post_implications", primary_key: ["post_id", "parent_post_id"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "post_id", null: false
     t.bigint "parent_post_id", null: false
@@ -311,23 +326,12 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_13_000000) do
     t.check_constraint "`begin_ms` >= 0", name: "chk_post_tag_sections_begin_ms_natural"
   end
 
-  create_table "post_tags", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+  create_table "post_tags", primary_key: ["post_id", "tag_id"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "post_id", null: false
     t.bigint "tag_id", null: false
     t.bigint "created_user_id"
-    t.bigint "deleted_user_id"
     t.datetime "created_at", null: false
-    t.datetime "updated_at", null: false
-    t.datetime "discarded_at"
-    t.virtual "is_active", type: :boolean, as: "(`discarded_at` is null)", stored: true
-    t.virtual "active_unique_key", type: :string, as: "(case when (`discarded_at` is null) then concat(`post_id`,_utf8mb4':',`tag_id`) else NULL end)", stored: true
-    t.index ["active_unique_key"], name: "idx_post_tags_active_unique", unique: true
     t.index ["created_user_id"], name: "index_post_tags_on_created_user_id"
-    t.index ["deleted_user_id"], name: "index_post_tags_on_deleted_user_id"
-    t.index ["discarded_at"], name: "index_post_tags_on_discarded_at"
-    t.index ["post_id", "discarded_at"], name: "index_post_tags_on_post_id_and_discarded_at"
-    t.index ["post_id"], name: "index_post_tags_on_post_id"
-    t.index ["tag_id", "discarded_at"], name: "index_post_tags_on_tag_id_and_discarded_at"
     t.index ["tag_id"], name: "index_post_tags_on_tag_id"
   end
 
@@ -349,6 +353,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_13_000000) do
     t.string "url", limit: 768, null: false
     t.string "thumbnail_base", limit: 2000
     t.text "tags", null: false
+    t.json "tags_json", null: false
     t.text "parent_post_ids", null: false
     t.datetime "original_created_from"
     t.datetime "original_created_before"
@@ -362,6 +367,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_13_000000) do
     t.check_constraint "(`video_ms` is null) or (`video_ms` > 0)", name: "chk_post_versions_video_ms_positive"
     t.check_constraint "`event_type` in (_utf8mb4'create',_utf8mb4'update',_utf8mb4'discard',_utf8mb4'restore')", name: "post_versions_event_type_valid"
     t.check_constraint "`version_no` > 0", name: "post_versions_version_no_positive"
+    t.check_constraint "json_schema_valid(_utf8mb4'{\"type\":\"array\",\"items\":{\"oneOf\":[{\"type\":\"object\",\"properties\":{\"tag_id\":{\"type\":\"integer\",\"minimum\":1},\"version_no\":{\"type\":\"integer\",\"minimum\":1},\"name\":{\"type\":\"string\",\"minLength\":1},\"category\":{\"type\":\"string\",\"enum\":[\"deerjikist\",\"meme\",\"character\",\"general\",\"material\",\"meta\"]},\"sections\":{\"type\":\"array\",\"items\":{\"type\":\"object\",\"properties\":{\"begin_ms\":{\"type\":\"integer\",\"minimum\":0},\"end_ms\":{\"type\":[\"integer\",\"null\"],\"minimum\":0}},\"required\":[\"begin_ms\",\"end_ms\"],\"additionalProperties\":false}}},\"required\":[\"tag_id\",\"version_no\",\"sections\"],\"additionalProperties\":false},{\"type\":\"object\",\"properties\":{\"external_tag_id\":{\"type\":\"integer\",\"minimum\":1}},\"required\":[\"external_tag_id\"],\"additionalProperties\":false}]}}',`tags_json`)", name: "chk_post_versions_tags_json_schema"
   end
 
   create_table "posts", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
@@ -393,19 +399,6 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_13_000000) do
     t.index ["user_id"], name: "index_settings_on_user_id", unique: true
   end
 
-  create_table "wiki_assets", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
-    t.bigint "wiki_page_id", null: false
-    t.integer "no", null: false
-    t.string "alt_text"
-    t.binary "sha256", limit: 32, null: false
-    t.bigint "created_by_user_id", null: false
-    t.datetime "created_at", null: false
-    t.datetime "updated_at", null: false
-    t.index ["created_by_user_id"], name: "index_wiki_assets_on_created_by_user_id"
-    t.index ["wiki_page_id", "no"], name: "index_wiki_assets_on_wiki_page_id_and_no", unique: true
-    t.index ["wiki_page_id", "sha256"], name: "index_wiki_assets_on_wiki_page_id_and_sha256", unique: true
-  end
-
   create_table "tag_implications", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "tag_id", null: false
     t.bigint "parent_tag_id", null: false
@@ -431,9 +424,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_13_000000) do
     t.bigint "canonical_id"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
-    t.datetime "discarded_at"
     t.index ["canonical_id"], name: "index_tag_names_on_canonical_id"
-    t.index ["discarded_at"], name: "index_tag_names_on_discarded_at"
     t.index ["name"], name: "index_tag_names_on_name", unique: true
   end
 
@@ -470,10 +461,8 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_13_000000) do
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
     t.integer "post_count", default: 0, null: false
-    t.datetime "discarded_at"
     t.integer "version_no", null: false
     t.index ["deprecated_at"], name: "index_tags_on_deprecated_at"
-    t.index ["discarded_at"], name: "index_tags_on_discarded_at"
     t.index ["tag_name_id"], name: "index_tags_on_tag_name_id", unique: true
     t.check_constraint "(`deprecated_at` is null) or (`category` <> _utf8mb4'nico')", name: "chk_tags_deprecated_at_not_nico"
     t.check_constraint "`version_no` > 0", name: "chk_tags_version_no_positive"
@@ -614,6 +603,19 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_13_000000) do
     t.index ["banned_at"], name: "index_users_on_banned_at"
   end
 
+  create_table "wiki_assets", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "wiki_page_id", null: false
+    t.integer "no", null: false
+    t.string "alt_text"
+    t.binary "sha256", limit: 32, null: false
+    t.bigint "created_by_user_id", null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["created_by_user_id"], name: "index_wiki_assets_on_created_by_user_id"
+    t.index ["wiki_page_id", "no"], name: "index_wiki_assets_on_wiki_page_id_and_no", unique: true
+    t.index ["wiki_page_id", "sha256"], name: "index_wiki_assets_on_wiki_page_id_and_sha256", unique: true
+  end
+
   create_table "wiki_lines", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "sha256", limit: 64, null: false
     t.text "body", null: false
@@ -707,27 +709,25 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_13_000000) do
   add_foreign_key "material_sync_suppressions", "users", column: "created_by_user_id"
   add_foreign_key "material_versions", "materials"
   add_foreign_key "material_versions", "materials", column: "parent_id"
-  add_foreign_key "material_versions", "tags"
   add_foreign_key "material_versions", "users", column: "created_by_user_id"
   add_foreign_key "material_versions", "users", column: "updated_by_user_id"
   add_foreign_key "materials", "materials", column: "parent_id"
   add_foreign_key "materials", "tags"
   add_foreign_key "materials", "users", column: "created_by_user_id"
   add_foreign_key "materials", "users", column: "updated_by_user_id"
+  add_foreign_key "nico_tag_relations", "external_tags", column: "nico_tag_id"
   add_foreign_key "nico_tag_relations", "tags"
-  add_foreign_key "nico_tag_relations", "tags", column: "nico_tag_id"
-  add_foreign_key "nico_tag_versions", "tags"
   add_foreign_key "nico_tag_versions", "users", column: "created_by_user_id"
+  add_foreign_key "post_external_tags", "external_tags"
+  add_foreign_key "post_external_tags", "posts"
   add_foreign_key "post_implications", "posts"
   add_foreign_key "post_implications", "posts", column: "parent_post_id"
   add_foreign_key "post_similarities", "posts"
   add_foreign_key "post_similarities", "posts", column: "target_post_id"
-  add_foreign_key "post_tag_sections", "posts"
-  add_foreign_key "post_tag_sections", "tags"
+  add_foreign_key "post_tag_sections", "post_tags", column: ["post_id", "tag_id"], primary_key: ["post_id", "tag_id"], on_delete: :cascade
   add_foreign_key "post_tags", "posts"
   add_foreign_key "post_tags", "tags"
   add_foreign_key "post_tags", "users", column: "created_user_id"
-  add_foreign_key "post_tags", "users", column: "deleted_user_id"
   add_foreign_key "post_versions", "posts"
   add_foreign_key "post_versions", "users", column: "created_by_user_id"
   add_foreign_key "posts", "users", column: "uploaded_user_id"
@@ -737,7 +737,6 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_13_000000) do
   add_foreign_key "tag_names", "tag_names", column: "canonical_id"
   add_foreign_key "tag_similarities", "tags"
   add_foreign_key "tag_similarities", "tags", column: "target_tag_id"
-  add_foreign_key "tag_versions", "tags"
   add_foreign_key "tag_versions", "users", column: "created_by_user_id"
   add_foreign_key "tags", "tag_names"
   add_foreign_key "theatre_comments", "theatres"

backend/lib/tasks/sync_nico.rake

@@ -16,7 +16,7 @@ namespace :nico do
     end
 
     def sync_post_tags! post, desired_tag_ids, current_tag_ids: nil
-      current_tag_ids ||= PostTag.kept.where(post_id: post.id).pluck(:tag_id).to_set
+      current_tag_ids ||= PostTag.where(post_id: post.id).pluck(:tag_id).to_set
       desired_tag_ids = desired_tag_ids.compact.to_set
 
       to_add = desired_tag_ids - current_tag_ids
@@ -30,9 +30,28 @@ namespace :nico do
         end
       end
 
-      PostTag.where(post_id: post.id, tag_id: to_remove.to_a).kept.find_each do |pt|
-        pt.discard_by!(nil)
+      PostTag.where(post_id: post.id, tag_id: to_remove.to_a).find_each(&:destroy!)
+    end
+
+    def sync_post_external_tags! post, desired_external_tag_ids, current_external_tag_ids: nil
+      current_external_tag_ids ||=
+          PostExternalTag.where(post_id: post.id).pluck(:external_tag_id).to_set
+      desired_external_tag_ids = desired_external_tag_ids.compact.to_set
+
+      to_add = desired_external_tag_ids - current_external_tag_ids
+      to_remove = current_external_tag_ids - desired_external_tag_ids
+
+      ExternalTag.where(id: to_add.to_a).find_each do |external_tag|
+        begin
+          PostExternalTag.create!(post:, external_tag:)
+        rescue ActiveRecord::RecordNotUnique
+          ;
+        end
       end
+
+      PostExternalTag
+          .where(post_id: post.id, external_tag_id: to_remove.to_a)
+          .find_each(&:destroy!)
     end
 
     mysql_user = ENV['MYSQL_USER']
@@ -111,58 +130,63 @@ namespace :nico do
         sync_post_tags!(post, [Tag.tagme.id, Tag.bot.id, Tag.niconico.id, Tag.video.id])
       end
 
-      tags = post.tags
       # 既存のタグ Id. 集合
-      kept_tag_ids = tags.pluck(:id).to_set
-      # うち内部タグ Id. 集合
-      kept_non_nico_tag_ids = tags.not_nico.pluck(:id).to_set
+      kept_tag_ids = post.tags.pluck(:id).to_set
+
+      # 既存の外部タグ Id. 集合
+      kept_external_tag_ids = post.external_tags.nico.pluck(:id).to_set
+
+      # 記載すべき外部タグ Id. のリスト
+      desired_external_tag_ids = []
 
-      # 記載すべき外部タグ Id. および連携される内部タグ Id. のリスト
-      desired_nico_tag_based_ids = []
       # 記載すべき内部タグ Id. のリスト
-      desired_non_nico_tag_ids = []
+      desired_tag_ids = kept_tag_ids.to_a
 
       datum['tags'].each do |raw|
-        name = TagNameSanitisationRule.sanitise("nico:#{ raw }")
-        tag = Tag.find_or_create_by_tag_name!(name, category: :nico)
+        tag = ExternalTag.find_or_create_by!(platform: :nico, name: raw)
 
-        event_type = tag.nico_tag_versions.exists? ? :update : :create
-        NicoTagVersionRecorder.record!(tag:, event_type:, created_by_user: nil)
+        unless tag.nico_tag_versions.exists?
+          NicoTagVersionRecorder.record!(external_tag: tag,
+                                         event_type: :create,
+                                         created_by_user: nil)
+        end
 
-        desired_nico_tag_based_ids << tag.id
+        desired_external_tag_ids << tag.id
 
         # 新たに記載される外部タグと連携される内部タグを記載
-        unless tag.id.in?(kept_tag_ids)
-          linked_ids = tag.linked_tags.pluck(:id)
-          desired_non_nico_tag_ids.concat(linked_ids)
-          desired_nico_tag_based_ids.concat(linked_ids)
+        # 連携タグは記載すれども消除せず．
+        unless tag.id.in?(kept_external_tag_ids)
+          desired_tag_ids.concat(tag.linked_tags.pluck(:id))
         end
       end
 
       deerjikist = Deerjikist.find_by(platform: :nico, code: datum['user'])
       if deerjikist
-        desired_non_nico_tag_ids << deerjikist.tag_id
-        desired_nico_tag_based_ids << deerjikist.tag_id
-      elsif !(Tag.where(id: kept_non_nico_tag_ids).where(category: :deerjikist).exists?)
-        desired_non_nico_tag_ids << Tag.no_deerjikist.id
-        desired_nico_tag_based_ids << Tag.no_deerjikist.id
+        desired_tag_ids << deerjikist.tag_id
+      elsif !(Tag.where(id: kept_tag_ids).where(category: :deerjikist).exists?)
+        desired_tag_ids << Tag.no_deerjikist.id
       end
 
-      desired_nico_tag_based_ids.uniq!
+      desired_external_tag_ids.uniq!
+      desired_tag_ids.uniq!
 
-      desired_all_tag_ids = kept_non_nico_tag_ids.to_a + desired_nico_tag_based_ids
-      desired_non_nico_tag_ids.concat(kept_non_nico_tag_ids.to_a)
-      desired_non_nico_tag_ids.uniq!
-      if kept_non_nico_tag_ids != desired_non_nico_tag_ids.to_set
-        desired_all_tag_ids << Tag.bot.id
+      # 外部タグの記載に際しては “bot 操作” タグを記載しなぃ．
+      if kept_tag_ids != desired_tag_ids.to_set
+        desired_tag_ids << Tag.bot.id
+        desired_tag_ids.uniq!
       end
-      desired_all_tag_ids.uniq!
 
-      sync_post_tags!(post, desired_all_tag_ids, current_tag_ids: kept_tag_ids)
+      tags_changed =
+          kept_tag_ids != desired_tag_ids.to_set ||
+          kept_external_tag_ids != desired_external_tag_ids.to_set
+
+      sync_post_tags!(post, desired_tag_ids, current_tag_ids: kept_tag_ids)
+      sync_post_external_tags!(post, desired_external_tag_ids,
+                               current_external_tag_ids: kept_external_tag_ids)
 
       if post_created
         PostVersionRecorder.record!(post:, event_type: :create, created_by_user: nil)
-      elsif post_changed || kept_tag_ids != desired_all_tag_ids.to_set
+      elsif post_changed || tags_changed
         PostVersionRecorder.ensure_snapshot!(post, created_by_user: nil)
         PostVersionRecorder.record!(post:, event_type: :update, created_by_user: nil)
       end

backend/spec/db/delete_discarded_tags_spec.rb

@@ -0,0 +1,123 @@
+require 'rails_helper'
+require_relative '../../db/migrate/20260921020000_delete_discarded_records_from_tags'
+require_relative '../../db/migrate/20260921030000_delete_discarded_records_from_tag_names'
+
+RSpec.describe 'discarded tag cleanup migrations' do
+  [DeleteDiscardedRecordsFromTags, DeleteDiscardedRecordsFromTagNames].each do |migration_class|
+    it "rejects rollback of #{ migration_class.name }" do
+      expect { migration_class.new.down }
+          .to raise_error(ActiveRecord::IrreversibleMigration)
+    end
+  end
+
+  context 'with legacy records' do
+    self.use_transactional_tests = false
+
+    before do
+      record_class = Class.new(ActiveRecord::Base) do
+        self.abstract_class = true
+      end
+      stub_const('TagCleanupMigrationRecord', record_class)
+      config = ActiveRecord::Base.connection_db_config.configuration_hash
+      @database = "btrc_hub_test_tag_cleanup_#{ Process.pid }_#{ SecureRandom.hex(4) }"
+      record_class.establish_connection(config.merge(database: nil))
+      @connection = record_class.lease_connection
+      @connection.create_database(@database)
+      @database_created = true
+      @connection.execute("USE #{ @connection.quote_table_name(@database) }")
+    end
+
+    after do
+      @connection.drop_database(@database) if @database_created
+    ensure
+      TagCleanupMigrationRecord.remove_connection
+    end
+
+    before do
+      @connection.create_table(:tag_names) do |t|
+        t.string :name, null: false, index: { unique: true }
+        t.bigint :canonical_id
+        t.datetime :discarded_at, index: true
+      end
+      @connection.add_foreign_key(:tag_names, :tag_names, column: :canonical_id)
+      @connection.create_table(:tags) do |t|
+        t.references :tag_name, null: false, foreign_key: true, index: { unique: true }
+        t.datetime :discarded_at, index: true
+      end
+      @connection.create_table(:nico_tag_relations) do |t|
+        t.references :tag, null: false, foreign_key: true
+        t.references :nico_tag, null: false, foreign_key: { to_table: :tags }
+      end
+      @connection.create_table(:tag_implications) do |t|
+        t.references :tag, null: false, foreign_key: true
+        t.references :parent_tag, null: false, foreign_key: { to_table: :tags }
+      end
+      [:tag_versions, :nico_tag_versions, :material_versions].each do |table|
+        @connection.create_table(table) do |t|
+          t.references :tag, null: false, foreign_key: true
+        end
+      end
+
+      @connection.execute(<<~SQL)
+        INSERT INTO tag_names (id, name, canonical_id, discarded_at) VALUES
+          (1, 'kept', NULL, NULL),
+          (2, 'merged_alias', 1, NULL),
+          (3, 'nico:deleted', NULL, '2026-09-20'),
+          (4, 'nico:kept', NULL, NULL),
+          (5, 'deleted_name', NULL, '2026-09-20')
+      SQL
+      @connection.execute(<<~SQL)
+        INSERT INTO tags (id, tag_name_id, discarded_at) VALUES
+          (1, 1, NULL), (2, 2, '2026-09-20'),
+          (3, 3, '2026-09-20'), (4, 4, NULL)
+      SQL
+      @connection.execute(<<~SQL)
+        INSERT INTO nico_tag_relations (id, tag_id, nico_tag_id) VALUES
+          (1, 1, 4), (2, 2, 4), (3, 1, 3), (4, 2, 3)
+      SQL
+      @connection.execute(<<~SQL)
+        INSERT INTO tag_implications (id, tag_id, parent_tag_id) VALUES
+          (1, 1, 4), (2, 2, 1), (3, 1, 2), (4, 2, 3)
+      SQL
+      @connection.execute('INSERT INTO tag_versions (tag_id) VALUES (1), (2)')
+      @connection.execute('INSERT INTO nico_tag_versions (tag_id) VALUES (3), (4)')
+      @connection.execute('INSERT INTO material_versions (tag_id) VALUES (1), (2)')
+    end
+
+    it 'removes discarded records and their links while retaining aliases and history' do
+      [DeleteDiscardedRecordsFromTags, DeleteDiscardedRecordsFromTagNames].each do |klass|
+        migration = klass.new
+        allow(migration).to receive(:connection).and_return(@connection)
+        migration.suppress_messages { migration.up }
+      end
+
+      expect(@connection.select_values('SELECT id FROM tags ORDER BY id')).to eq([1, 4])
+      expect(@connection.select_rows('SELECT id, canonical_id FROM tag_names ORDER BY id'))
+          .to eq([[1, nil], [2, 1], [4, nil]])
+      expect(@connection.select_values('SELECT id FROM nico_tag_relations')).to eq([1])
+      expect(@connection.select_values('SELECT id FROM tag_implications')).to eq([1])
+      expect(@connection.select_values('SELECT tag_id FROM tag_versions ORDER BY tag_id'))
+          .to eq([1, 2])
+      expect(@connection.select_values('SELECT tag_id FROM nico_tag_versions ORDER BY tag_id'))
+          .to eq([3, 4])
+      expect(@connection.select_values('SELECT tag_id FROM material_versions ORDER BY tag_id'))
+          .to eq([1, 2])
+
+      [:tags, :tag_names].each do |table|
+        expect(@connection.column_exists?(table, :discarded_at)).to be(false)
+        expect(@connection.index_exists?(table, :discarded_at)).to be(false)
+      end
+      [:tag_versions, :nico_tag_versions, :material_versions].each do |table|
+        expect(@connection.foreign_key_exists?(table, :tags, column: :tag_id)).to be(false)
+      end
+      expect(@connection.foreign_key_exists?(:tags, :tag_names)).to be(true)
+      expect(@connection.foreign_key_exists?(:tag_names, :tag_names, column: :canonical_id))
+          .to be(true)
+      expect(@connection.index_exists?(:tag_names, :name, unique: true)).to be(true)
+      expect(@connection.index_exists?(:tags, :tag_name_id, unique: true)).to be(true)
+      [:nico_tag_relations, :tag_implications].each do |table|
+        expect(@connection.foreign_keys(table).map(&:to_table)).to eq(['tags', 'tags'])
+      end
+    end
+  end
+end

backend/spec/db/external_tags_spec.rb

@@ -0,0 +1,191 @@
+require 'rails_helper'
+require_relative '../../db/migrate/20260921040000_create_external_tags'
+require_relative '../../db/migrate/20260921050000_create_post_external_tags'
+require_relative '../../db/migrate/20260921060000_change_foreign_key_on_nico_tag_relations'
+require_relative '../../db/migrate/20260921230000_migrate_external_tags'
+
+RSpec.describe 'external tag migrations' do
+  self.use_transactional_tests = false
+
+  before do
+    record_class = Class.new(ActiveRecord::Base) do
+      self.abstract_class = true
+    end
+    stub_const('ExternalTagMigrationRecord', record_class)
+    config = ActiveRecord::Base.connection_db_config.configuration_hash
+    @database = "btrc_hub_test_external_tags_#{ Process.pid }_#{ SecureRandom.hex(4) }"
+    record_class.establish_connection(config.merge(database: nil))
+    @connection = record_class.lease_connection
+    @connection.create_database(@database)
+    @database_created = true
+    @connection.execute("USE #{ @connection.quote_table_name(@database) }")
+    version_class = Class.new(record_class) do
+      self.table_name = 'post_versions'
+    end
+    stub_const('MigrateExternalTags::MigrationPostVersion', version_class)
+    create_legacy_tables
+    seed_legacy_records
+  end
+
+  after do
+    @connection.drop_database(@database) if @database_created
+  ensure
+    ExternalTagMigrationRecord.remove_connection
+  end
+
+  def migrate klass
+    migration = klass.new
+    allow(migration).to receive(:connection).and_return(@connection)
+    migration.suppress_messages { migration.migrate(:up) }
+  end
+
+  def create_legacy_tables
+    @connection.create_table(:tag_names) { |t| t.string :name }
+    @connection.create_table(:tags) do |t|
+      t.references :tag_name, foreign_key: true
+      t.string :category
+      t.integer :post_count
+      t.datetime :created_at
+    end
+    @connection.create_table(:posts)
+    @connection.create_table(:post_tags) do |t|
+      t.references :post, foreign_key: true
+      t.references :tag, foreign_key: true
+      t.datetime :created_at
+    end
+    @connection.create_table(:nico_tag_versions) do |t|
+      t.bigint :tag_id
+      t.integer :version_no
+      t.string :name
+      t.datetime :created_at
+    end
+    @connection.create_table(:nico_tag_relations) do |t|
+      t.references :tag, foreign_key: true
+      t.references :nico_tag, foreign_key: { to_table: :tags }
+    end
+    [:post_tag_sections, :materials, :theatre_skip_event_tags].each do |table|
+      @connection.create_table(table) { |t| t.references :tag, foreign_key: true }
+    end
+    @connection.create_table(:tag_implications) do |t|
+      t.references :tag, foreign_key: true
+      t.references :parent_tag, foreign_key: { to_table: :tags }
+    end
+    @connection.create_table(:tag_similarities) do |t|
+      t.references :tag, foreign_key: true
+      t.references :target_tag, foreign_key: { to_table: :tags }
+    end
+    @connection.create_table(:post_versions) { |t| t.json :tags_json, null: false }
+    @connection.add_check_constraint(:post_versions, 'JSON_VALID(tags_json)',
+                                     name: 'chk_post_versions_tags_json_schema')
+  end
+
+  def seed_legacy_records
+    @connection.execute(<<~SQL)
+      INSERT INTO tag_names (id, name) VALUES (1, 'internal'), (2, 'nico:raw tag[]');
+    SQL
+    @connection.execute(<<~SQL)
+      INSERT INTO tags (id, tag_name_id, category, post_count, created_at) VALUES
+        (1, 1, 'general', 1, '2026-09-01'), (2, 2, 'nico', 1, '2026-09-02')
+    SQL
+    @connection.execute('INSERT INTO posts (id) VALUES (1)')
+    @connection.execute(<<~SQL)
+      INSERT INTO post_tags (post_id, tag_id, created_at) VALUES
+        (1, 1, '2026-09-03'), (1, 2, '2026-09-04')
+    SQL
+    @connection.execute(<<~SQL)
+      INSERT INTO nico_tag_versions (tag_id, version_no, name, created_at) VALUES
+        (2, 1, 'nico:raw tag[]', '2026-09-02'),
+        (3, 1, 'nico:deleted', '2026-09-01'),
+        (3, 2, 'nico:deleted_later', '2026-09-02')
+    SQL
+    @connection.execute('INSERT INTO nico_tag_relations (tag_id, nico_tag_id) VALUES (1, 2)')
+    @connection.execute(<<~SQL)
+      INSERT INTO tag_similarities (tag_id, target_tag_id) VALUES (1, 1), (1, 2), (2, 1)
+    SQL
+    @connection.execute('INSERT INTO theatre_skip_event_tags (tag_id) VALUES (1), (2)')
+    @legacy_internal = { 'id' => 1, 'version_no' => 2,
+                         'name' => 'internal', 'category' => 'general',
+                         'sections' => [{ 'begin_ms' => 1000, 'end_ms' => nil }] }
+    snapshots = [@legacy_internal,
+                 { 'id' => 2, 'category' => 'nico' },
+                 { 'id' => 3, 'category' => 'nico' }]
+    @connection.execute(<<~SQL)
+      INSERT INTO post_versions (tags_json) VALUES
+        (#{ @connection.quote(snapshots.to_json) }), ('[]')
+    SQL
+  end
+
+  def prepare_external_tables
+    migrate(CreateExternalTags)
+    migrate(CreatePostExternalTags)
+    migrate(ChangeForeignKeyOnNicoTagRelations)
+  end
+
+  it 'preserves ids, raw names, counts, timestamps, post links and historical-only tags' do
+    prepare_external_tables
+
+    rows = @connection.select_rows(<<~SQL)
+      SELECT id, platform, name, post_count, DATE_FORMAT(created_at, '%Y-%m-%d')
+      FROM external_tags ORDER BY id
+    SQL
+    expect(rows).to eq([
+        [2, 'nico', 'raw tag[]', 1, '2026-09-02'],
+        [3, 'nico', 'deleted', 0, '2026-09-01']])
+    expect(@connection.select_rows(<<~SQL)).to eq([[1, 2, '2026-09-04']])
+      SELECT post_id, external_tag_id, DATE_FORMAT(created_at, '%Y-%m-%d')
+      FROM post_external_tags
+    SQL
+    expect(@connection.foreign_key_exists?(:nico_tag_relations, :external_tags,
+                                           column: :nico_tag_id)).to be(true)
+    expect(@connection.foreign_key_exists?(:nico_tag_relations, :tags,
+                                           column: :tag_id)).to be(true)
+  end
+
+  it 'converts snapshots and removes only obsolete internal rows and derived links' do
+    prepare_external_tables
+    migrate(MigrateExternalTags)
+
+    expect(@connection.select_values('SELECT id FROM tags')).to eq([1])
+    expect(@connection.select_values('SELECT id FROM tag_names')).to eq([1])
+    expect(@connection.select_values('SELECT tag_id FROM post_tags')).to eq([1])
+    expect(@connection.select_rows('SELECT tag_id, nico_tag_id FROM nico_tag_relations'))
+        .to eq([[1, 2]])
+    expect(@connection.select_rows('SELECT tag_id, target_tag_id FROM tag_similarities'))
+        .to eq([[1, 1]])
+    expect(@connection.select_values('SELECT tag_id FROM theatre_skip_event_tags')).to eq([1])
+    expect(@connection.select_value('SELECT COUNT(*) FROM nico_tag_versions')).to eq(3)
+    versions = MigrateExternalTags::MigrationPostVersion.order(:id)
+    expect(versions.first.tags_json).to eq([
+        @legacy_internal.except('id').merge('tag_id' => 1),
+        { 'external_tag_id' => 2 }, { 'external_tag_id' => 3 }])
+    expect(versions.last.tags_json).to eq([])
+
+    invalid_snapshots = [@legacy_internal,
+                         { 'external_tag_id' => 0 },
+                         { 'external_tag_id' => 2, 'tag_id' => 1 },
+                         { 'external_tag_id' => 2, 'name' => 'extra' },
+                         { 'tag_id' => 1, 'version_no' => 1 }]
+    invalid_snapshots.each do |snapshot|
+      expect {
+        MigrateExternalTags::MigrationPostVersion.create!(tags_json: [snapshot])
+      }.to raise_error(ActiveRecord::StatementInvalid, /check constraint/i)
+    end
+    expect { MigrateExternalTags.new.down }
+        .to raise_error(ActiveRecord::IrreversibleMigration)
+  end
+
+  { post_tag_sections: '(tag_id) VALUES (2)',
+    materials: '(tag_id) VALUES (2)',
+    tag_implications: '(tag_id, parent_tag_id) VALUES (1, 2)' }.each do |table, values|
+    it "aborts before deleting data when #{ table } references a legacy nico tag" do
+      prepare_external_tables
+      @connection.execute("INSERT INTO #{ table } #{ values }")
+
+      expect { migrate(MigrateExternalTags) }.to raise_error(RuntimeError, /#{ table }/)
+
+      expect(@connection.select_values('SELECT id FROM tags ORDER BY id')).to eq([1, 2])
+      expect(MigrateExternalTags::MigrationPostVersion.first.tags_json.first)
+          .to eq(@legacy_internal)
+    end
+  end
+end

backend/spec/factories/external_tags.rb

@@ -0,0 +1,6 @@
+FactoryBot.define do
+  factory :external_tag do
+    platform { :nico }
+    sequence(:name) { |n| "external_tag_#{ n }" }
+  end
+end

backend/spec/factories/tags.rb

@@ -11,12 +11,5 @@ FactoryBot.define do
     after(:build) do |tag, evaluator|
       tag.name = evaluator.name if evaluator.name.present?
     end
-
-    trait :nico do
-      category { :nico }
-      transient do
-        name { "nico:#{ SecureRandom.hex(4) }" }
-      end
-    end
   end
 end

backend/spec/models/external_tag_spec.rb

@@ -0,0 +1,38 @@
+require 'rails_helper'
+
+RSpec.describe ExternalTag, type: :model do
+  it 'preserves names without internal tag sanitisation or TagName records' do
+    external = nil
+
+    expect {
+      external = described_class.create!(platform: :nico, name: 'raw tag[]')
+    }.not_to change(TagName, :count)
+
+    expect(external.reload.name).to eq('raw tag[]')
+  end
+
+  it 'deletes post associations without deleting posts or version history' do
+    external = create(:external_tag)
+    post = create(:post)
+    PostExternalTag.create!(post:, external_tag: external)
+    version = NicoTagVersionRecorder.record!(
+        external_tag: external, event_type: :create, created_by_user: nil)
+
+    external.destroy!
+
+    expect(PostExternalTag.where(external_tag_id: external.id)).to be_empty
+    expect(post.reload.external_tags).to be_empty
+    expect(version.reload.tag_id).to eq(external.id)
+  end
+
+  it 'deletes external links without deleting linked internal tags' do
+    external = create(:external_tag)
+    tag = create(:tag)
+    NicoTagRelation.create!(nico_tag: external, tag:)
+
+    external.destroy!
+
+    expect(NicoTagRelation.where(nico_tag_id: external.id)).to be_empty
+    expect(Tag.exists?(tag.id)).to be(true)
+  end
+end

backend/spec/models/nico_tag_relation_spec.rb

@@ -0,0 +1,27 @@
+require 'rails_helper'
+
+RSpec.describe NicoTagRelation, type: :model do
+  it 'does not constrain the ExternalTag association to the nico platform' do
+    external_tag = create(:external_tag)
+    tag = create(:tag)
+    allow(external_tag).to receive(:platform).and_return('registered_external')
+    allow(external_tag).to receive(:nico?).and_return(false)
+
+    expect {
+      described_class.create!(nico_tag: external_tag, tag:)
+    }.to change(described_class, :count).by(1)
+    expect(tag.linked_nico_tags).to contain_exactly(external_tag)
+  end
+
+  it 'rejects an internal Tag through the external association type' do
+    expect {
+      described_class.new(nico_tag: create(:tag), tag: create(:tag))
+    }.to raise_error(ActiveRecord::AssociationTypeMismatch)
+  end
+
+  it 'rejects an ExternalTag through the internal association type' do
+    expect {
+      described_class.new(nico_tag: create(:external_tag), tag: create(:external_tag))
+    }.to raise_error(ActiveRecord::AssociationTypeMismatch)
+  end
+end

backend/spec/models/post_external_tag_spec.rb

@@ -0,0 +1,25 @@
+require 'rails_helper'
+
+RSpec.describe PostExternalTag, type: :model do
+  it 'exposes both sides and enforces a unique post and external tag pair' do
+    post = create(:post)
+    external_tag = create(:external_tag)
+    described_class.create!(post:, external_tag:)
+
+    expect(post.external_tags).to contain_exactly(external_tag)
+    expect(external_tag.posts).to contain_exactly(post)
+    expect { described_class.create!(post:, external_tag:) }
+        .to raise_error(ActiveRecord::RecordNotUnique)
+  end
+
+  it 'removes associations when the post is deleted while retaining the external tag' do
+    post = create(:post)
+    external_tag = create(:external_tag)
+    described_class.create!(post:, external_tag:)
+
+    post.destroy!
+
+    expect(described_class.where(post_id: post.id)).to be_empty
+    expect(external_tag.reload.posts).to be_empty
+  end
+end

backend/spec/models/post_spec.rb

@@ -6,6 +6,25 @@ RSpec.describe Post, type: :model do
     PostUrlSanitisationRule.unscoped.delete_all
   end
 
+  describe '#snapshot_tags_json' do
+    it 'keeps internal snapshots and external identifiers distinct, even with the same id' do
+      post = create(:post)
+      tag = create(:tag)
+      external = create(:external_tag, id: tag.id)
+      create(:post_tag, post:, tag:)
+      create(:post_tag_section, post:, tag:, begin_ms: 2000, end_ms: nil)
+      PostExternalTag.create!(post:, external_tag: external)
+
+      expect(post.snapshot_tags_json).to eq([
+          { 'tag_id' => tag.id,
+            'version_no' => tag.version_no,
+            'name' => tag.name,
+            'category' => tag.category,
+            'sections' => [{ 'begin_ms' => 2000, 'end_ms' => nil }] },
+          { 'external_tag_id' => external.id }])
+    end
+  end
+
   describe 'URL normalisation' do
     it 'normalises the HTTP URL before applying sanitisation rules' do
       PostUrlSanitisationRule.create!(
@@ -110,7 +129,7 @@ RSpec.describe Post, type: :model do
     end
 
     describe '.resized_thumbnail_attachment' do
-      it 'centre-crops a wide image to 180x180 without distorting it' do
+      it 'fits a wide image within 180x180 without distorting it' do
         blob = image_blob(
             width: 360,
             height: 180,
@@ -123,13 +142,13 @@ RSpec.describe Post, type: :model do
         resized = described_class.resized_thumbnail_attachment(upload_for(blob))
         image = read_image(resized)
 
-        expect(image.dimensions).to eq([180, 180])
-        expect_green(colour_at(image, 0, 90))
-        expect_green(colour_at(image, 90, 90))
-        expect_green(colour_at(image, 179, 90))
+        expect(image.dimensions).to eq([180, 90])
+        expect_red(colour_at(image, 0, 45))
+        expect_green(colour_at(image, 90, 45))
+        expect_red(colour_at(image, 179, 45))
       end
 
-      it 'centre-crops a tall image to 180x180 without distorting it' do
+      it 'fits a tall image within 180x180 without distorting it' do
         blob = image_blob(
             width: 180,
             height: 360,
@@ -142,10 +161,10 @@ RSpec.describe Post, type: :model do
         resized = described_class.resized_thumbnail_attachment(upload_for(blob))
         image = read_image(resized)
 
-        expect(image.dimensions).to eq([180, 180])
-        expect_green(colour_at(image, 90, 0))
-        expect_green(colour_at(image, 90, 90))
-        expect_green(colour_at(image, 90, 179))
+        expect(image.dimensions).to eq([90, 180])
+        expect_red(colour_at(image, 45, 0))
+        expect_green(colour_at(image, 45, 90))
+        expect_red(colour_at(image, 45, 179))
       end
 
       it 'keeps a square image square without distortion' do
@@ -192,7 +211,7 @@ RSpec.describe Post, type: :model do
 
         expect(post.thumbnail).to be_attached
         image = read_image(post.thumbnail)
-        expect(image.dimensions).to eq([180, 180])
+        expect(image.dimensions).to eq([180, 135])
       end
 
       it 'does not attach anything when thumbnail conversion fails' do

backend/spec/models/post_tag_spec.rb

@@ -1,5 +1,73 @@
+require 'rails_helper'
+
 RSpec.describe PostTag, type: :model do
+  describe 'uniqueness' do
+    it 'rejects duplicate post and tag pairs but allows either to be reused' do
+      post_tag = create(:post_tag)
+      duplicate = build(:post_tag, post: post_tag.post, tag: post_tag.tag)
+
+      expect(duplicate).not_to be_valid
+      expect(duplicate.errors.of_kind?(:post_id, :taken)).to be(true)
+      expect(build(:post_tag, post: post_tag.post, tag: create(:tag))).to be_valid
+      expect(build(:post_tag, post: create(:post), tag: post_tag.tag)).to be_valid
+    end
+
+    it 'enforces uniqueness in the database when validation is bypassed' do
+      post_tag = create(:post_tag)
+      duplicate = build(:post_tag, post: post_tag.post, tag: post_tag.tag)
+
+      expect { duplicate.save!(validate: false) }
+          .to raise_error(ActiveRecord::RecordNotUnique)
+    end
+  end
+
+  describe '#destroy!' do
+    it 'deletes only the selected pair and its sections and updates the counter' do
+      post_tag = create(:post_tag)
+      same_post = create(:post_tag, post: post_tag.post)
+      same_tag = create(:post_tag, tag: post_tag.tag)
+      sections = [post_tag, same_post, same_tag].map do |link|
+        create(:post_tag_section, post: link.post, tag: link.tag,
+                                  begin_ms: 1000, end_ms: 2000)
+      end
+
+      expect { post_tag.destroy! }.to change(described_class, :count).by(-1)
+          .and change(PostTagSection, :count).by(-1)
+          .and change { post_tag.tag.reload.post_count }.from(2).to(1)
+
+      expect(described_class.exists?(post: post_tag.post, tag: post_tag.tag)).to be(false)
+      expect(same_post.reload).to be_persisted
+      expect(same_tag.reload).to be_persisted
+      expect(PostTagSection.all).to contain_exactly(*sections.drop(1))
+      expect(post_tag.post.reload.tags).to contain_exactly(same_post.tag)
+      expect(post_tag.tag.reload.posts).to contain_exactly(same_tag.post)
+    end
+
+    it 'allows a removed tag to be added again without restoring old sections' do
+      post_tag = create(:post_tag)
+      create(:post_tag_section, post: post_tag.post, tag: post_tag.tag,
+                                begin_ms: 1000, end_ms: 2000)
+      post_tag.destroy!
+
+      replacement = create(:post_tag, post: post_tag.post, tag: post_tag.tag)
+
+      expect(replacement.reload.sections).to be_empty
+      expect(replacement.tag.reload.post_count).to eq(1)
+    end
+  end
+
   describe '#sections' do
+    it 'loads the owning post_tag from a section using both keys' do
+      post_tag = create(:post_tag)
+      create(:post_tag, post: post_tag.post)
+      create(:post_tag, tag: post_tag.tag)
+      section = create(:post_tag_section, post: post_tag.post,
+                                         tag: post_tag.tag,
+                                         begin_ms: 1000, end_ms: 2000)
+
+      expect(section.reload.post_tag).to eq(post_tag)
+    end
+
     it 'loads sections by post_id and tag_id' do
       post_tag = create(:post_tag)
       section = create(:post_tag_section,
@@ -12,18 +80,25 @@ RSpec.describe PostTag, type: :model do
     end
 
     it 'does not load sections for another tag on the same post' do
-      post = create(:post)
-      tag = create(:tag)
+      post_tag = create(:post_tag)
+      post = post_tag.post
       other_tag = create(:tag)
 
-      post_tag = create(:post_tag, post:, tag:)
+      own_section = create(:post_tag_section,
+                           post:,
+                           tag: post_tag.tag,
+                           begin_ms: 1000,
+                           end_ms: 2000)
+
+      create(:post_tag, post:, tag: other_tag)
+
       create(:post_tag_section,
              post:,
              tag: other_tag,
              begin_ms: 1000,
              end_ms: 2000)
 
-      expect(post_tag.sections).to be_empty
+      expect(post_tag.reload.sections).to contain_exactly(own_section)
     end
 
     it 'allows open-ended sections' do

backend/spec/models/post_version_spec.rb

@@ -19,6 +19,7 @@ RSpec.describe PostVersion, type: :model do
       url: post_record.url,
       thumbnail_base: post_record.thumbnail_base,
       tags: post_record.snapshot_tag_names.join(' '),
+      tags_json: post_record.snapshot_tags_json,
       parent_post_ids: post_record.snapshot_parent_post_ids.join(' '),
       original_created_from: post_record.original_created_from,
       original_created_before: post_record.original_created_before,

backend/spec/models/tag_name_sanitisation_rule_spec.rb

@@ -57,7 +57,7 @@ RSpec.describe TagNameSanitisationRule, type: :model do
 
       it 'deletes the source tag_name' do
         described_class.apply!
-        expect(TagName.exists?(source.id)).to be(false)
+        expect(TagName.unscoped.exists?(source.id)).to be(false)
         expect(existing.reload.name).to eq('foobar')
       end
     end
@@ -75,7 +75,27 @@ RSpec.describe TagNameSanitisationRule, type: :model do
         described_class.apply!
         expected_tag_name_id = existing.canonical_id || existing.id
         expect(source_tag.reload.tag_name_id).to eq(expected_tag_name_id)
-        expect(TagName.exists?(source_tag_name_id)).to be(false)
+        expect(TagName.unscoped.exists?(source_tag_name_id)).to be(false)
+      end
+    end
+
+    context 'when the sanitised name is an alias of an existing tag' do
+      let!(:existing_tag) { create(:tag) }
+      let!(:alias_name) do
+        TagName.create!(name: 'foobar', canonical: existing_tag.tag_name)
+      end
+      let!(:source) do
+        TagName.create!(name: 'tmp').tap do |tn|
+          tn.update_columns(name: 'foo_bar', updated_at: Time.current)
+        end
+      end
+
+      it 'deletes only the source and preserves the alias and its canonical tag' do
+        described_class.apply!
+
+        expect(TagName.unscoped.exists?(source.id)).to be(false)
+        expect(alias_name.reload.canonical).to eq(existing_tag.tag_name)
+        expect(Tag.find(existing_tag.id)).to eq(existing_tag)
       end
     end
 
@@ -92,13 +112,15 @@ RSpec.describe TagNameSanitisationRule, type: :model do
       end
 
       it 'merges the source tag into the existing tag and deletes the source tag_name' do
-        expect(TagName.find_by(name: 'foobar')&.tag&.id).to eq(existing_tag.id)
-        expect(TagName.find_by(name: 'foo_bar')&.tag&.id).to eq(source_tag.id)
+        post = create(:post)
+        PostTag.create!(post:, tag: source_tag)
 
         described_class.apply!
 
-        expect(Tag.exists?(source_tag.id)).to be(false)
-        expect(TagName.exists?(source_tag.tag_name_id)).to be(false)
+        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
+        expect(TagName.unscoped.exists?(source_tag_name_id)).to be(false)
+        expect(post.reload.tags).to contain_exactly(existing_tag)
+        expect(existing_tag.reload.name).to eq('foobar')
       end
     end
   end

backend/spec/models/tag_spec.rb

@@ -1,6 +1,29 @@
 require 'rails_helper'
 
 RSpec.describe Tag, type: :model do
+  describe 'external tag separation' do
+    ['nico:reserved', 'NiCo:reserved'].each do |name|
+      it "rejects the reserved prefix #{ name } for internal tags" do
+        tag = build(:tag, name:)
+
+        expect(tag).to be_invalid
+        expect(tag.errors[:name]).to be_present
+        expect {
+          described_class.normalise_tags!([name])
+        }.to raise_error(Tag::NicoTagNormalisationError)
+      end
+    end
+
+    it 'finds external links by the internal tag id even when ids differ' do
+      tag = create(:tag)
+      external = create(:external_tag, id: tag.id + 10_000)
+      # The migration retains existing links without running model validation.
+      NicoTagRelation.insert_all!([{ tag_id: tag.id, nico_tag_id: external.id }])
+
+      expect(tag.linked_nico_tags).to contain_exactly(external)
+    end
+  end
+
   describe '.normalise_tags!' do
     it 'rejects deprecated tags when deny_deprecated is enabled' do
       tag_name = TagName.create!(name: 'normalise deprecated tag')
@@ -159,17 +182,44 @@ RSpec.describe Tag, type: :model do
     end
   end
 
-  describe 'deprecated validation' do
-    it 'rejects deprecated nico tags' do
-      tag = build(
-        :tag,
-        name: 'nico:deprecated_validation',
-        category: :nico,
-        deprecated_at: Time.current
-      )
+  describe '.find_or_create_by_tag_name!' do
+    it 'creates a tag and name with the requested category after stripping whitespace' do
+      tag = nil
 
-      expect(tag).not_to be_valid
-      expect(tag.errors[:deprecated_at]).to include('ニコタグは廃止できません．')
+      expect {
+        tag = described_class.find_or_create_by_tag_name!(
+            '  lookup_new  ', category: :character)
+      }.to change(Tag, :count).by(1).and change(TagName, :count).by(1)
+
+      expect(tag.name).to eq('lookup_new')
+      expect(tag.category).to eq('character')
+    end
+
+    it 'reuses the canonical tag for an alias without changing its category' do
+      tag = create(:tag, category: :character)
+      alias_name = TagName.create!(name: 'lookup_alias', canonical: tag.tag_name)
+      found = nil
+
+      expect {
+        found = described_class.find_or_create_by_tag_name!(
+            alias_name.name, category: :general)
+      }.to change(Tag, :count).by(0).and change(TagName, :count).by(0)
+
+      expect(found).to eq(tag)
+      expect(found.category).to eq('character')
+    end
+
+    it 'creates a tag for an existing canonical name reached through an alias' do
+      canonical = create(:tag_name)
+      alias_name = TagName.create!(name: 'lookup_alias', canonical:)
+      tag = nil
+
+      expect {
+        tag = described_class.find_or_create_by_tag_name!(
+            alias_name.name, category: :general)
+      }.to change(Tag, :count).by(1).and change(TagName, :count).by(0)
+
+      expect(tag.tag_name).to eq(canonical)
     end
   end
 
@@ -185,18 +235,14 @@ RSpec.describe Tag, type: :model do
     context 'when merging a simple source tag' do
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
 
-      it 'discards the source post_tag, creates an active target post_tag, discards the source tag, and aliases the source tag_name' do
+      it 'deletes the source tag, moves its post link, and keeps its name as an alias' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        source_pt = PostTag.with_discarded.find(source_post_tag.id)
-        active_target = PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)
+        target_link = PostTag.find_by(post: post_record, tag: target_tag)
 
-        expect(source_pt.discarded_at).to be_present
-        expect(source_pt.tag_id).to eq(source_tag.id)
-        expect(active_target).to be_present
-
-        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
-        expect(TagName.with_discarded.find(source_tag_name.id)).not_to be_discarded
+        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
+        expect(target_link).to be_present
+        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
         expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
         expect(target_tag.reload.post_count).to eq(1)
       end
@@ -206,38 +252,101 @@ RSpec.describe Tag, type: :model do
       let!(:target_post_tag) { PostTag.create!(post: post_record, tag: target_tag) }
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
 
-      it 'discards the source post_tag, keeps one active target post_tag, discards the source tag, and aliases the source tag_name' do
+      it 'deletes the source link and preserves the existing target link' do
+        create(:post_tag_section, post: post_record, tag: source_tag,
+                                  begin_ms: 1000, end_ms: 2000)
+        target_section = create(:post_tag_section, post: post_record,
+                                                   tag: target_tag,
+                                                   begin_ms: 3000, end_ms: nil)
+
         described_class.merge_tags!(target_tag, [source_tag])
 
-        source_pt = PostTag.with_discarded.find(source_post_tag.id)
-        active = PostTag.kept.where(post_id: post_record.id, tag_id: target_tag.id)
+        target_links = PostTag.where(post: post_record, tag: target_tag)
 
-        expect(source_pt.discarded_at).to be_present
-        expect(source_pt.tag_id).to eq(source_tag.id)
-        expect(active.count).to eq(1)
-        expect(active.first.id).to eq(target_post_tag.id)
+        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
+        expect(target_links).to contain_exactly(target_post_tag)
+        expect(PostTagSection.where(post: post_record, tag: source_tag)).to be_empty
+        expect(target_post_tag.reload.sections).to contain_exactly(target_section)
 
-        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
-        expect(TagName.with_discarded.find(source_tag_name.id)).not_to be_discarded
+        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
         expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
         expect(target_tag.reload.post_count).to eq(1)
       end
     end
 
+    it 'keeps source history and records the new target alias after deleting the source' do
+      user = create_member_user!
+      source_name = source_tag.name
+      source_alias = TagName.create!(name: 'merge_alias', canonical: source_tag_name)
+      TagVersioning.ensure_snapshot!(source_tag, created_by_user: user)
+      original_version = source_tag.tag_versions.first
+
+      described_class.merge_tags!(target_tag, [source_tag], created_by_user: user)
+
+      versions = TagVersion.where(tag_id: source_tag.id).order(:version_no)
+      expect(versions.pluck(:version_no, :event_type))
+          .to eq([[1, 'create'], [2, 'discard']])
+      expect(versions.first).to eq(original_version)
+      expect(versions.last).to have_attributes(
+          name: source_name, aliases: source_alias.name, created_by_user: user)
+      expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
+
+      target_versions = target_tag.tag_versions.order(:version_no)
+      expect(target_versions.pluck(:event_type)).to eq(['create', 'update'])
+      expect(target_versions.last.aliases.split).to eq([source_name])
+    end
+
+    it 'deletes source relationships while preserving unrelated relationships' do
+      parent = create(:tag)
+      child = create(:tag)
+      nico_tag = create(:external_tag)
+      TagImplication.create!(tag: source_tag, parent_tag: parent)
+      TagImplication.create!(tag: child, parent_tag: source_tag)
+      kept_implication = TagImplication.create!(tag: target_tag, parent_tag: parent)
+      NicoTagRelation.create!(tag: source_tag, nico_tag:)
+      kept_relation = NicoTagRelation.create!(tag: target_tag, nico_tag:)
+      TagSimilarity.create!(tag: source_tag, target_tag:, cos: 0.5)
+      TagSimilarity.create!(tag: target_tag, target_tag: source_tag, cos: 0.5)
+      kept_similarity = TagSimilarity.create!(tag: target_tag, target_tag: parent,
+                                               cos: 0.5)
+
+      described_class.merge_tags!(target_tag, [source_tag])
+
+      expect(TagImplication.all).to contain_exactly(kept_implication)
+      expect(NicoTagRelation.all).to contain_exactly(kept_relation)
+      expect(TagSimilarity.all).to contain_exactly(kept_similarity)
+      expect(TagVersion.where(tag_id: source_tag.id).order(:version_no).last.parent_tag_ids)
+          .to eq(parent.id.to_s)
+    end
+
+    it 'preserves material history referencing the deleted source tag' do
+      source_tag.update!(category: :material)
+      target_tag.update!(category: :material)
+      material = Material.create!(tag: source_tag, url: 'https://example.com/material')
+      version = MaterialVersionRecorder.record!(
+          material:, event_type: :create, created_by_user: nil)
+      material.update!(tag: target_tag)
+
+      described_class.merge_tags!(target_tag, [source_tag])
+
+      expect(version.reload).to have_attributes(
+          tag_id: source_tag.id, tag_name: source_tag_name.name, tag_category: 'material')
+      expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
+      expect(material.reload.tag).to eq(target_tag)
+    end
+
     context 'when source_tags includes the target itself' do
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
 
       it 'ignores the target in source_tags while still merging the source tag' do
         described_class.merge_tags!(target_tag, [source_tag, target_tag])
 
-        source_pt = PostTag.with_discarded.find(source_post_tag.id)
-        active_target = PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)
+        target_link = PostTag.find_by(post: post_record, tag: target_tag)
 
         expect(Tag.find(target_tag.id)).to be_present
-        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
-        expect(source_pt.discarded_at).to be_present
-        expect(source_pt.tag_id).to eq(source_tag.id)
-        expect(active_target).to be_present
+        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
+        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
+        expect(target_link).to be_present
         expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
         expect(target_tag.reload.post_count).to eq(1)
       end
@@ -260,18 +369,16 @@ RSpec.describe Tag, type: :model do
         )
       end
 
-      it 'still merges, but discards the source tag_name instead of aliasing it' do
+      it 'still merges and keeps the source name as an alias without validating it' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        source_pt = PostTag.with_discarded.find(source_post_tag.id)
-        active_target = PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)
-        discarded_source_tag_name = TagName.with_discarded.find(source_tag_name.id)
+        target_link = PostTag.find_by(post: post_record, tag: target_tag)
 
-        expect(source_pt.discarded_at).to be_present
-        expect(source_pt.tag_id).to eq(source_tag.id)
-        expect(active_target).to be_present
+        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
+        expect(target_link).to be_present
 
-        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
+        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
+        expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
         expect(target_tag.reload.post_count).to eq(1)
       end
     end
@@ -288,38 +395,32 @@ RSpec.describe Tag, type: :model do
             message: 'init')
       end
 
-      it 'rolls back the transaction' do
+      it 'rolls back earlier deletions, links, and history when a later source has a wiki' do
+        earlier_source = create(:tag)
+        earlier_name = earlier_source.tag_name
+        source_section = create(:post_tag_section, post: post_record,
+                                                   tag: source_tag,
+                                                   begin_ms: 1000, end_ms: 2000)
+
         expect {
-          described_class.merge_tags!(target_tag, [source_tag])
+          described_class.merge_tags!(target_tag, [earlier_source, source_tag])
         }.to raise_error(ActiveRecord::RecordInvalid)
 
-        expect(Tag.with_discarded.find(source_tag.id)).not_to be_discarded
-        expect(TagName.with_discarded.find(source_tag_name.id)).not_to be_discarded
-        expect(PostTag.kept.find(source_post_tag.id).tag_id).to eq(source_tag.id)
-        expect(PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)).to be_nil
+        expect(Tag.unscoped.exists?(earlier_source.id)).to be(true)
+        expect(earlier_name.reload.canonical_id).to be_nil
+        expect(TagVersion.where(tag_id: [earlier_source.id, source_tag.id, target_tag.id]))
+            .to be_empty
+        expect(Tag.unscoped.exists?(source_tag.id)).to be(true)
+        expect(TagName.unscoped.exists?(source_tag_name.id)).to be(true)
+        expect(source_post_tag.reload.tag_id).to eq(source_tag.id)
+        expect(source_post_tag.sections).to contain_exactly(source_section)
+        expect(PostTag.find_by(post: post_record, tag: target_tag)).to be_nil
+        expect(source_tag.reload.post_count).to eq(1)
         expect(source_tag_name.reload.canonical_id).to be_nil
         expect(target_tag.reload.post_count).to eq(0)
       end
     end
 
-    context 'when merging a nico source tag' do
-      let!(:target_tag) { create(:tag, category: :nico, name: 'nico:foo') }
-      let!(:source_tag) { create(:tag, category: :nico, name: 'nico:bar') }
-      let!(:source_tag_name_id) { source_tag.tag_name_id }
-
-      it 'discards the source tag_name instead of aliasing it' do
-        described_class.merge_tags!(target_tag, [source_tag])
-
-        discarded_source_tag = Tag.with_discarded.find(source_tag.id)
-        discarded_source_tag_name = TagName.with_discarded.find(source_tag_name_id)
-
-        expect(discarded_source_tag).to be_discarded
-        expect(discarded_source_tag_name).to be_discarded
-        expect(discarded_source_tag_name.canonical_id).to be_nil
-        expect(target_tag.reload.post_count).to eq(0)
-      end
-    end
-
     def snapshot_tags(post)
       post.snapshot_tag_names.join(' ')
     end
@@ -333,6 +434,7 @@ RSpec.describe Tag, type: :model do
         url: post.url,
         thumbnail_base: post.thumbnail_base,
         tags: snapshot_tags(post),
+        tags_json: post.snapshot_tags_json,
         parent_post_ids: post.snapshot_parent_post_ids.join(' '),
         original_created_from: post.original_created_from,
         original_created_before: post.original_created_before,
@@ -364,12 +466,15 @@ RSpec.describe Tag, type: :model do
         expect(latest.event_type).to eq('update')
         expect(latest.created_by_user).to be_nil
         expect(latest.tags).to eq(snapshot_tags(post_record.reload))
+        expect(latest.tags_json.map { |item| item.fetch('tag_id') }).to eq([target_tag.id])
+        expect(affected_versions.first.tags_json.map { |item| item.fetch('tag_id') })
+            .to eq([source_tag.id])
 
         expect(unaffected_post.reload.post_versions.count).to eq(1)
       end
     end
 
-    context 'when the source tag has no active post_tags' do
+    context 'when the source tag has no post_tags' do
       let!(:another_post) do
         Post.create!(url: 'https://example.com/posts/3', title: 'another post')
       end

backend/spec/models/version_record_spec.rb

@@ -2,7 +2,7 @@ require 'rails_helper'
 
 RSpec.describe VersionRecord, type: :model do
   let!(:tag) { create(:tag, name: 'version_record_tag') }
-  let!(:nico_tag) { create(:tag, :nico, name: 'nico:version_record_tag') }
+  let!(:nico_tag) { create(:external_tag, name: 'version_record_tag') }
 
   it 'makes TagVersion read only after create' do
     version = TagVersion.create!(
@@ -42,10 +42,10 @@ RSpec.describe VersionRecord, type: :model do
 
   it 'makes NicoTagVersion read only after create' do
     version = NicoTagVersion.create!(
-      tag: nico_tag,
+      external_tag: nico_tag,
       version_no: 1,
       event_type: 'create',
-      name: nico_tag.name,
+      name: "nico:#{ nico_tag.name }",
       linked_tags: '',
       created_at: Time.current,
       created_by_user: nil
@@ -58,10 +58,10 @@ RSpec.describe VersionRecord, type: :model do
 
   it 'prevents NicoTagVersion destroy' do
     version = NicoTagVersion.create!(
-      tag: nico_tag,
+      external_tag: nico_tag,
       version_no: 1,
       event_type: 'create',
-      name: nico_tag.name,
+      name: "nico:#{ nico_tag.name }",
       linked_tags: '',
       created_at: Time.current,
       created_by_user: nil

backend/spec/requests/nico_tags_spec.rb

@@ -3,10 +3,34 @@ require 'rails_helper'
 
 RSpec.describe 'NicoTags', type: :request do
   describe 'GET /tags/nico' do
-    it 'returns paginated tags and total count' do
-      create_list(:tag, 3, :nico)
+    it 'returns the legacy Tag-compatible external fields' do
+      external = create(:external_tag, name: 'legacy_external', post_count: 3)
 
-      get '/tags/nico', params: { page: 2, limit: 2 }
+      get '/tags/nico', params: { name: 'legacy_external' }
+
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('count')).to eq(1)
+      expect(json.fetch('tags')).to contain_exactly(
+          a_hash_including(
+              'id' => external.id,
+              'name' => 'nico:legacy_external',
+              'category' => 'nico',
+              'post_count' => 3,
+              'created_at' => external.created_at.as_json,
+              'updated_at' => external.created_at.as_json,
+              'deprecated_at' => nil,
+              'aliases' => [],
+              'parents' => [],
+              'has_wiki' => false,
+              'material_id' => nil,
+              'has_deerjikists' => false,
+              'linked_tags' => []))
+    end
+
+    it 'returns paginated tags and total count' do
+      3.times { |i| create(:external_tag, name: "pagination_#{ i }") }
+
+      get '/tags/nico', params: { page: 2, limit: 2, name: 'pagination_' }
 
       expect(response).to have_http_status(:ok)
       expect(json['tags'].size).to eq(1)
@@ -14,12 +38,12 @@ RSpec.describe 'NicoTags', type: :request do
     end
 
     it 'filters by nico tag name, linked tag name, and link status' do
-      linked = create(:tag, :nico)
-      linked.tag_name.update!(name: 'nico:search_linked')
-      unlinked = create(:tag, :nico)
-      unlinked.tag_name.update!(name: 'nico:search_unlinked')
-      other = create(:tag, :nico)
-      other.tag_name.update!(name: 'nico:other')
+      linked = create(:external_tag)
+      linked.update!(name: 'search_linked')
+      unlinked = create(:external_tag)
+      unlinked.update!(name: 'search_unlinked')
+      other = create(:external_tag)
+      other.update!(name: 'other')
       destination = create(:tag, :general)
       destination.tag_name.update!(name: 'destination_search')
       NicoTagRelation.create!(nico_tag: linked, tag: destination)
@@ -41,27 +65,41 @@ RSpec.describe 'NicoTags', type: :request do
       expect(json.fetch('tags').map { |tag| tag['id'] }).to eq([unlinked.id])
     end
 
+    it 'filters by the qualified legacy name as well as the raw name' do
+      external = create(:external_tag, name: 'qualified_filter')
+      create(:external_tag, name: 'unrelated_filter')
+
+      ['qualified_filter', 'nico:qualified_filter'].each do |name|
+        get '/tags/nico', params: { name: }
+
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch('count')).to eq(1)
+        expect(json.fetch('tags')).to contain_exactly(
+            a_hash_including('id' => external.id, 'name' => 'nico:qualified_filter'))
+      end
+    end
+
     it 'sorts by name and timestamps' do
-      older = create(:tag, :nico)
-      older.tag_name.update!(name: 'nico:a')
+      older = create(:external_tag)
+      older.update!(name: 'ordered_a')
       older.update_columns(created_at: 2.days.ago)
-      newer = create(:tag, :nico)
-      newer.tag_name.update!(name: 'nico:b')
+      newer = create(:external_tag)
+      newer.update!(name: 'ordered_b')
       newer.update_columns(created_at: 1.day.ago)
       older_post_tag =
-        PostTag.create!(post: Post.create!(url: 'https://example.com/nico-older'), tag: older)
+        PostExternalTag.create!(post: create(:post), external_tag: older)
       older_post_tag.update_columns(created_at: 1.hour.ago)
       newer_post_tag =
-        PostTag.create!(post: Post.create!(url: 'https://example.com/nico-newer'), tag: newer)
+        PostExternalTag.create!(post: create(:post), external_tag: newer)
       newer_post_tag.update_columns(created_at: 2.hours.ago)
 
-      get '/tags/nico', params: { order: 'name:desc' }
+      get '/tags/nico', params: { order: 'name:desc', name: 'ordered_' }
       expect(json.fetch('tags').map { |tag| tag['id'] }).to eq([newer.id, older.id])
 
-      get '/tags/nico', params: { order: 'created_at:asc' }
+      get '/tags/nico', params: { order: 'created_at:asc', name: 'ordered_' }
       expect(json.fetch('tags').map { |tag| tag['id'] }).to eq([older.id, newer.id])
 
-      get '/tags/nico', params: { order: 'updated_at:desc' }
+      get '/tags/nico', params: { order: 'updated_at:desc', name: 'ordered_' }
       expect(json.fetch('tags').map { |tag| tag['id'] }).to eq([older.id, newer.id])
       expect(Time.zone.parse(json.fetch('tags').first.fetch('recent_post_tag_created_at')))
         .to be_within(1.second).of(older_post_tag.created_at)
@@ -71,7 +109,7 @@ RSpec.describe 'NicoTags', type: :request do
   describe 'PATCH /tags/nico/:id' do
     let(:member) { create(:user, :member) }
     let(:admin) { create(:user, :admin) }
-    let(:nico_tag) { create(:tag, :nico) }
+    let(:nico_tag) { create(:external_tag) }
 
     it '401 when not logged in' do
       sign_out
@@ -85,18 +123,18 @@ RSpec.describe 'NicoTags', type: :request do
       expect(response).to have_http_status(:forbidden)
     end
 
-    it '400 when target is not nico category' do
+    it '404 when only an internal tag exists for the target id' do
       sign_in_as(member)
       non_nico = create(:tag, :general)
+      expect(ExternalTag.exists?(non_nico.id)).to be(false)
       patch "/tags/nico/#{non_nico.id}", params: { tags: 'a b' }
-      expect(response).to have_http_status(:bad_request)
+      expect(response).to have_http_status(:not_found)
     end
 
     it '200 and updates linked tags while recording tag versions' do
       sign_in_as(admin)
 
-      nico_tag_name = TagName.create!(name: 'nico:nico_tags_spec_source')
-      nico_tag = Tag.create!(tag_name: nico_tag_name, category: :nico)
+      nico_tag = create(:external_tag, name: 'nico_tags_spec_source')
 
       linked_a_name = TagName.create!(name: 'nico_linked_a')
       linked_a = Tag.create!(tag_name: linked_a_name, category: :general)
@@ -104,7 +142,8 @@ RSpec.describe 'NicoTags', type: :request do
       linked_b_name = TagName.create!(name: 'nico_linked_b')
       linked_b = Tag.create!(tag_name: linked_b_name, category: :general)
 
-      TagVersioning.ensure_snapshot!(nico_tag, created_by_user: admin)
+      NicoTagVersionRecorder.record!(external_tag: nico_tag,
+                                     event_type: :create, created_by_user: admin)
 
       expect {
         patch "/tags/nico/#{nico_tag.id}", params: {
@@ -126,26 +165,26 @@ RSpec.describe 'NicoTags', type: :request do
       expect(versions.map(&:event_type)).to eq(['create', 'update'])
       expect(versions.last.linked_tags.split).to match_array([
         'nico_linked_a',
-        'nico_linked_b'
-      ])
+        'nico_linked_b'])
       expect(versions.last.created_by_user_id).to eq(admin.id)
     end
 
-    it 'returns 422 when linked tag normalises to nico tag' do
+    it 'clears existing links and records the empty mapping for a member' do
       sign_in_as(member)
-
-      other_nico = create(:tag, :nico, name: 'nico:linked_ng')
-      TagName.create!(name: 'linked_ng_alias', canonical: other_nico.tag_name)
-
-      TagVersioning.ensure_snapshot!(nico_tag, created_by_user: member)
+      linked = create(:tag)
+      NicoTagRelation.insert_all!([{ nico_tag_id: nico_tag.id, tag_id: linked.id }])
+      NicoTagVersionRecorder.record!(external_tag: nico_tag,
+                                     event_type: :create, created_by_user: member)
 
       expect {
-        patch "/tags/nico/#{nico_tag.id}", params: { tags: 'linked_ng_alias' }
-      }.not_to change(NicoTagVersion, :count)
+        patch "/tags/nico/#{ nico_tag.id }", params: { tags: '' }
+      }.to change(NicoTagVersion, :count).by(1)
 
-      expect(response).to have_http_status(:unprocessable_entity)
-      expect(json.fetch('errors')).to include(
-          'tags' => ['ニコニコ・タグ同士は連携できません．'])
+      expect(response).to have_http_status(:ok)
+      expect(json).to eq([])
+      expect(nico_tag.reload.linked_tags).to be_empty
+      expect(nico_tag.nico_tag_versions.order(:version_no).last)
+          .to have_attributes(linked_tags: '', created_by_user: member)
     end
 
     it 'returns the tags field error when a nico tag is specified directly' do

backend/spec/requests/post_imports_spec.rb

@@ -1,184 +0,0 @@
-require 'rails_helper'
-
-RSpec.describe 'Post imports API', type: :request do
-  let(:member) { create(:user, :member) }
-
-  before do
-    allow(Preview::UrlSafety).to receive(:validate) do |url|
-      [URI.parse(url), ['8.8.8.8']]
-    end
-    allow(PostMetadataFetcher).to receive(:fetch).and_return(
-      title: 'fetched title',
-      thumbnail_base: nil,
-      tags: ''
-    )
-  end
-
-  describe 'POST /posts/import/preview' do
-    it 'requires a member' do
-      sign_out
-      post '/posts/import/preview', params: { source: 'https://example.com/post' }
-      expect(response).to have_http_status(:unauthorized)
-
-      sign_in_as(create(:user, :guest))
-      post '/posts/import/preview', params: { source: 'https://example.com/post' }
-      expect(response).to have_http_status(:forbidden)
-    end
-
-    it 'parses a URL list and returns preview rows' do
-      sign_in_as(member)
-
-      post '/posts/import/preview', params: {
-        source: " https://example.com/one \r\n\r\nhttps://example.com/two"
-      }
-
-      expect(response).to have_http_status(:ok)
-      expect(json.fetch('rows').map { _1.fetch('source_row') }).to eq([1, 3])
-      expect(json.fetch('rows').map { _1.fetch('url') }).to eq([
-        'https://example.com/one',
-        'https://example.com/two'
-      ])
-    end
-
-    it 'returns a safe 400 response for an invalid source' do
-      sign_in_as(member)
-
-      post '/posts/import/preview', params: { source: '' }
-
-      expect(response).to have_http_status(:bad_request)
-      expect(json.fetch('message')).to eq('URL を入力してください．')
-    end
-  end
-
-  describe 'POST /posts/import/validate' do
-    it 'accepts camel-case row properties and returns their warnings' do
-      sign_in_as(member)
-
-      post '/posts/import/validate', params: {
-        rows: [{
-          sourceRow: '1',
-          url: 'https://example.com/post',
-          metadataUrl: 'https://example.com/post',
-          attributes: { title: 'manual title' },
-          provenance: { url: 'manual', title: 'manual' },
-          tagSources: { automatic: '', manual: '' },
-          fieldWarnings: { title: ['old warning'] },
-          baseWarnings: ['base warning']
-        }],
-        changed_row: -1
-      }
-
-      expect(response).to have_http_status(:ok)
-      result = json.fetch('rows').first
-      expect(result.fetch('source_row')).to eq(1)
-      expect(result.fetch('tag_sources')).to eq('automatic' => '', 'manual' => '')
-      expect(result.fetch('field_warnings')).to eq('title' => ['old warning'])
-      expect(result.fetch('base_warnings')).to eq(['base warning'])
-    end
-
-    it 'rejects a non-array rows value with 400' do
-      sign_in_as(member)
-
-      post '/posts/import/validate', params: { rows: { sourceRow: 1 }, changed_row: -1 }
-
-      expect(response).to have_http_status(:bad_request)
-      expect(json.fetch('message')).to eq('取込行の形式が不正です．')
-    end
-
-    it 'returns original created datetime validation errors for minute precision' do
-      sign_in_as(member)
-
-      post '/posts/import/validate', params: {
-        rows: [{
-          sourceRow: 1,
-          url: 'https://example.com/post',
-          metadataUrl: 'https://example.com/post',
-          attributes: {
-            originalCreatedFrom: '2020-01-01T00:00:30Z',
-            originalCreatedBefore: '2020-01-01T00:01Z' },
-          provenance: {
-            url: 'manual',
-            originalCreatedFrom: 'manual',
-            originalCreatedBefore: 'manual' },
-          tagSources: { automatic: '', manual: '' }
-        }],
-        changed_row: -1
-      }
-
-      expect(response).to have_http_status(:ok)
-      expect(json.fetch('rows').first.fetch('validation_errors')).to include(
-        'original_created_from' => ['オリジナルの作成日時は分単位で入力してください．']
-      )
-      expect(json.fetch('rows').first.fetch('validation_errors'))
-          .not_to have_key('original_created_at')
-    end
-  end
-
-  describe 'POST /posts/import' do
-    it 'keeps the duration string contract through preview, validate, and import' do
-      sign_in_as(member)
-      allow(PostMetadataFetcher).to receive(:fetch).and_return(
-        title: 'fetched title',
-        thumbnail_base: nil,
-        duration: '2.5',
-        tags: '動画'
-      )
-
-      post '/posts/import/preview', params: {
-        source: 'https://example.com/video'
-      }
-      preview_row = json.fetch('rows').first
-      expect(preview_row.dig('attributes', 'duration')).to eq('2.5')
-
-      post '/posts/import/validate', params: {
-        rows: [{
-          sourceRow: preview_row.fetch('source_row'),
-          url: preview_row.fetch('url'),
-          attributes: preview_row.fetch('attributes'),
-          provenance: preview_row.fetch('provenance'),
-          tagSources: preview_row.fetch('tag_sources'),
-          metadataUrl: preview_row.fetch('metadata_url')
-        }],
-        changed_row: -1
-      }
-      validated_row = json.fetch('rows').first
-      expect(validated_row.dig('attributes', 'duration')).to eq('2.5')
-
-      post '/posts/import', params: {
-        rows: [{
-          sourceRow: validated_row.fetch('source_row'),
-          url: validated_row.fetch('url'),
-          attributes: validated_row.fetch('attributes'),
-          provenance: validated_row.fetch('provenance'),
-          tagSources: validated_row.fetch('tag_sources'),
-          metadataUrl: validated_row.fetch('metadata_url')
-        }]
-      }
-
-      expect(response).to have_http_status(:ok)
-      expect(Post.order(:id).last.video_ms).to eq(2_500)
-    end
-
-    it 'returns a formal skipped result for an existing post' do
-      existing = create(:post, url: 'https://example.com/existing')
-      sign_in_as(member)
-
-      post '/posts/import', params: {
-        rows: [{
-          sourceRow: 1,
-          url: existing.url,
-          attributes: { title: 'ignored' },
-          provenance: { url: 'manual', title: 'manual' },
-          tagSources: { automatic: '', manual: '' }
-        }]
-      }
-
-      expect(response).to have_http_status(:ok)
-      expect(json).to include('created' => 0, 'skipped' => 1, 'failed' => 0)
-      expect(json.fetch('rows').first).to include(
-        'status' => 'skipped',
-        'existing_post_id' => existing.id
-      )
-    end
-  end
-end

backend/spec/requests/posts_spec.rb

@@ -9,20 +9,19 @@ RSpec.describe 'Posts API', type: :request do
   # resized_thumbnail! が MiniMagick 依存でコケやすいので request spec ではスタブしとくのが無難。
   before do
     allow_any_instance_of(Post).to receive(:resized_thumbnail!).and_return(true)
-    allow(Post).to receive(:resized_thumbnail_attachment).and_return(
-      io: StringIO.new('dummy'),
-      filename: 'resized_thumbnail.jpg',
-      content_type: 'image/jpeg'
-    )
+    allow(Post).to receive(:resized_thumbnail_attachment) do
+      { io: StringIO.new('dummy'),
+        filename: 'resized_thumbnail.jpg',
+        content_type: 'image/jpeg' }
+    end
   end
 
   def create_nico_tag!(name)
-    Tag.find_or_create_by_tag_name!(name, category: :nico)
+    ExternalTag.find_or_create_by!(platform: :nico, name: name.delete_prefix('nico:'))
   end
 
   def dummy_upload
-    # 中身は何でもいい（加工処理はスタブしてる）
-    Rack::Test::UploadedFile.new(StringIO.new('dummy'), 'image/jpeg', original_filename: 'dummy.jpg')
+    real_thumbnail_upload
   end
 
   def real_thumbnail_upload
@@ -56,6 +55,7 @@ RSpec.describe 'Posts API', type: :request do
             thumbnail_base: post.thumbnail_base,
             video_ms: post.video_ms,
             tags: post.snapshot_tag_names.join(' '),
+            tags_json: post.snapshot_tags_json,
             parent_post_ids: post.snapshot_parent_post_ids.join(' '),
             original_created_from: post.original_created_from,
             original_created_before: post.original_created_before,
@@ -93,6 +93,26 @@ RSpec.describe 'Posts API', type: :request do
     count
   end
 
+  def expect_external_tag_json tag_json, external_tag
+    external_tag.reload
+
+    expect(tag_json).to include(
+        'id' => external_tag.id,
+        'name' => "#{ external_tag.platform }:#{ external_tag.name }",
+        'category' => 'nico',
+        'created_at' => external_tag.created_at.as_json,
+        'updated_at' => external_tag.created_at.as_json,
+        'deprecated_at' => nil,
+        'aliases' => [],
+        'parents' => [],
+        'post_count' => external_tag.post_count,
+        'has_wiki' => false,
+        'material_id' => nil,
+        'has_deerjikists' => false,
+        'children' => [],
+        'sections' => [])
+  end
+
   let!(:tag_name) { TagName.create!(name: 'spec_tag') }
   let!(:tag)      { Tag.create!(tag_name: tag_name, category: :general) }
 
@@ -102,34 +122,34 @@ RSpec.describe 'Posts API', type: :request do
     end
   end
 
-  describe "GET /posts" do
+  describe 'GET /posts' do
     let!(:user) { create_member_user! }
 
-    let!(:tag_name) { TagName.create!(name: "spec_tag") }
+    let!(:tag_name) { TagName.create!(name: 'spec_tag') }
     let!(:tag) { Tag.create!(tag_name:, category: :general) }
     let!(:tag_name2) { TagName.create!(name: 'unko') }
     let!(:tag2) { Tag.create!(tag_name: tag_name2, category: :deerjikist) }
     let!(:alias_tag_name) { TagName.create!(name: 'manko', canonical: tag_name) }
 
     let!(:hit_post) do
-      Post.create!(uploaded_user: user, title: "hello spec world",
+      Post.create!(uploaded_user: user, title: 'hello spec world',
                    url: 'https://example.com/spec2').tap do |p|
         PostTag.create!(post: p, tag:)
       end
     end
 
     let!(:miss_post) do
-      Post.create!(uploaded_user: user, title: "unrelated title",
+      Post.create!(uploaded_user: user, title: 'unrelated title',
                    url: 'https://example.com/spec3').tap do |p|
         PostTag.create!(post: p, tag: tag2)
       end
     end
 
-    it "returns posts with tag name in JSON" do
-      get "/posts"
+    it 'returns posts with tag name in JSON' do
+      get '/posts'
 
       expect(response).to have_http_status(:ok)
-      posts = json.fetch("posts")
+      posts = json.fetch('posts')
 
       # 全postの全tagが name を含むこと
       expect(posts).not_to be_empty
@@ -142,18 +162,21 @@ RSpec.describe 'Posts API', type: :request do
       expect(json['count']).to be_an(Integer)
 
       # spec_tag を含む投稿が存在すること
-      all_tag_names = posts.flat_map { |p| p["tags"].map { |t| t["name"] } }
-      expect(all_tag_names).to include("spec_tag")
+      all_tag_names = posts.flat_map { |p| p['tags'].map { |t| t['name'] } }
+      expect(all_tag_names).to include('spec_tag')
     end
 
     it 'keeps children and sections keys in non-detail tag responses' do
       PostTagSection.create!(post: hit_post, tag:, begin_ms: 1_000, end_ms: nil)
+      deprecated_tag = create(:tag, deprecated_at: Time.current)
+      create(:post_tag, post: hit_post, tag: deprecated_tag)
 
       get '/posts'
 
       expect(response).to have_http_status(:ok)
 
       hit_json = json.fetch('posts').find { |post| post['id'] == hit_post.id }
+      expect(hit_json.fetch('tags').map { |item| item.fetch('id') }).to eq([tag.id])
       tag_json = hit_json.fetch('tags').find { |item| item['name'] == 'spec_tag' }
 
       expect(tag_json.fetch('children')).to eq([])
@@ -162,9 +185,29 @@ RSpec.describe 'Posts API', type: :request do
       ])
     end
 
-    context "when q is provided" do
-      it "filters posts by q (hit case)" do
-        get "/posts", params: { tags: "spec_tag" }
+    it 'preloads tag details and sections as the number of posts grows' do
+      5.times do
+        link = create(:post_tag, post: create(:post, uploaded_user: user))
+        create(:post_tag_section, post: link.post, tag: link.tag,
+                                  begin_ms: 1000, end_ms: 2000)
+      end
+      get '/posts', params: { limit: 1 }
+
+      one_post_queries = count_sql_queries do
+        get '/posts', params: { limit: 1 }
+      end
+      many_post_queries = count_sql_queries do
+        get '/posts', params: { limit: 20 }
+      end
+
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('posts').size).to eq(8)
+      expect(many_post_queries).to be <= one_post_queries
+    end
+
+    context 'when q is provided' do
+      it 'filters posts by q (hit case)' do
+        get '/posts', params: { tags: 'spec_tag' }
 
         expect(response).to have_http_status(:ok)
         posts = json.fetch('posts')
@@ -182,8 +225,8 @@ RSpec.describe 'Posts API', type: :request do
         end
       end
 
-      it "filters posts by q (hit case by alias)" do
-        get "/posts", params: { tags: "manko" }
+      it 'filters posts by q (hit case by alias)' do
+        get '/posts', params: { tags: 'manko' }
 
         expect(response).to have_http_status(:ok)
         posts = json.fetch('posts')
@@ -201,15 +244,102 @@ RSpec.describe 'Posts API', type: :request do
         end
       end
 
-      it "returns empty posts when nothing matches" do
-        get "/posts", params: { tags: "no_such_keyword_12345" }
+      it 'returns empty posts when nothing matches' do
+        get '/posts', params: { tags: 'no_such_keyword_12345' }
 
         expect(response).to have_http_status(:ok)
-        expect(json.fetch("posts")).to eq([])
+        expect(json.fetch('posts')).to eq([])
         expect(json.fetch('count')).to eq(0)
       end
     end
 
+    context 'with legacy external tag name searches' do
+      let!(:external_tag) { create(:external_tag, id: tag.id, name: 'search_external') }
+      let!(:both_post) do
+        create(:post).tap do |post|
+          PostTag.create!(post:, tag:)
+          PostExternalTag.create!(post:, external_tag:)
+        end
+      end
+
+      before do
+        PostExternalTag.create!(post: miss_post, external_tag:)
+      end
+
+      it 'keeps internal name searches independent of colliding external ids' do
+        get '/posts', params: { tags: tag.name }
+
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch('count')).to eq(3)
+        expect(json.fetch('posts').map { _1.fetch('id') })
+            .to contain_exactly(post_record.id, hit_post.id, both_post.id)
+      end
+
+      it 'finds posts through PostExternalTag by the qualified legacy name' do
+        get '/posts', params: { tags: 'nico:search_external' }
+
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch('count')).to eq(2)
+        expect(json.fetch('posts').map { _1.fetch('id') })
+            .to contain_exactly(miss_post.id, both_post.id)
+      end
+
+      [nil, 'all'].each do |match|
+        it "intersects internal and external matches with match=#{ match || 'omitted' }" do
+          params = { tags: "#{ tag.name } nico:search_external" }
+          params[:match] = match if match
+
+          get '/posts', params: params
+
+          expect(response).to have_http_status(:ok)
+          expect(json.fetch('count')).to eq(1)
+          expect(json.fetch('posts').map { _1.fetch('id') }).to eq([both_post.id])
+        end
+      end
+
+      it 'unions internal alias and external matches without duplicate posts' do
+        get '/posts', params: { tags: 'manko nico:search_external', match: 'any' }
+
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch('count')).to eq(4)
+        expect(json.fetch('posts').map { _1.fetch('id') })
+            .to contain_exactly(post_record.id, hit_post.id, miss_post.id, both_post.id)
+      end
+
+      it 'excludes external matches from an internal tag search' do
+        get '/posts', params: { tags: "#{ tag.name } not:nico:search_external" }
+
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch('count')).to eq(2)
+        expect(json.fetch('posts').map { _1.fetch('id') })
+            .to contain_exactly(post_record.id, hit_post.id)
+      end
+
+      it 'unions an external match with a negated internal match' do
+        get '/posts', params: { tags: "nico:search_external not:#{ tag.name }", match: 'any' }
+
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch('count')).to eq(2)
+        expect(json.fetch('posts').map { _1.fetch('id') })
+            .to contain_exactly(miss_post.id, both_post.id)
+      end
+
+      it 'keeps a missing qualified external name empty' do
+        get '/posts', params: { tags: 'nico:missing_search_external' }
+
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch('count')).to eq(0)
+        expect(json.fetch('posts')).to be_empty
+      end
+
+      it 'applies the same mixed name search to the existing random endpoint' do
+        get '/posts/random', params: { tags: "#{ tag.name } nico:search_external", match: 'all' }
+
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch('id')).to eq(both_post.id)
+      end
+    end
+
     context 'when tags contain not:' do
       let!(:foo_tag_name) { TagName.create!(name: 'not_spec_foo') }
       let!(:foo_tag) { Tag.create!(tag_name: foo_tag_name, category: :general) }
@@ -460,6 +590,71 @@ RSpec.describe 'Posts API', type: :request do
       end
     end
 
+    context 'when update times include version history' do
+      let(:t0) { Time.zone.parse('2020-01-01 12:00:00') }
+      let(:t1) { t0 + 1.day }
+      let(:t2) { t0 + 2.days }
+      let(:t3) { t0 + 3.days }
+      let!(:history_post) do
+        create(:post, url: 'https://example.com/version-time/history',
+                      created_at: t0, updated_at: t0)
+      end
+      let!(:plain_post) do
+        create(:post, url: 'https://example.com/version-time/plain',
+                      created_at: t1, updated_at: t1)
+      end
+      let!(:newer_post) do
+        create(:post, url: 'https://example.com/version-time/newer',
+                      created_at: t0, updated_at: t3)
+      end
+
+      before do
+        link = create(:post_tag, post: history_post, tag:)
+        travel_to(t0) do
+          PostVersionRecorder.record!(post: history_post,
+                                      event_type: :create, created_by_user: nil)
+          PostVersionRecorder.record!(post: newer_post,
+                                      event_type: :create, created_by_user: nil)
+        end
+        travel_to(t2) do
+          link.destroy!
+          PostVersionRecorder.record!(post: history_post,
+                                      event_type: :update, created_by_user: nil)
+        end
+        create(:post_tag, post: plain_post, tag:, created_at: t3)
+      end
+
+      ['asc', 'desc'].each do |direction|
+        it "sorts by the later of post update and latest version time (#{ direction })" do
+          get '/posts', params: { url: '/version-time/', order: "updated_at:#{ direction }" }
+
+          expect(response).to have_http_status(:ok)
+          expected_ids = [plain_post.id, history_post.id, newer_post.id]
+          expected_ids.reverse! if direction == 'desc'
+          expect(json.fetch('posts').map { |item| item.fetch('id') }).to eq(expected_ids)
+          expect(json.fetch('count')).to eq(3)
+
+          times = json.fetch('posts').to_h do |item|
+            [item.fetch('id'), Time.zone.parse(item.fetch('updated_at'))]
+          end
+          expect(times).to eq({ plain_post.id => t1,
+                               history_post.id => t2,
+                               newer_post.id => t3 })
+          expect(history_post.reload.updated_at).to eq(t0)
+        end
+      end
+
+      it 'filters inclusively by the latest version time after a tag is deleted' do
+        get '/posts', params: { url: '/version-time/',
+                               updated_from: t2.iso8601,
+                               updated_to: t2.iso8601 }
+
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch('posts').map { |item| item.fetch('id') }).to eq([history_post.id])
+        expect(json.fetch('count')).to eq(1)
+      end
+    end
+
     context 'when original_created_from/original_created_to are provided' do
       # 注意: controller の現状ロジックに合わせてる
       # original_created_from は `original_created_before > ?`
@@ -524,6 +719,30 @@ RSpec.describe 'Posts API', type: :request do
         expect(json.fetch('count')).to eq(2)
       end
     end
+
+    it 'returns internal and external tags with colliding ids in the legacy tags array' do
+      external_tag = create(:external_tag, id: tag.id, name: 'post_index_external')
+      PostExternalTag.create!(post: hit_post, external_tag:)
+
+      get '/posts'
+
+      expect(response).to have_http_status(:ok)
+
+      post_json =
+          json
+          .fetch('posts')
+          .find { _1.fetch('id') == hit_post.id }
+
+      external_json =
+          post_json
+          .fetch('tags')
+          .find { _1['name'] == 'nico:post_index_external' }
+
+      expect(post_json.fetch('tags')).to include(
+          a_hash_including('id' => tag.id, 'name' => tag.name, 'category' => 'general'))
+      expect(external_json).not_to be_nil
+      expect_external_tag_json(external_json, external_tag)
+    end
   end
 
   describe 'GET /posts/:id' do
@@ -681,6 +900,25 @@ RSpec.describe 'Posts API', type: :request do
         expect(response).to have_http_status(:ok)
         expect(query_count).to be <= 45
       end
+
+      it 'returns external tags as root nodes in the legacy tag tree' do
+        external_tag = create(:external_tag, id: tag.id, name: 'post_detail_external')
+        PostExternalTag.create!(post: post_record, external_tag:)
+
+        request
+
+        expect(response).to have_http_status(:ok)
+
+        expect(json.fetch('tags')).to include(
+            a_hash_including('id' => tag.id, 'name' => tag.name, 'category' => 'general'))
+        external_json =
+            json
+            .fetch('tags')
+            .find { _1['name'] == 'nico:post_detail_external' }
+
+        expect(external_json).not_to be_nil
+        expect_external_tag_json(external_json, external_tag)
+      end
     end
 
     context 'when post does not exist' do
@@ -693,6 +931,73 @@ RSpec.describe 'Posts API', type: :request do
     end
   end
 
+  describe 'GET /posts/metadata' do
+    let(:member) { create(:user, :member) }
+
+    it 'returns compact existing post data without fetching external metadata' do
+      sign_in_as(member)
+      existing = create(
+          :post,
+          title: 'existing post',
+          url: 'https://example.com/existing')
+      existing.thumbnail.attach(
+          io: StringIO.new('thumbnail'),
+          filename: 'thumbnail.jpg',
+          content_type: 'image/jpeg')
+      expect(PostMetadataFetcher).not_to receive(:fetch)
+
+      get '/posts/metadata', params: { url: 'https://example.com/existing' }
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include(
+          'url' => existing.url,
+          'existing_post_id' => existing.id,
+          'field_warnings' => { })
+      expect(json.fetch('existing_post')).to include(
+          'id' => existing.id,
+          'title' => existing.title,
+          'url' => existing.url,
+          'thumbnail_base' => existing.thumbnail_base)
+      expect(json.dig('existing_post', 'thumbnail'))
+          .to include('/rails/active_storage/blobs/proxy/')
+    end
+
+    it 'returns fetched metadata and structured display tags' do
+      sign_in_as(member)
+      allow(Preview::UrlSafety).to receive(:validate)
+      allow(PostMetadataFetcher).to receive(:fetch).and_return(
+          title: 'fetched title',
+          thumbnail_base: 'https://example.com/thumbnail.jpg',
+          tags: 'character:虹夏',
+          display_tags: [{ name: '虹夏', category: 'character' }],
+          original_created_from: nil,
+          original_created_before: nil,
+          duration: '1:00',
+          video_ms: 60_000)
+
+      get '/posts/metadata', params: { url: 'https://example.com/new' }
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include(
+          'url' => 'https://example.com/new',
+          'title' => 'fetched title',
+          'duration' => '1:00',
+          'video_ms' => 60_000,
+          'field_warnings' => { })
+      expect(json.fetch('display_tags')).to eq(
+          [{ 'name' => '虹夏', 'category' => 'character' }])
+    end
+
+    it 'returns URL validation errors as 422' do
+      sign_in_as(member)
+
+      get '/posts/metadata', params: { url: 'file:///etc/passwd' }
+
+      expect(response).to have_http_status(:unprocessable_entity)
+      expect(json.fetch('errors')).to have_key('url')
+    end
+  end
+
   describe 'POST /posts' do
     let(:member) { create(:user, :member) }
     let!(:alias_tag_name) { TagName.create!(name: 'manko', canonical: tag_name) }
@@ -712,6 +1017,46 @@ RSpec.describe 'Posts API', type: :request do
       expect(response).to have_http_status(:forbidden)
     end
 
+    it 'dry-runs without persisting posts or new tags' do
+      sign_in_as(member)
+      counts = [Post.count, Tag.count, TagName.count]
+
+      post '/posts?dry=1', params: post_write_params(
+          title: 'dry-run post',
+          url: 'https://example.com/dry-run',
+          tags: 'character:new_dry_run_tag')
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include(
+          'url' => 'https://example.com/dry-run',
+          'tags' => 'new_dry_run_tag',
+          'existing_post_id' => nil)
+      expect(json.fetch('display_tags')).to eq(
+          [{ 'name' => 'new_dry_run_tag',
+             'category' => 'character',
+             'section_literals' => [] }])
+      expect([Post.count, Tag.count, TagName.count]).to eq(counts)
+    end
+
+    it 'dry-runs an existing URL without validating its upload' do
+      sign_in_as(member)
+      existing = create(:post, url: 'https://example.com/dry-existing')
+      invalid_upload = Rack::Test::UploadedFile.new(
+          StringIO.new('<svg></svg>'),
+          'image/png',
+          original_filename: 'thumbnail.png')
+
+      post '/posts?dry=1', params: post_write_params(
+          title: '',
+          url: existing.url,
+          tags: '',
+          thumbnail: invalid_upload)
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include('existing_post_id' => existing.id)
+      expect(json.fetch('field_warnings')).not_to have_key('thumbnail_base')
+    end
+
     it '201 and creates post + tags when member' do
       sign_in_as(member)
 
@@ -769,7 +1114,8 @@ RSpec.describe 'Posts API', type: :request do
       )
 
       expect(response).to have_http_status(:created)
-      expect(open_transactions).to eq([baseline_open_transactions])
+      expect(open_transactions).to eq(
+          [baseline_open_transactions, baseline_open_transactions])
     end
 
     it 'returns 422 and does not create a post when thumbnail resize fails' do
@@ -827,7 +1173,7 @@ RSpec.describe 'Posts API', type: :request do
 
       expect(response).to have_http_status(:unprocessable_entity)
       expect(json.fetch('errors')).to include(
-        'tags' => ['廃止済みタグは付与できません．']
+        'tags' => ['廃止済みタグがあります: deprecated_direct_tag']
       )
     end
 
@@ -1054,11 +1400,9 @@ RSpec.describe 'Posts API', type: :request do
       )
     end
 
-    context "when nico tag already exists in tags" do
+    context 'when the external nico tag already exists' do
       before do
-        Tag.find_undiscard_or_create_by!(
-            tag_name: TagName.find_undiscard_or_create_by!(name: 'nico:nico_tag'),
-            category: :nico)
+        create(:external_tag, name: 'nico_tag')
       end
 
       it 'returns 422 with tag field errors' do
@@ -1251,6 +1595,47 @@ RSpec.describe 'Posts API', type: :request do
     end
   end
 
+  describe 'POST /posts/bulk' do
+    let(:member) { create(:user, :member) }
+
+    it 'parses the manifest and indexed thumbnail parts' do
+      sign_in_as(member)
+      manifest = [{ 'url' => 'https://example.com/bulk',
+                    'title' => 'bulk post',
+                    'tags' => 'spec_tag',
+                    'parent_post_ids' => '' }]
+      creator = instance_double(
+          PostBulkCreator,
+          run: { results: [{ status: 'created', post: { id: 123 } }] })
+      allow(PostBulkCreator).to receive(:new).and_return(creator)
+
+      post '/posts/bulk', params: {
+          posts: JSON.generate(manifest),
+          thumbnails: { '0' => real_thumbnail_upload } }
+
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('results')).to eq(
+          [{ 'status' => 'created', 'post' => { 'id' => 123 } }])
+      expect(PostBulkCreator).to have_received(:new) do |arguments|
+        expect(arguments[:actor]).to eq(member)
+        expect(arguments[:posts]).to eq(manifest)
+        expect(arguments[:thumbnails].keys).to eq([0])
+        expect(arguments[:host]).to eq('http://www.example.com')
+      end
+    end
+
+    it 'rejects malformed manifests as a request-level error' do
+      sign_in_as(member)
+
+      post '/posts/bulk', params: {
+          posts: '{',
+          thumbnails: { '0' => real_thumbnail_upload } }
+
+      expect(response).to have_http_status(:bad_request)
+      expect(json.fetch('message')).to eq('posts manifest の JSON が不正です．')
+    end
+  end
+
   describe 'PUT /posts/:id' do
     let(:member) { create(:user, :member) }
 
@@ -1270,9 +1655,11 @@ RSpec.describe 'Posts API', type: :request do
 
     it '200 and updates title + resync tags when member' do
       sign_in_as(member)
+      create(:post_tag_section, post: post_record, tag:,
+                                begin_ms: 1000, end_ms: 2000)
 
       tn2 = TagName.create!(name: 'spec_tag_2')
-      Tag.create!(tag_name: tn2, category: :general)
+      replacement_tag = Tag.create!(tag_name: tn2, category: :general)
 
       put "/posts/#{post_record.id}", params: post_update_params(
           post_record,
@@ -1285,6 +1672,38 @@ RSpec.describe 'Posts API', type: :request do
 
       names = json['tags'].map { |n| n['name'] }
       expect(names).to include('spec_tag_2')
+      expect(names).not_to include('spec_tag')
+      expect(PostTag.exists?(post: post_record, tag:)).to be(false)
+      expect(PostTagSection.exists?(post: post_record, tag:)).to be(false)
+      expect(tag.reload.post_count).to eq(0)
+      expect(replacement_tag.reload.post_count).to eq(1)
+
+      versions = post_record.post_versions.order(:version_no)
+      expect(versions.first.tags_json).to include(
+          a_hash_including('tag_id' => tag.id,
+                           'sections' => [{ 'begin_ms' => 1000, 'end_ms' => 2000 }]))
+      expect(versions.last.tags_json.map { |item| item.fetch('tag_id') })
+          .not_to include(tag.id)
+    end
+
+    it 'can add a removed tag again and records both changes' do
+      sign_in_as(member)
+
+      put "/posts/#{ post_record.id }", params: post_update_params(post_record, tags: '')
+      expect(response).to have_http_status(:ok)
+      expect(PostTag.exists?(post: post_record, tag:)).to be(false)
+
+      put "/posts/#{ post_record.id }", params: post_update_params(
+          post_record, tags: 'spec_tag')
+
+      expect(response).to have_http_status(:ok)
+      expect(PostTag.where(post: post_record, tag:).count).to eq(1)
+      expect(PostTag.find_by!(post: post_record, tag:).created_user).to eq(member)
+      expect(tag.reload.post_count).to eq(1)
+      snapshots = post_record.post_versions.order(:version_no).map do |version|
+        version.tags_json.map { |item| item.fetch('tag_id') }
+      end
+      expect(snapshots.map { |ids| ids.include?(tag.id) }).to eq([true, false, true])
     end
 
     it 'rejects a deprecated tag specified directly' do
@@ -1307,11 +1726,9 @@ RSpec.describe 'Posts API', type: :request do
       )
     end
 
-    context "when nico tag already exists in tags" do
+    context 'when the external nico tag already exists' do
       before do
-        Tag.find_undiscard_or_create_by!(
-            tag_name: TagName.find_undiscard_or_create_by!(name: 'nico:nico_tag'),
-            category: :nico)
+        create(:external_tag, name: 'nico_tag')
       end
 
       it 'returns 422 with tag field errors' do
@@ -1564,7 +1981,7 @@ RSpec.describe 'Posts API', type: :request do
         expect(post_record.reload.title).to eq('updated by other user')
       end
 
-      it 'returns 409 with mergeable true when stale tag changes do not conflict but merge is not requested' do
+      it 'returns mergeable 409 for stale non-conflicting tag changes without merge' do
         sign_in_as(member)
 
         base_version = create_post_version_for!(post_record.reload)
@@ -1628,7 +2045,7 @@ RSpec.describe 'Posts API', type: :request do
         base_version = create_post_version_for!(post_record.reload)
 
         nico_tag = create_nico_tag!('nico:optimistic_lock_nico')
-        PostTag.create!(post: post_record, tag: nico_tag, created_user: member)
+        PostExternalTag.create!(post: post_record, external_tag: nico_tag)
 
         PostVersionRecorder.record!(
             post: post_record.reload,
@@ -1648,14 +2065,14 @@ RSpec.describe 'Posts API', type: :request do
 
         expect(names).to include('spec_tag')
         expect(names).to include(Tag.no_deerjikist.name)
-        expect(names).to include(nico_tag.name)
+        expect(post_record.external_tags).to contain_exactly(nico_tag)
       end
 
       it 'keeps nico tags even when they are not included in PUT tags' do
         sign_in_as(member)
 
         nico_tag = create_nico_tag!('nico:readonly_update_nico')
-        PostTag.create!(post: post_record, tag: nico_tag, created_user: member)
+        PostExternalTag.create!(post: post_record, external_tag: nico_tag)
 
         base_version = create_post_version_for!(post_record.reload)
 
@@ -1670,7 +2087,15 @@ RSpec.describe 'Posts API', type: :request do
 
         expect(names).to include('spec_tag')
         expect(names).to include(Tag.no_deerjikist.name)
-        expect(names).to include(nico_tag.name)
+        expect(post_record.external_tags).to contain_exactly(nico_tag)
+
+        external_json =
+            json
+            .fetch('tags')
+            .find { _1['name'] == "nico:#{ nico_tag.name }" }
+
+        expect(external_json).not_to be_nil
+        expect_external_tag_json(external_json, nico_tag)
       end
 
       it 'allows non-nico tags linked from nico tags to be removed by normal post update' do
@@ -1680,7 +2105,7 @@ RSpec.describe 'Posts API', type: :request do
         linked_tag = Tag.find_or_create_by_tag_name!('relation_linked_tag', category: :general)
 
         NicoTagRelation.create!(nico_tag:, tag: linked_tag)
-        PostTag.create!(post: post_record, tag: nico_tag, created_user: member)
+        PostExternalTag.create!(post: post_record, external_tag: nico_tag)
         PostTag.create!(post: post_record, tag: linked_tag, created_user: member)
 
         base_version = create_post_version_for!(post_record.reload)
@@ -1694,7 +2119,7 @@ RSpec.describe 'Posts API', type: :request do
 
         names = post_record.reload.tags.map(&:name)
 
-        expect(names).to include(nico_tag.name)
+        expect(post_record.external_tags).to contain_exactly(nico_tag)
         expect(names).to include('spec_tag')
         expect(names).to include(Tag.no_deerjikist.name)
         expect(names).not_to include(linked_tag.name)
@@ -1730,123 +2155,29 @@ RSpec.describe 'Posts API', type: :request do
       expect(response).to have_http_status(:not_found)
     end
 
-    it '200 and returns viewed boolean' do
+    it 'returns viewed state and current tags with their sections' do
+      create(:post_tag_section, post: post_record, tag:,
+                                begin_ms: 1000, end_ms: nil)
+      deprecated_tag = create(:tag, deprecated_at: Time.current)
+      create(:post_tag, post: post_record, tag: deprecated_tag)
+
       get '/posts/random'
+
       expect(response).to have_http_status(:ok)
       expect(json).to have_key('viewed')
       expect([true, false]).to include(json['viewed'])
+      expect(json.fetch('tags')).to contain_exactly(
+          a_hash_including('id' => tag.id,
+                           'children' => [],
+                           'sections' => [{ 'begin_ms' => 1000, 'end_ms' => nil }]))
     end
   end
 
   describe 'GET /posts/changes' do
-    let(:member) { create(:user, :member) }
+    it 'returns 404 for the retired history endpoint' do
+      get '/posts/changes'
 
-    it 'returns add/remove events (history) for a post' do
-      # add
-      tn2 = TagName.create!(name: 'spec_tag2')
-      tag2 = Tag.create!(tag_name: tn2, category: :general)
-      pt = PostTag.create!(post: post_record, tag: tag2, created_user: member)
-
-      # remove (discard)
-      pt.discard_by!(member)
-
-      get '/posts/changes', params: { id: post_record.id }
-
-      expect(response).to have_http_status(:ok)
-      expect(json).to include('changes', 'count')
-      expect(json['changes']).to be_an(Array)
-      expect(json['count']).to be >= 2
-
-      types = json['changes'].map { |e| e['change_type'] }.uniq
-      expect(types).to include('add')
-      expect(types).to include('remove')
-    end
-
-    it 'filters history by tag' do
-      tn2 = TagName.create!(name: 'history_tag_hit')
-      tag2 = Tag.create!(tag_name: tn2, category: :general)
-
-      tn3 = TagName.create!(name: 'history_tag_miss')
-      tag3 = Tag.create!(tag_name: tn3, category: :general)
-
-      other_post = Post.create!(
-        title: 'other post',
-        url: 'https://example.com/history-other'
-      )
-
-      # hit: add
-      PostTag.create!(post: post_record, tag: tag2, created_user: member)
-
-      # hit: add + remove
-      pt2 = PostTag.create!(post: other_post, tag: tag2, created_user: member)
-      pt2.discard_by!(member)
-
-      # miss: add + remove
-      pt3 = PostTag.create!(post: post_record, tag: tag3, created_user: member)
-      pt3.discard_by!(member)
-
-      get '/posts/changes', params: { tag: tag2.id }
-
-      expect(response).to have_http_status(:ok)
-      expect(json).to include('changes', 'count')
-      expect(json['count']).to eq(3)
-
-      changes = json.fetch('changes')
-
-      expect(changes.map { |e| e.dig('tag', 'id') }.uniq).to eq([tag2.id])
-      expect(changes.map { |e| e['change_type'] }).to match_array(%w[add add remove])
-      expect(changes.map { |e| e.dig('post', 'id') }).to match_array([
-        post_record.id,
-        other_post.id,
-        other_post.id
-      ])
-    end
-
-    it 'filters history by post and tag together' do
-      tn2 = TagName.create!(name: 'history_tag_combo_hit')
-      tag2 = Tag.create!(tag_name: tn2, category: :general)
-
-      tn3 = TagName.create!(name: 'history_tag_combo_miss')
-      tag3 = Tag.create!(tag_name: tn3, category: :general)
-
-      other_post = Post.create!(
-        title: 'other combo post',
-        url: 'https://example.com/history-combo-other'
-      )
-
-      # hit
-      PostTag.create!(post: post_record, tag: tag2, created_user: member)
-
-      # miss by post
-      pt2 = PostTag.create!(post: other_post, tag: tag2, created_user: member)
-      pt2.discard_by!(member)
-
-      # miss by tag
-      pt3 = PostTag.create!(post: post_record, tag: tag3, created_user: member)
-      pt3.discard_by!(member)
-
-      get '/posts/changes', params: { id: post_record.id, tag: tag2.id }
-
-      expect(response).to have_http_status(:ok)
-      expect(json).to include('changes', 'count')
-      expect(json['count']).to eq(1)
-
-      changes = json.fetch('changes')
-      expect(changes.size).to eq(1)
-      expect(changes[0]['change_type']).to eq('add')
-      expect(changes[0].dig('post', 'id')).to eq(post_record.id)
-      expect(changes[0].dig('tag', 'id')).to eq(tag2.id)
-    end
-
-    it 'returns empty history when tag does not match' do
-      tn2 = TagName.create!(name: 'history_tag_no_hit')
-      tag2 = Tag.create!(tag_name: tn2, category: :general)
-
-      get '/posts/changes', params: { tag: tag2.id }
-
-      expect(response).to have_http_status(:ok)
-      expect(json.fetch('changes')).to eq([])
-      expect(json.fetch('count')).to eq(0)
+      expect(response).to have_http_status(:not_found)
     end
   end
 
@@ -1876,6 +2207,7 @@ RSpec.describe 'Posts API', type: :request do
         url: post.url,
         thumbnail_base: post.thumbnail_base,
         tags: snapshot_tags(post),
+        tags_json: post.snapshot_tags_json,
         parent_post_ids: post.snapshot_parent_post_ids.join(' '),
         original_created_from: post.original_created_from,
         original_created_before: post.original_created_before,
@@ -1897,7 +2229,7 @@ RSpec.describe 'Posts API', type: :request do
     end
 
     let!(:v2) do
-      post_record.post_tags.kept.find_by!(tag: tag).discard_by!(member)
+      post_record.post_tags.find_by!(tag: tag).destroy!
       PostTag.create!(post: post_record, tag: tag2, created_user: member)
       post_record.update!(
         title: 'updated spec post',
@@ -2006,6 +2338,88 @@ RSpec.describe 'Posts API', type: :request do
       expect(first.fetch('created_at')).to eq(t_v1.iso8601)
     end
 
+    context 'with external tag history' do
+      let(:external_id) { tag.id }
+      let(:external) { create(:external_tag, id: external_id) }
+      let(:external_post) { create(:post) }
+
+      before do
+        PostExternalTag.create!(post: external_post, external_tag: external)
+        PostVersionRecorder.record!(
+            post: external_post, event_type: :create, created_by_user: member)
+
+        unrelated_post = create(:post)
+        PostExternalTag.create!(post: unrelated_post, external_tag: create(:external_tag))
+        PostVersionRecorder.record!(
+            post: unrelated_post, event_type: :create, created_by_user: member)
+      end
+
+      it 'prefers Tag over ExternalTag for the legacy tag parameter' do
+        get '/posts/versions', params: { tag: tag.id }
+
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch('count')).to eq(3)
+        expect(json.fetch('versions').map { [_1.fetch('post_id'), _1.fetch('version_no')] })
+            .to contain_exactly(
+                [post_record.id, 1], [post_record.id, 2], [other_post_version.post_id, 1])
+      end
+
+      it 'explicitly filters ExternalTag even when its id collides with Tag' do
+        get '/posts/versions', params: { external_tag: external.id }
+
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch('count')).to eq(1)
+        expect(json.fetch('versions')).to contain_exactly(
+            a_hash_including('post_id' => external_post.id, 'version_no' => 1))
+      end
+
+      # Temporary compatibility shim until the frontend sends external_tag explicitly.
+      context 'with legacy tag fallback and no internal Tag with the external id' do
+        let(:external_id) { Tag.maximum(:id).to_i + 10_000 }
+
+        it 'falls back to ExternalTag for the legacy tag parameter' do
+          expect(Tag.exists?(external.id)).to be(false)
+
+          get '/posts/versions', params: { tag: external.id }
+
+          expect(response).to have_http_status(:ok)
+          expect(json.fetch('count')).to eq(1)
+          expect(json.fetch('versions')).to contain_exactly(
+              a_hash_including('post_id' => external_post.id, 'version_no' => 1))
+        end
+
+        [:tag, :external_tag].each do |parameter|
+          it "includes external removal history through the legacy API's #{ parameter }" do
+            external_post.post_external_tags.destroy_all
+            PostVersionRecorder.record!(
+                post: external_post.reload, event_type: :update, created_by_user: member)
+
+            get '/posts/versions', params: { parameter => external.id }
+
+            expect(response).to have_http_status(:ok)
+            expect(json.fetch('count')).to eq(2)
+            expect(json.fetch('versions')).to contain_exactly(
+                a_hash_including('post_id' => external_post.id, 'version_no' => 1),
+                a_hash_including('post_id' => external_post.id, 'version_no' => 2))
+          end
+        end
+      end
+    end
+
+    it 'can render history containing external identifiers' do
+      PostExternalTag.create!(post: post_record, external_tag: create(:external_tag))
+      post_record.update_columns(version_no: 2)
+      PostVersionRecorder.record!(post: post_record,
+                                  event_type: :update, created_by_user: member)
+
+      get '/posts/versions', params: { post: post_record.id }
+
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('count')).to eq(3)
+      expect(json.fetch('versions').first.fetch('tags')).to include(
+          'name' => tag2.name, 'type' => 'context')
+    end
+
     it 'filters versions by tag when the current snapshot includes the tag' do
       get '/posts/versions', params: { post: post_record.id, tag: tag2.id }
 
@@ -2271,9 +2685,9 @@ RSpec.describe 'Posts API', type: :request do
 
       expect(response).to have_http_status(:created)
       expect(Time.iso8601(json.fetch('original_created_from')))
-          .to eq(Time.iso8601('2020-01-01T00:00Z'))
+          .to eq(Time.utc(2020, 1, 1, 0, 0))
       expect(Time.iso8601(json.fetch('original_created_before')))
-          .to eq(Time.iso8601('2020-01-01T00:01Z'))
+          .to eq(Time.utc(2020, 1, 1, 0, 1))
     end
 
     it 'rejects unparseable original created timestamps on PUT /posts/:id' do

backend/spec/requests/tag_children_spec.rb

@@ -80,38 +80,6 @@ RSpec.describe "TagChildren", type: :request do
         expect(response).to have_http_status(:not_found)
       end
     end
-
-    context 'when parent is nico' do
-      before { stub_current_user(admin) }
-
-      let!(:parent) { create(:tag, :nico, name: 'nico:parent_ng') }
-      let(:parent_id) { parent.id }
-      let(:child_id) { child.id }
-
-      it 'returns 400 and does not create relation' do
-        expect {
-          do_request
-        }.not_to change(TagImplication, :count)
-
-        expect(response).to have_http_status(:bad_request)
-      end
-    end
-
-    context 'when child is nico' do
-      before { stub_current_user(admin) }
-
-      let!(:child) { create(:tag, :nico, name: 'nico:child_ng') }
-      let(:parent_id) { parent.id }
-      let(:child_id) { child.id }
-
-      it 'returns 400 and does not create relation' do
-        expect {
-          do_request
-        }.not_to change(TagImplication, :count)
-
-        expect(response).to have_http_status(:bad_request)
-      end
-    end
   end
 
   describe "DELETE /tag_children" do
@@ -186,31 +154,5 @@ RSpec.describe "TagChildren", type: :request do
         expect(response).to have_http_status(:not_found)
       end
     end
-
-    context 'when parent is nico' do
-      before { stub_current_user(admin) }
-
-      let!(:parent) { create(:tag, :nico, name: 'nico:parent_ng_delete') }
-      let(:parent_id) { parent.id }
-      let(:child_id) { child.id }
-
-      it 'returns 400' do
-        do_request
-        expect(response).to have_http_status(:bad_request)
-      end
-    end
-
-    context 'when child is nico' do
-      before { stub_current_user(admin) }
-
-      let!(:child) { create(:tag, :nico, name: 'nico:child_ng_delete') }
-      let(:parent_id) { parent.id }
-      let(:child_id) { child.id }
-
-      it 'returns 400' do
-        do_request
-        expect(response).to have_http_status(:bad_request)
-      end
-    end
   end
 end

backend/spec/requests/tags_deerjikists_spec.rb

@@ -226,6 +226,14 @@ RSpec.describe 'Tags deerjikists API', type: :request do
               [platform2, code2],
             )
         end
+
+        it 'locks the tag before replacing the complete list' do
+          expect_any_instance_of(Tag).to receive(:lock!).and_call_original
+
+          do_request
+
+          expect(response).to have_http_status(:ok)
+        end
       end
 
       context 'when tag already has deerjikists' do
@@ -299,6 +307,81 @@ RSpec.describe 'Tags deerjikists API', type: :request do
         end
       end
 
+      context 'when platform is outside the enum' do
+        let(:payload) do
+          [
+            { platform: 'invalid', code: code1 },
+          ]
+        end
+
+        it 'returns 422 with an indexed platform error without changing the list' do
+          Deerjikist.create!(platform: platform1, code: code1, tag: tag)
+
+          expect {
+            do_request
+          }.not_to change { Deerjikist.where(tag: tag).map { |d| [d.platform, d.code] } }
+
+          expect(response).to have_http_status(:unprocessable_entity)
+          expect(json.fetch('errors')).to include(
+            'deerjikists.0.platform' => [be_present],
+          )
+        end
+      end
+
+      context 'when a requested deerjikist belongs to another tag' do
+        let!(:other_tag) { create(:tag, category: :deerjikist) }
+        let!(:owned_deerjikist) do
+          Deerjikist.create!(platform: platform1, code: code1, tag: tag)
+        end
+        let!(:conflicting_deerjikist) do
+          Deerjikist.create!(platform: platform2, code: code2, tag: other_tag)
+        end
+        let(:payload) do
+          [
+            { platform: 'nico', code: 'new-code' },
+            { platform: platform2, code: code2 },
+          ]
+        end
+
+        before do
+          other_tag.tag_name.update!(name: 'existing-deerjikist')
+        end
+
+        it 'returns an indexed 422 error and rolls back the complete replacement' do
+          expect {
+            do_request
+          }.not_to change { Deerjikist.order(:platform, :code).pluck(:platform, :code, :tag_id) }
+
+          expect(response).to have_http_status(:unprocessable_entity)
+          expect(json.fetch('errors')).to include(
+            'deerjikists.1.code' => [include('existing-deerjikist')],
+          )
+          expect(owned_deerjikist.reload.tag_id).to eq(tag.id)
+          expect(conflicting_deerjikist.reload.tag_id).to eq(other_tag.id)
+        end
+      end
+
+      context 'when a requested deerjikist already belongs to the same tag' do
+        let!(:existing_deerjikist) do
+          Deerjikist.create!(platform: platform1, code: code1, tag: tag)
+        end
+        let(:payload) do
+          [
+            { platform: platform1, code: code1 },
+          ]
+        end
+
+        it 'keeps the existing row' do
+          expect {
+            do_request
+          }.not_to change { existing_deerjikist.reload.created_at }
+
+          expect(response).to have_http_status(:ok)
+          expect(Deerjikist.where(tag: tag).pluck(:platform, :code))
+            .to eq([[platform1, code1]])
+        end
+      end
+
       context 'when youtube code is handle' do
         let(:channel_id) { 'UCabcdefghijklmnopqrstuv' }
         let(:payload) do

backend/spec/requests/tags_spec.rb

@@ -30,6 +30,30 @@ RSpec.describe 'Tags API', type: :request do
   end
 
   describe 'GET /tags' do
+    it 'includes legacy external JSON alongside an internal tag with the same id' do
+      external = create(:external_tag, id: tag.id, name: 'spec_external', post_count: 3)
+
+      get '/tags', params: { name: 'spec_' }
+
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('count')).to eq(2)
+      expect(response_tags).to contain_exactly(
+          a_hash_including('id' => tag.id, 'name' => tag.name, 'category' => 'general'),
+          a_hash_including(
+              'id' => external.id,
+              'name' => 'nico:spec_external',
+              'category' => 'nico',
+              'post_count' => 3,
+              'created_at' => external.created_at.as_json,
+              'updated_at' => external.created_at.as_json,
+              'deprecated_at' => nil,
+              'aliases' => [],
+              'parents' => [],
+              'has_wiki' => false,
+              'material_id' => nil,
+              'has_deerjikists' => false))
+    end
+
     it 'returns tags with count and metadata' do
       get '/tags'
 
@@ -164,20 +188,79 @@ RSpec.describe 'Tags API', type: :request do
       Tag.create!(tag_name: TagName.create!(name: 'cat_general'), category: :general)
       Tag.create!(tag_name: TagName.create!(name: 'cat_material'), category: :material)
       Tag.create!(tag_name: TagName.create!(name: 'cat_meta'), category: :meta)
-      Tag.create!(tag_name: TagName.create!(name: 'nico:cat_nico'), category: :nico)
+      create(:external_tag, name: 'cat_nico')
 
       get '/tags', params: { name: 'cat_', order: 'category:asc', limit: 20 }
 
       expect(response).to have_http_status(:ok)
-      expect(response_names).to eq(%w[
-          cat_deerjikist
-          cat_meme
-          cat_character
-          cat_general
-          cat_material
-          cat_meta
-          nico:cat_nico
-      ])
+      expect(response_names).to eq([
+          'cat_deerjikist', 'cat_meme', 'cat_character',
+          'cat_general', 'cat_material', 'cat_meta', 'nico:cat_nico'])
+      expect(json.fetch('count')).to eq(7)
+    end
+
+    context 'with mixed legacy pagination' do
+      let!(:first_tag) do
+        create(:tag,
+               tag_name: create(:tag_name, name: 'a_mixed_page'),
+               category: :meme)
+      end
+
+      let!(:middle_tag) do
+        create(:tag,
+               tag_name: create(:tag_name, name: 'm_mixed_page'),
+               category: :meta)
+      end
+
+      let!(:last_tag) do
+        create(:tag,
+               tag_name: create(:tag_name, name: 'z_mixed_page'),
+               category: :general)
+      end
+
+      let!(:first_external) do
+        create(:external_tag, id: first_tag.id, name: 'a_mixed_page')
+      end
+      let!(:last_external) do
+        create(:external_tag, id: last_tag.id, name: 'z_mixed_page')
+      end
+
+      let(:name_order) do
+        [
+            [first_tag.id, 'a_mixed_page'], [middle_tag.id, 'm_mixed_page'],
+            [first_external.id, 'nico:a_mixed_page'], [last_external.id, 'nico:z_mixed_page'],
+            [last_tag.id, 'z_mixed_page']]
+      end
+      let(:category_order) do
+        [
+            [first_tag.id, 'a_mixed_page'], [last_tag.id, 'z_mixed_page'],
+            [middle_tag.id, 'm_mixed_page'], [first_external.id, 'nico:a_mixed_page'],
+            [last_external.id, 'nico:z_mixed_page']]
+      end
+
+      ['name', 'category'].each do |order|
+        ['asc', 'desc'].each do |direction|
+          it "orders the combined records by #{ order }:#{ direction } before paging" do
+            ascending = order == 'name' ? name_order : category_order
+            expected = direction == 'asc' ? ascending : ascending.reverse
+
+            [2, 3].each do |limit|
+              pages = expected.each_slice(limit).to_a + [[]]
+              pages.each.with_index(1) do |expected_page, page|
+                get '/tags', params: {
+                    name: 'mixed_page', order: "#{ order }:#{ direction }", page:, limit: }
+
+                expect(response).to have_http_status(:ok)
+                expect(json.fetch('count')).to eq(5)
+                expect(response_tags.size).to eq(expected_page.size)
+                expect(response_tags.size).to be <= limit
+                expect(response_tags.map { [_1.fetch('id'), _1.fetch('name')] })
+                    .to eq(expected_page)
+              end
+            end
+          end
+        end
+      end
     end
 
     it 'paginates and keeps total count' do
@@ -299,6 +382,89 @@ RSpec.describe 'Tags API', type: :request do
   end
 
   describe 'GET /tags/autocomplete' do
+    it 'combines internal and external matches without conflating equal ids' do
+      internal = Tag.create!(category: :general, name: 'mixed_internal', post_count: 2)
+      external = create(:external_tag, id: internal.id,
+                                       name: 'mixed_external', post_count: 3)
+
+      get '/tags/autocomplete', params: { q: 'not:mixed' }
+
+      expect(response).to have_http_status(:ok)
+      expect(json.map { |row| row.fetch('name') })
+          .to eq(['nico:mixed_external', 'mixed_internal'])
+      expect(json.first).to include(
+          'id' => external.id, 'category' => 'nico', 'post_count' => 3,
+          'created_at' => external.created_at.as_json,
+          'updated_at' => external.created_at.as_json,
+          'deprecated_at' => nil, 'matched_alias' => nil,
+          'aliases' => [], 'parents' => [], 'has_wiki' => false,
+          'material_id' => nil, 'has_deerjikists' => false)
+    end
+
+    it 'matches an external tag by its platform prefix' do
+      create(:external_tag, name: 'prefix_match', post_count: 1)
+
+      get '/tags/autocomplete', params: { q: 'nico:prefix' }
+
+      expect(json.map { |row| row.fetch('name') }).to eq(['nico:prefix_match'])
+    end
+
+    it 'excludes unused external tags unless present is false' do
+      create(:external_tag, name: 'unused_external')
+
+      get '/tags/autocomplete', params: { q: 'unused' }
+      expect(json).to be_empty
+
+      get '/tags/autocomplete', params: { q: 'unused', present: '0' }
+      expect(json.map { |row| row.fetch('name') }).to eq(['nico:unused_external'])
+    end
+
+    it 'limits the combined results to 20 and sorts ties by displayed name' do
+      11.times do |i|
+        name = "combined_#{ i.to_s.rjust(2, '0') }"
+        Tag.create!(category: :general, name:, post_count: 1)
+        create(:external_tag, name:, post_count: 1)
+      end
+
+      get '/tags/autocomplete', params: { q: 'combined' }
+
+      expected = 11.times.map { |i| "combined_#{ i.to_s.rjust(2, '0') }" }
+      expected += expected.map { |name| "nico:#{ name }" }
+      expect(json.map { |row| row.fetch('name') }).to eq(expected.first(20))
+    end
+
+    it 'excludes external tags but preserves internal alias matches when nico is false' do
+      internal = Tag.create!(category: :general, name: 'switch_internal', post_count: 1)
+      alias_target = Tag.create!(category: :general, name: 'alias_target', post_count: 1)
+      TagName.create!(name: 'switch_alias', canonical: alias_target.tag_name)
+      create(:external_tag, name: 'switch_external', post_count: 1)
+
+      get '/tags/autocomplete', params: { q: 'switch', nico: '0' }
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to contain_exactly(
+          a_hash_including('id' => internal.id, 'name' => internal.name),
+          a_hash_including('id' => alias_target.id, 'name' => alias_target.name,
+                           'matched_alias' => 'switch_alias'))
+    end
+
+    ['%', '_'].each do |wildcard|
+      it "treats #{ wildcard } literally for canonical, alias, and external names" do
+        literal = "literal#{ wildcard }match"
+        Tag.create!(category: :general, name: literal, post_count: 1)
+        alias_target = Tag.create!(category: :general, name: 'literal_alias_target', post_count: 1)
+        TagName.create!(name: "#{ literal }_alias", canonical: alias_target.tag_name)
+        create(:external_tag, name: literal, post_count: 1)
+        Tag.create!(category: :general, name: 'literalXmatch', post_count: 2)
+        create(:external_tag, name: 'literalXmatch', post_count: 2)
+
+        get '/tags/autocomplete', params: { q: "literal#{ wildcard }" }
+
+        expect(json.map { |row| row.fetch('name') })
+            .to contain_exactly(literal, alias_target.name, "nico:#{ literal }")
+      end
+    end
+
     it 'returns matching tags by q' do
       get '/tags/autocomplete', params: { q: 'spec' }
 
@@ -340,6 +506,27 @@ RSpec.describe 'Tags API', type: :request do
   end
 
   describe 'GET /tags/name/:name' do
+    it 'preserves qualified external name lookup used by wiki pages' do
+      external = create(:external_tag, name: 'detail_external', post_count: 3)
+
+      get "/tags/name/#{ CGI.escape('nico:detail_external') }"
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include(
+          'id' => external.id,
+          'name' => 'nico:detail_external',
+          'category' => 'nico',
+          'post_count' => 3,
+          'created_at' => external.created_at.as_json,
+          'updated_at' => external.created_at.as_json,
+          'deprecated_at' => nil,
+          'aliases' => [],
+          'parents' => [],
+          'has_wiki' => false,
+          'material_id' => nil,
+          'has_deerjikists' => false)
+    end
+
     it 'returns tag by name' do
       get "/tags/name/#{ CGI.escape('spec_tag') }"
 
@@ -487,18 +674,6 @@ RSpec.describe 'Tags API', type: :request do
         expect(json.fetch('deprecated_at')).to be_present
       end
 
-      it 'rejects deprecating a nico tag' do
-        nico_tag = Tag.create!(name: 'nico:deprecated_update', category: :nico)
-
-        patch "/tags/#{ nico_tag.id }", params: { deprecated: '1' }
-
-        expect(response).to have_http_status(:unprocessable_entity)
-        expect(nico_tag.reload.deprecated_at).to be_nil
-        expect(json.fetch('errors')).to include(
-          'deprecated' => ['ニコタグは廃止できません．']
-        )
-      end
-
       it 'returns 422 when changing normal tag category to nico' do
         expect {
           patch "/tags/#{tag.id}", params: { category: 'nico' }
@@ -508,32 +683,6 @@ RSpec.describe 'Tags API', type: :request do
         expect(tag.reload.category).to eq('general')
       end
 
-      it 'returns 422 when updating nico tag name' do
-        nico_tag_name = TagName.create!(name: 'nico:tags_spec_source')
-        nico_tag = Tag.create!(tag_name: nico_tag_name, category: :nico)
-
-        expect {
-          patch "/tags/#{ nico_tag.id }", params: { name: 'nico:tags_spec_renamed' }
-        }.not_to change(NicoTagVersion, :count)
-
-        expect(response).to have_http_status(:unprocessable_entity)
-
-        expect(nico_tag.reload.name).to eq('nico:tags_spec_source')
-        expect(nico_tag.category).to eq('nico')
-      end
-
-      it 'returns 422 when changing nico tag category to normal category' do
-        nico_tag_name = TagName.create!(name: 'nico:category_change_ng')
-        nico_tag = Tag.create!(tag_name: nico_tag_name, category: :nico)
-
-        expect {
-          patch "/tags/#{nico_tag.id}", params: { category: 'general' }
-        }.not_to change(NicoTagVersion, :count)
-
-        expect(response).to have_http_status(:unprocessable_entity)
-        expect(nico_tag.reload.category).to eq('nico')
-      end
-
       it 'PATCH で tag の name を変更すると対応する wiki version を作成する' do
         wiki_page =
             Wiki::Commit.create_content!(
@@ -1169,28 +1318,6 @@ RSpec.describe 'Tags API', type: :request do
         expect(tag.category).to eq('general')
       end
 
-      it 'nico tag は更新できない' do
-        nico_tag = Tag.create!(
-          tag_name: TagName.create!(name: 'nico:put_update_all_ng'),
-          category: :nico
-        )
-
-        expect {
-          put "/tags/#{ nico_tag.id }", params: {
-            name: 'nico:put_update_all_renamed',
-            category: 'nico',
-            aliases: '',
-            parent_tags: '',
-            deprecated: '0',
-          }
-        }.not_to change(NicoTagVersion, :count)
-
-        expect(response).to have_http_status(:unprocessable_entity)
-
-        expect(nico_tag.reload.name).to eq('nico:put_update_all_ng')
-        expect(nico_tag.category).to eq('nico')
-      end
-
       it 'system tag の name は変更できない' do
         system_tag = Tag.tagme
         old_name = system_tag.name

backend/spec/requests/wiki_body_search_pending_spec.rb

@@ -1,27 +0,0 @@
-require 'rails_helper'
-
-RSpec.describe 'Wiki body search', type: :request do
-  let!(:user) { create_member_user! }
-
-  it 'searches wiki pages by body text' do
-    pending '#336 で対応予定'
-
-    Wiki::Commit.create_content!(
-        tag_name: TagName.create!(name: 'wiki_body_search_hit'),
-        body: 'unique body keyword for wiki search',
-        created_by_user: user,
-        message: 'init')
-
-    Wiki::Commit.create_content!(
-        tag_name: TagName.create!(name: 'wiki_body_search_miss'),
-        body: 'ordinary body',
-        created_by_user: user,
-        message: 'init')
-
-    get '/wiki/search', params: { body: 'unique body keyword' }
-
-    expect(response).to have_http_status(:ok)
-    expect(json.map { |page| page['title'] }).to include('wiki_body_search_hit')
-    expect(json.map { |page| page['title'] }).not_to include('wiki_body_search_miss')
-  end
-end

backend/spec/requests/wiki_restore_pending_spec.rb

@@ -1,37 +0,0 @@
-require 'rails_helper'
-
-RSpec.describe 'Wiki restore', type: :request do
-  let!(:user) { create_member_user! }
-
-  def auth_headers user
-    { 'X-Transfer-Code' => user.inheritance_code }
-  end
-
-  it 'restores wiki page to previous version' do
-    pending '#337 で対応予定'
-
-    page =
-        Wiki::Commit.create_content!(
-            tag_name: TagName.create!(name: 'wiki_restore_page'),
-            body: 'v1',
-            created_by_user: user,
-            message: 'init')
-
-    v1 = page.wiki_versions.order(:version_no).last
-
-    Wiki::Commit.content!(
-        page:,
-        body: 'v2',
-        created_user: user,
-        message: 'edit',
-        base_revision_id: page.current_revision.id)
-
-    post "/wiki/#{ page.id }/restore",
-         params: { version_no: v1.version_no },
-         headers: auth_headers(user)
-
-    expect(response).to have_http_status(:ok)
-    expect(page.reload.body).to eq('v1')
-    expect(page.wiki_versions.order(:version_no).last.event_type).to eq('restore')
-  end
-end

backend/spec/services/nico_tag_version_recorder_spec.rb

@@ -0,0 +1,76 @@
+require 'rails_helper'
+
+RSpec.describe NicoTagVersionRecorder do
+  let(:external_tag) { create(:external_tag, name: 'raw tag[]') }
+  let(:member) { create(:user, :member) }
+
+  def record event_type
+    described_class.record!(external_tag:, event_type:, created_by_user: member)
+  end
+
+  it 'records the external association and platform-qualified name' do
+    version = record(:create)
+
+    expect(version).to have_attributes(
+        external_tag:, version_no: 1, event_type: 'create',
+        name: 'nico:raw tag[]', linked_tags: '', created_by_user: member)
+    expect(external_tag.reload.nico_tag_versions).to contain_exactly(version)
+  end
+
+  it 'uses the latest history number when the record has no version_no column' do
+    first = record(:create)
+    external_tag.update!(name: 'changed')
+    second = record(:update)
+
+    expect(second).to have_attributes(version_no: 2, name: 'nico:changed')
+    expect(first.reload.name).to eq('nico:raw tag[]')
+    expect(external_tag.reload.has_attribute?(:version_no)).to be(false)
+  end
+
+  it 'builds the qualified name from the registered platform value' do
+    external = create(:external_tag, name: 'foo')
+    locked_scope = instance_double(ActiveRecord::Relation)
+    allow(ExternalTag).to receive(:unscoped).and_return(locked_scope)
+    allow(locked_scope).to receive(:lock).and_return(locked_scope)
+    allow(locked_scope).to receive(:find).with(external.id).and_return(external)
+    allow(external).to receive(:platform).and_return('registered_external')
+
+    version = described_class.record!(
+        external_tag: external, event_type: :create, created_by_user: member)
+
+    expect(version).to have_attributes(
+        external_tag: external, name: 'registered_external:foo', version_no: 1,
+        event_type: 'create', linked_tags: '', created_by_user: member)
+  end
+
+  it 'returns the latest version without appending an unchanged snapshot' do
+    first = record(:create)
+
+    expect { expect(record(:update)).to eq(first) }
+        .not_to change(NicoTagVersion, :count)
+  end
+
+  it 'still requires a create event before any update' do
+    expect { record(:update) }
+        .to raise_error(RuntimeError, 'NicoTagVersion first event must be create')
+    expect(external_tag.nico_tag_versions).to be_empty
+  end
+
+  it 'still rejects a second create event' do
+    first = record(:create)
+
+    expect { record(:create) }
+        .to raise_error(RuntimeError, 'NicoTagVersion create event already exists')
+    expect(external_tag.nico_tag_versions).to contain_exactly(first)
+  end
+
+  it 'records sorted linked internal names and later link removal' do
+    tags = ['z_link', 'a_link'].map { |name| Tag.create!(name:, category: :general) }
+    tags.each { |tag| NicoTagRelation.create!(nico_tag: external_tag, tag:) }
+
+    expect(record(:create).linked_tags).to eq('a_link z_link')
+    external_tag.linked_tags = [tags.first]
+
+    expect(record(:update)).to have_attributes(version_no: 2, linked_tags: 'z_link')
+  end
+end

backend/spec/services/post_bulk_creator_spec.rb

@@ -0,0 +1,59 @@
+require 'rails_helper'
+
+RSpec.describe PostBulkCreator do
+  it 'limits workers to two and keeps failures in their request slots' do
+    actor = instance_double(User, id: 123)
+    allow(User).to receive(:find).with(123) {
+      instance_double(User, id: 123)
+    }
+    mutex = Mutex.new
+    active = 0
+    maximum_active = 0
+
+    allow(PostCreatePreflight).to receive(:new) do |attributes:, **|
+      preflight = instance_double(PostCreatePreflight)
+      allow(preflight).to receive(:run) do
+        mutex.synchronize do
+          active += 1
+          maximum_active = [maximum_active, active].max
+        end
+        sleep 0.02
+        mutex.synchronize { active -= 1 }
+        attributes.symbolize_keys.merge(
+            existing_post_id: nil,
+            field_warnings: { },
+            base_warnings: [])
+      end
+      preflight
+    end
+    allow(PostCreator).to receive(:new) do |attributes:, **|
+      creator = instance_double(PostCreator)
+      if attributes[:title] == 'broken'
+        allow(creator).to receive(:create!).and_raise(StandardError, 'broken')
+      else
+        post = instance_double(Post, id: attributes[:title].delete_prefix('post ').to_i)
+        allow(creator).to receive(:create!).and_return(post)
+      end
+      creator
+    end
+    posts = [
+        { 'title' => 'post 1', 'url' => 'https://example.com/1' },
+        { 'title' => 'broken', 'url' => 'https://example.com/2' },
+        { 'title' => 'post 3', 'url' => 'https://example.com/3' },
+        { 'title' => 'post 4', 'url' => 'https://example.com/4' }]
+
+    results = described_class.new(
+        actor:,
+        posts:,
+        thumbnails: { }).run.fetch(:results)
+
+    expect(maximum_active).to eq(2)
+    expect(results.length).to eq(posts.length)
+    expect(results.map { _1[:status] }).to eq(
+        ['created', 'failed', 'created', 'created'])
+    expect(results[0].dig(:post, :id)).to eq(1)
+    expect(results[1]).to include(status: 'failed', recoverable: false)
+    expect(results[2].dig(:post, :id)).to eq(3)
+    expect(results[3].dig(:post, :id)).to eq(4)
+  end
+end

backend/spec/services/post_create_plan_spec.rb

@@ -0,0 +1,95 @@
+require 'rails_helper'
+
+RSpec.describe PostCreatePlan do
+  def create_tag! name, category
+    Tag.create!(name:, category:)
+  end
+
+  before do
+    create_tag!('タグ希望', :meta)
+    create_tag!('ニジラー情報不詳', :meta)
+  end
+
+  it 'plans direct and existing default tags without persisting records' do
+    counts = [TagName.count, Tag.count]
+
+    plan = described_class.new(
+        attributes: {
+            url: 'https://example.com/post',
+            title: 'title',
+            tags: 'character:new_character',
+            parent_post_ids: '' }).build!
+
+    expect(plan[:tags]).to eq('new_character')
+    expect(plan[:direct_tag_specs]).to eq(
+        [{ name: 'new_character', category: :character }])
+    expect(plan[:default_tag_specs]).to include(
+        { name: 'タグ希望', category: :meta },
+        { name: 'ニジラー情報不詳', category: :meta })
+    expect([TagName.count, Tag.count]).to eq(counts)
+  end
+
+  it 'resolves aliases and keeps tag sections separate from canonical names' do
+    canonical = create_tag!('虹夏', :character)
+    TagName.create!(name: 'にじか', canonical: canonical.tag_name)
+    create_tag!('動画', :meta)
+
+    plan = described_class.new(
+        attributes: {
+            url: 'https://example.com/video',
+            title: 'video',
+            tags: '動画 にじか[0:10-0:20]',
+            duration: '1:00',
+            parent_post_ids: '' }).build!
+
+    expect(plan[:tags].split).to include('動画', '虹夏[0:10-0:20]')
+    expect(plan[:display_tags]).to include(
+        { name: '虹夏',
+          category: 'character',
+          section_literals: ['[0:10-0:20]'] })
+    expect(plan[:video_ms]).to eq(60_000)
+  end
+
+  it 'rejects direct external tag input without creating internal records' do
+    create(:external_tag, name: 'reserved')
+    counts = [Tag.count, TagName.count, ExternalTag.count]
+
+    expect {
+      described_class.new(attributes: { tags: 'NiCo:reserved' }).build!
+    }.to raise_error(ActiveRecord::RecordInvalid) { |error|
+      expect(error.record.errors[:tags]).to be_present
+    }
+
+    expect([Tag.count, TagName.count, ExternalTag.count]).to eq(counts)
+  end
+
+  it 'validates a new tag name without persisting it' do
+    long_name = 'a' * 256
+    counts = [TagName.count, Tag.count]
+
+    expect {
+      described_class.new(
+          attributes: {
+              url: 'https://example.com/post',
+              title: 'title',
+              tags: long_name,
+              parent_post_ids: '' }).build!
+    }.to raise_error(ActiveRecord::RecordInvalid) { |error|
+      expect(error.record.errors[:tags]).not_to be_empty
+    }
+    expect([TagName.count, Tag.count]).to eq(counts)
+  end
+
+  it 'ignores duration when the planned tags do not include video' do
+    plan = described_class.new(
+        attributes: {
+            url: 'https://example.com/post',
+            title: 'title',
+            tags: 'ordinary_tag',
+            duration: 'invalid',
+            parent_post_ids: '' }).build!
+
+    expect(plan[:duration]).to eq('invalid')
+    expect(plan[:video_ms]).to be_nil
+  end
+end

backend/spec/services/post_creator_spec.rb

@@ -20,12 +20,12 @@ RSpec.describe PostCreator do
     allow(PostVersionRecorder).to receive(:record!)
   end
 
-  it 'prefers an explicit upload over thumbnail_base' do
-    allow(Post).to receive(:resized_thumbnail_attachment).and_return(
-        io: StringIO.new('upload'),
+  it 'prefers thumbnail_base over an explicit upload' do
+    expect(Post).not_to receive(:resized_thumbnail_attachment)
+    allow(Post).to receive(:remote_thumbnail_attachment).and_return(
+        io: StringIO.new('remote'),
         filename: 'resized_thumbnail.jpg',
         content_type: 'image/jpeg')
-    expect_any_instance_of(Post).not_to receive(:attach_thumbnail_from_url!)
 
     post = described_class.new(
         actor:,
@@ -41,13 +41,12 @@ RSpec.describe PostCreator do
   end
 
   it 'uses the common remote thumbnail attach path when thumbnail_base is given' do
-    expect_any_instance_of(Post).to receive(:attach_thumbnail_from_url!)
-        .with('https://example.com/thumb.jpg') do |post, _url|
-      post.thumbnail.attach(
-          io: StringIO.new('thumbnail'),
-          filename: 'thumbnail.jpg',
-          content_type: 'image/jpeg')
-    end
+    expect(Post).to receive(:remote_thumbnail_attachment)
+        .with('https://example.com/thumb.jpg')
+        .and_return(
+            io: StringIO.new('thumbnail'),
+            filename: 'thumbnail.jpg',
+            content_type: 'image/jpeg')
 
     post = described_class.new(
         actor:,
@@ -61,8 +60,8 @@ RSpec.describe PostCreator do
     expect(post.thumbnail).to be_attached
   end
 
-  it 'keeps creating the post and records a warning when remote thumbnail fetch fails' do
-    allow_any_instance_of(Post).to receive(:attach_thumbnail_from_url!)
+  it 'does not create a post when remote thumbnail fetch fails' do
+    allow(Post).to receive(:remote_thumbnail_attachment)
         .and_raise(Post::RemoteThumbnailFetchFailed, 'サムネール画像を取得できませんでした．')
     creator = described_class.new(
         actor:,
@@ -72,11 +71,8 @@ RSpec.describe PostCreator do
           thumbnail_base: 'https://example.com/thumb.jpg',
           tags: '' })
 
-    post = creator.create!
-
-    expect(post.thumbnail_base).to eq('https://example.com/thumb.jpg')
-    expect(post.thumbnail).not_to be_attached
-    expect(creator.field_warnings).to eq(
-        thumbnail_base: ['サムネール画像を取得できませんでした．'])
+    post_count = Post.count
+    expect { creator.create! }.to raise_error(Post::RemoteThumbnailFetchFailed)
+    expect(Post.count).to eq(post_count)
   end
 end

backend/spec/services/post_import_row_normaliser_spec.rb

@@ -1,77 +0,0 @@
-require 'rails_helper'
-
-RSpec.describe PostImportRowNormaliser do
-  def valid_row(overrides = { })
-    {
-      sourceRow: '1',
-      url: 'https://example.com/post',
-      metadataUrl: 'https://example.com/post',
-      attributes: { title: 'title', duration: '1' },
-      provenance: { url: 'manual', title: 'automatic' },
-      tagSources: { automatic: 'tag', manual: '' }
-    }.deep_merge(overrides)
-  end
-
-  describe '.normalise!' do
-    it 'normalises aliases and source rows into a permitted plain hash' do
-      row = ActionController::Parameters.new(valid_row)
-
-      expect(described_class.normalise!([row])).to eq([
-        {
-          'source_row' => 1,
-          'url' => 'https://example.com/post',
-          'metadata_url' => 'https://example.com/post',
-          'attributes' => { 'title' => 'title', 'duration' => '1' },
-          'provenance' => { 'url' => 'manual', 'title' => 'automatic' },
-          'tag_sources' => { 'automatic' => 'tag', 'manual' => '' }
-        }
-      ])
-    end
-
-    it 'normalises source rows before checking duplicates' do
-      rows = [valid_row, valid_row(sourceRow: 1, url: 'https://example.com/other')]
-
-      expect { described_class.normalise!(rows) }
-          .to raise_error(ArgumentError, '元行番号が重複しています．')
-    end
-
-    it 'rejects non-array batches and non-hash rows' do
-      expect { described_class.normalise!({}) }
-          .to raise_error(ArgumentError, '取込行の形式が不正です．')
-      expect { described_class.normalise!(['row']) }
-          .to raise_error(ArgumentError, '取込行の形式が不正です．')
-    end
-
-    it 'rejects unknown attributes and invalid field types' do
-      expect { described_class.normalise!([valid_row(attributes: { unknown: 'x' })]) }
-          .to raise_error(ArgumentError, '取込項目が不正です．')
-      expect { described_class.normalise!([valid_row(attributes: { title: [] })]) }
-          .to raise_error(ArgumentError, '取込項目の型が不正です．')
-      expect { described_class.normalise!([valid_row(attributes: { duration: false })]) }
-          .to raise_error(ArgumentError, '取込項目の型が不正です．')
-    end
-
-    it 'rejects unknown provenance and tag-source values' do
-      expect { described_class.normalise!([valid_row(provenance: { title: 'mapped' })]) }
-          .to raise_error(ArgumentError, '値の由来が不正です．')
-      expect { described_class.normalise!([valid_row(tagSources: { mapped: 'tag' })]) }
-          .to raise_error(ArgumentError, 'タグ由来の形式が不正です．')
-    end
-
-    it 'accepts warning fields only at the validation boundary' do
-      row = valid_row.merge(
-        fieldWarnings: { title: ['取得できませんでした．'] },
-        baseWarnings: ['確認してください．']
-      )
-
-      without_warnings = described_class.normalise!([row]).first
-      with_warnings = described_class.normalise!([row], allow_warning_fields: true).first
-
-      expect(without_warnings).not_to include('field_warnings', 'base_warnings')
-      expect(with_warnings).to include(
-        'field_warnings' => { 'title' => ['取得できませんでした．'] },
-        'base_warnings' => ['確認してください．']
-      )
-    end
-  end
-end

backend/spec/services/post_import_runner_spec.rb

@@ -1,152 +0,0 @@
-require 'rails_helper'
-
-RSpec.describe PostImportRunner do
-  let(:actor) { create(:user, :member) }
-
-  def row(source_row: 1, url: 'https://example.com/post')
-    {
-      sourceRow: source_row,
-      url:,
-      attributes: { title: 'title', tags: '' },
-      provenance: { url: 'manual', title: 'manual', tags: 'manual' },
-      tagSources: { automatic: '', manual: '' }
-    }
-  end
-
-  def preview(source_row: 1, errors: { }, skip_reason: nil, existing_post_id: nil)
-    {
-      source_row:,
-      attributes: { 'title' => 'title', 'tags' => '' },
-      validation_errors: errors,
-      skip_reason:,
-      existing_post_id:
-    }
-  end
-
-  it 'previews the whole batch once before processing individual rows' do
-    rows = [row, row(source_row: 2, url: 'https://example.com/two')]
-    previewer = instance_double(PostImportPreviewer)
-    allow(PostImportPreviewer).to receive(:new).and_return(previewer)
-    expect(previewer).to receive(:preview_rows)
-        .with(
-          rows: satisfy { _1.map { |row_value| row_value['source_row'] } == [1, 2] },
-          fetch_metadata: false
-        )
-        .and_return([preview, preview(source_row: 2)])
-    allow(PostCreator).to receive(:new).and_return(
-      instance_double(PostCreator, create!: create(:post))
-    )
-
-    result = described_class.new(actor:, rows:).run
-
-    expect(result).to include(created: 2, skipped: 0, failed: 0)
-  end
-
-  it 'treats validation errors as failures before an existing skip' do
-    existing = create(:post)
-    previewer = instance_double(PostImportPreviewer)
-    allow(PostImportPreviewer).to receive(:new).and_return(previewer)
-    allow(previewer).to receive(:preview_rows).and_return([
-      preview(errors: { url: ['URL が重複しています．'] },
-              skip_reason: 'existing', existing_post_id: existing.id)
-    ])
-    expect(PostCreator).not_to receive(:new)
-
-    result = described_class.new(actor:, rows: [row]).run.fetch(:rows).first
-
-    expect(result).to include(
-      status: 'failed',
-      errors: { url: ['URL が重複しています．'] },
-      recoverable: true
-    )
-  end
-
-  it 'returns the existing post ID for skipped rows' do
-    existing = create(:post)
-    previewer = instance_double(PostImportPreviewer)
-    allow(PostImportPreviewer).to receive(:new).and_return(previewer)
-    allow(previewer).to receive(:preview_rows).and_return([
-      preview(skip_reason: 'existing', existing_post_id: existing.id)
-    ])
-
-    result = described_class.new(actor:, rows: [row]).run.fetch(:rows).first
-
-    expect(result).to eq(
-      source_row: 1,
-      status: 'skipped',
-      existing_post_id: existing.id
-    )
-  end
-
-  it 'converts a URL uniqueness validation race into a skip' do
-    existing = create(:post, url: 'https://example.com/race')
-    invalid = Post.new(url: existing.url)
-    invalid.errors.add(:url, :taken)
-    previewer = instance_double(PostImportPreviewer)
-    allow(PostImportPreviewer).to receive(:new).and_return(previewer)
-    allow(previewer).to receive(:preview_rows).and_return([preview])
-    creator = instance_double(PostCreator)
-    allow(PostCreator).to receive(:new).and_return(creator)
-    allow(creator).to receive(:create!).and_raise(ActiveRecord::RecordInvalid.new(invalid))
-
-    result = described_class.new(
-      actor:,
-      rows: [row(url: 'https://EXAMPLE.com/race/')]
-    ).run.fetch(:rows).first
-
-    expect(result).to include(status: 'skipped', existing_post_id: existing.id)
-  end
-
-  it 're-raises RecordNotUnique errors unrelated to the posts URL index' do
-    previewer = instance_double(PostImportPreviewer)
-    allow(PostImportPreviewer).to receive(:new).and_return(previewer)
-    allow(previewer).to receive(:preview_rows).and_return([preview])
-    creator = instance_double(PostCreator)
-    allow(PostCreator).to receive(:new).and_return(creator)
-    allow(creator).to receive(:create!)
-        .and_raise(ActiveRecord::RecordNotUnique, 'other_unique_index')
-
-    expect {
-      described_class.new(actor:, rows: [row]).run
-    }.to raise_error(ActiveRecord::RecordNotUnique)
-  end
-
-  it 'converts a posts URL index race into a skip' do
-    existing = create(:post, url: 'https://example.com/index-race')
-    previewer = instance_double(PostImportPreviewer)
-    allow(PostImportPreviewer).to receive(:new).and_return(previewer)
-    allow(previewer).to receive(:preview_rows).and_return([preview])
-    creator = instance_double(PostCreator)
-    allow(PostCreator).to receive(:new).and_return(creator)
-    allow(creator).to receive(:create!).and_raise(
-      ActiveRecord::RecordNotUnique,
-      'duplicate key index_posts_on_url'
-    )
-
-    result = described_class.new(
-      actor:,
-      rows: [row(url: 'https://EXAMPLE.com/index-race/')]
-    ).run.fetch(:rows).first
-
-    expect(result).to include(status: 'skipped', existing_post_id: existing.id)
-  end
-
-  it 'returns thumbnail warnings from PostCreator on a created row' do
-    previewer = instance_double(PostImportPreviewer)
-    allow(PostImportPreviewer).to receive(:new).and_return(previewer)
-    allow(previewer).to receive(:preview_rows).and_return([preview])
-    created_post = create(:post)
-    creator = instance_double(
-        PostCreator,
-        create!: created_post,
-        field_warnings: { thumbnail_base: ['サムネール画像を取得できませんでした．'] })
-    allow(PostCreator).to receive(:new).and_return(creator)
-
-    result = described_class.new(actor:, rows: [row]).run.fetch(:rows).first
-
-    expect(result).to include(
-      status: 'created',
-      field_warnings: { thumbnail_base: ['サムネール画像を取得できませんでした．'] }
-    )
-  end
-end

backend/spec/services/post_import_url_list_parser_spec.rb

@@ -1,40 +0,0 @@
-require 'rails_helper'
-
-RSpec.describe PostImportUrlListParser do
-  describe '.parse' do
-    it 'trims URLs, ignores blank lines, and preserves source line numbers' do
-      source = "  https://example.com/one  \r\n\r\nhttps://example.com/two\n"
-
-      expect(described_class.parse(source)).to eq([
-        { source_row: 1, url: 'https://example.com/one' },
-        { source_row: 3, url: 'https://example.com/two' }
-      ])
-    end
-
-    it 'rejects an empty URL list' do
-      expect { described_class.parse(" \n\r\n") }
-          .to raise_error(ArgumentError, 'URL を入力してください．')
-    end
-
-    it 'rejects more than 100 non-empty rows' do
-      source = 101.times.map { |index| "https://example.com/#{ index }" }.join("\n")
-
-      expect { described_class.parse(source) }
-          .to raise_error(ArgumentError, '取込件数は 100 件までです．')
-    end
-
-    it 'includes the original line number in an oversized URL error' do
-      source = "\n#{ 'a' * (described_class::MAX_URL_BYTES + 1) }"
-
-      expect { described_class.parse(source) }
-          .to raise_error(ArgumentError, '2 行目: URL が長すぎます．')
-    end
-
-    it 'rejects an oversized request before parsing rows' do
-      source = 'a' * (described_class::MAX_BYTES + 1)
-
-      expect { described_class.parse(source) }
-          .to raise_error(ArgumentError, '入力が大きすぎます．')
-    end
-  end
-end

backend/spec/services/post_thumbnail_upload_validator_spec.rb

@@ -0,0 +1,52 @@
+require 'rails_helper'
+require 'base64'
+require 'tempfile'
+
+RSpec.describe PostThumbnailUploadValidator do
+  def with_upload bytes, content_type:, filename:
+    tempfile = Tempfile.new(['thumbnail-upload', File.extname(filename)])
+    tempfile.binmode
+    tempfile.write(bytes)
+    tempfile.rewind
+    upload = ActionDispatch::Http::UploadedFile.new(
+        tempfile:,
+        filename:,
+        type: content_type)
+    yield upload
+  ensure
+    tempfile&.close!
+  end
+
+  it 'accepts a raster upload after decoding and rewinds it' do
+    gif = Base64.decode64('R0lGODdhAQABAIABAP///wAAACwAAAAAAQABAAACAkQBADs=')
+
+    with_upload(gif, content_type: 'image/gif', filename: 'thumbnail.gif') do |upload|
+      expect { described_class.validate!(upload) }.not_to raise_error
+      expect(upload.read(6)).to eq('GIF87a')
+    end
+  end
+
+  it 'rejects SVG content disguised as a raster MIME type' do
+    with_upload(
+        '<svg width="10" height="10"></svg>',
+        content_type: 'image/png',
+        filename: 'thumbnail.png') do |upload|
+      expect { described_class.validate!(upload) }
+          .to raise_error(
+              described_class::InvalidUpload,
+              'サムネイル画像の形式が不正です．')
+    end
+  end
+
+  it 'rejects non-raster content disguised as an image' do
+    with_upload(
+        '%PDF-1.7',
+        content_type: 'image/png',
+        filename: 'thumbnail.png') do |upload|
+      expect { described_class.validate!(upload) }
+          .to raise_error(
+              described_class::InvalidUpload,
+              'サムネイル画像の形式が不正です．')
+    end
+  end
+end

backend/spec/services/preview/thumbnail_fetcher_spec.rb

@@ -2,7 +2,7 @@ require 'rails_helper'
 
 RSpec.describe Preview::ThumbnailFetcher do
   describe '.fetch' do
-    it 'rejects svg thumbnails' do
+    it 'accepts svg thumbnails for the common safe rasterisation path' do
       page = Preview::HttpFetcher::Response.new(
           '<meta property="og:image" content="https://example.com/thumb.svg">',
           'text/html',
@@ -12,8 +12,9 @@ RSpec.describe Preview::ThumbnailFetcher do
           'image/svg+xml',
           'https://example.com/thumb.svg')
 
-      allow(Preview::UrlSafety).to receive(:validate)
-          .and_return([URI.parse('https://example.com/page'), ['203.0.113.10']])
+      allow(Preview::UrlSafety).to receive(:validate) do |url|
+        [URI.parse(url), ['203.0.113.10']]
+      end
       allow(Preview::HttpFetcher).to receive(:fetch)
           .with('https://example.com/page', max_bytes: described_class::HTML_MAX_BYTES)
           .and_return(page)
@@ -21,9 +22,7 @@ RSpec.describe Preview::ThumbnailFetcher do
           .with('https://example.com/thumb.svg')
           .and_return(svg)
 
-      expect {
-        described_class.fetch('https://example.com/page')
-      }.to raise_error(Preview::ThumbnailFetcher::GenerationFailed)
+      expect(described_class.fetch('https://example.com/page')).to eq('<svg></svg>')
     end
 
     it 'accepts allowed image content type with parameters' do
@@ -32,12 +31,13 @@ RSpec.describe Preview::ThumbnailFetcher do
           'text/html',
           'https://example.com/page')
       image = Preview::HttpFetcher::Response.new(
-          'jpeg-bytes',
+          "\xFF\xD8\xFFjpeg-bytes".b,
           'image/jpeg; charset=binary',
           'https://example.com/thumb.jpg')
 
-      allow(Preview::UrlSafety).to receive(:validate)
-          .and_return([URI.parse('https://example.com/page'), ['203.0.113.10']])
+      allow(Preview::UrlSafety).to receive(:validate) do |url|
+        [URI.parse(url), ['203.0.113.10']]
+      end
       allow(Preview::HttpFetcher).to receive(:fetch)
           .with('https://example.com/page', max_bytes: described_class::HTML_MAX_BYTES)
           .and_return(page)
@@ -45,7 +45,8 @@ RSpec.describe Preview::ThumbnailFetcher do
           .with('https://example.com/thumb.jpg')
           .and_return(image)
 
-      expect(described_class.fetch('https://example.com/page')).to eq('jpeg-bytes')
+      expect(described_class.fetch('https://example.com/page'))
+          .to eq("\xFF\xD8\xFFjpeg-bytes".b)
     end
   end
 

backend/spec/services/youtube/sync_spec.rb

@@ -268,6 +268,10 @@ RSpec.describe Youtube::Sync do
       expect(tag_ids).to include(deerjikist_tag.id)
       expect(tag_ids).not_to include(Tag.no_deerjikist.id)
 
+      expect(PostTag.exists?(post:, tag: Tag.no_deerjikist)).to be(false)
+      expect(Tag.no_deerjikist.reload.post_count).to eq(0)
+      expect(deerjikist_tag.reload.post_count).to eq(1)
+
       expect(PostVersionRecorder).to have_received(:ensure_snapshot!).with(
         post,
         created_by_user: nil

backend/spec/tasks/nico_sync_spec.rb

@@ -1,36 +1,35 @@
-require "rails_helper"
+require 'rails_helper'
 
 
-RSpec.describe "nico:sync" do
+RSpec.describe 'nico:sync' do
   def stub_python(json_array)
     status = instance_double(Process::Status, success?: true)
-    allow(Open3).to receive(:capture3).and_return([json_array.to_json, "", status])
+    allow(Open3).to receive(:capture3).and_return([json_array.to_json, '', status])
   end
 
   def create_tag!(name, category:)
-    tn = TagName.find_undiscard_or_create_by!(name: name.to_s.strip)
-    Tag.find_undiscard_or_create_by!(tag_name_id: tn.id) { |t| t.category = category }
+    Tag.find_or_create_by_tag_name!(name, category:)
   end
 
   def link_nico_to_tag!(nico_tag, tag)
     NicoTagRelation.create!(nico_tag_id: nico_tag.id, tag_id: tag.id)
   end
 
-  it "既存 post を見つけて、nico tag と linked tag を追加し、差分が出たら bot を付ける" do
+  it '既存 post を見つけて、nico tag と linked tag を追加し、差分が出たら bot を付ける' do
     # 既存 post（正規表現で拾われるURL）
     post = Post.create!(
-      title: "old",
-      url: "https://www.nicovideo.jp/watch/sm9",
+      title: 'old',
+      url: 'https://www.nicovideo.jp/watch/sm9',
       uploaded_user: nil
     )
 
     # 既存の非nicoタグ（kept_non_nico_ids）
-    kept_general = create_tag!("spec_kept", category: "general")
+    kept_general = create_tag!('spec_kept', category: 'general')
     PostTag.create!(post: post, tag: kept_general)
 
     # 追加される linked tag を準備（nico tag に紐付く一般タグ）
-    linked = create_tag!("spec_linked", category: "general")
-    nico = create_tag!("nico:AAA", category: "nico")
+    linked = create_tag!('spec_linked', category: 'general')
+    nico = create_external_tag!('AAA')
     link_nico_to_tag!(nico, linked)
 
     # bot / tagme は task 内で使うので作っておく（Tag.bot/tagme がある前提）
@@ -46,22 +45,22 @@ RSpec.describe "nico:sync" do
         'deleted_at' => '2026-01-31 00:00:00' }])
 
     # 外部HTTPは今回「既存 post なので呼ばれない」はずだが、念のため塞ぐ
-    allow(URI).to receive(:open).and_return(StringIO.new("<html></html>"))
+    allow(URI).to receive(:open).and_return(StringIO.new('<html></html>'))
 
-    run_rake_task("nico:sync")
+    run_rake_task('nico:sync')
 
     post.reload
-    active_tag_names = post.tags.joins(:tag_name).pluck("tag_names.name")
+    active_tag_names = post.tags.joins(:tag_name).pluck('tag_names.name')
 
-    expect(active_tag_names).to include("spec_kept")
-    expect(active_tag_names).to include("nico:AAA")
-    expect(active_tag_names).to include("spec_linked")
+    expect(active_tag_names).to include('spec_kept')
+    expect(post.external_tags).to contain_exactly(nico)
+    expect(active_tag_names).to include('spec_linked')
 
     expect(post.original_created_from).to eq(Time.iso8601('2026-01-01T03:34:00Z'))
     expect(post.original_created_before).to eq(Time.iso8601('2026-01-01T03:35:00Z'))
 
     # 差分が出るので bot が付く（kept_non_nico_ids != desired_non_nico_ids）
-    expect(active_tag_names).to include("bot操作")
+    expect(active_tag_names).to include('bot操作')
   end
 
   it '既存 post のサムネール取得に共通 attach 経路を使ふ' do
@@ -73,11 +72,11 @@ RSpec.describe "nico:sync" do
     Tag.tagme
 
     stub_python([{ 'code' => 'sm9', 'title' => 't', 'tags' => [] }])
-    allow(URI).to receive(:open)
-        .and_return(
-            StringIO.new(
-                '<meta name="thumbnail" content="https://example.com/thumb.jpg">'))
-    expect(post).to receive(:attach_thumbnail_from_url!)
+    allow(URI).to receive(:open) do
+      StringIO.new(
+          '<meta name="thumbnail" content="https://example.com/thumb.jpg">')
+    end
+    expect_any_instance_of(Post).to receive(:attach_thumbnail_from_url!)
         .with('https://example.com/thumb.jpg')
 
     run_rake_task('nico:sync')
@@ -92,53 +91,61 @@ RSpec.describe "nico:sync" do
     Tag.tagme
 
     stub_python([{ 'code' => 'sm9', 'title' => 't', 'tags' => [] }])
-    allow(URI).to receive(:open)
-        .and_return(
-            StringIO.new(
-                '<meta name="thumbnail" content="https://example.com/thumb.jpg">'))
-    expect(post).to receive(:attach_thumbnail_from_url!)
-        .with('https://example.com/thumb.jpg')
-        .twice
-        .and_raise(Post::RemoteThumbnailFetchFailed, 'failed')
+    allow(URI).to receive(:open) do
+      StringIO.new(
+          '<meta name="thumbnail" content="https://example.com/thumb.jpg">')
+    end
+    calls = 0
+    allow_any_instance_of(Post).to receive(:attach_thumbnail_from_url!) do
+      calls += 1
+      raise Post::RemoteThumbnailFetchFailed, 'failed'
+    end
 
     2.times do
       run_rake_task('nico:sync')
     end
+    expect(calls).to eq(2)
   end
 
-  it "既存 post にあった古い nico tag は active から外され、履歴として discard される" do
+  it '古い nico tag の関連を物理削除し、変更前後の履歴を version に残す' do
     post = Post.create!(
-      title: "old",
-      url: "https://www.nicovideo.jp/watch/sm9",
+      title: 'old',
+      url: 'https://www.nicovideo.jp/watch/sm9',
       uploaded_user: nil
     )
 
     # 旧nicoタグ（今回の同期結果に含まれない）
-    old_nico = create_tag!("nico:OLD", category: "nico")
-    old_pt = PostTag.create!(post: post, tag: old_nico)
-    expect(old_pt.discarded_at).to be_nil
+    old_nico = create_external_tag!('OLD')
+    PostExternalTag.create!(post:, external_tag: old_nico)
+    create_post_version_for!(post)
 
     # 今回は NEW のみ欲しい
-    new_nico = create_tag!("nico:NEW", category: "nico")
+    new_nico = create_external_tag!('NEW')
 
     # bot/tagme 念のため
     Tag.bot
     Tag.tagme
 
-    stub_python([{ "code" => "sm9", "title" => "t", "tags" => ["NEW"] }])
-    allow(URI).to receive(:open).and_return(StringIO.new("<html></html>"))
+    stub_python([{ 'code' => 'sm9', 'title' => 't', 'tags' => ['NEW'] }])
+    allow(URI).to receive(:open).and_return(StringIO.new('<html></html>'))
 
-    run_rake_task("nico:sync")
+    run_rake_task('nico:sync')
 
-    # OLD は active から外れる（discarded_at が入る）
-    old_pts = PostTag.where(post_id: post.id, tag_id: old_nico.id).order(:id).to_a
-    expect(old_pts.last.discarded_at).to be_present
+    expect(PostExternalTag.exists?(post:, external_tag: old_nico)).to be(false)
+    expect(old_nico.reload.post_count).to eq(0)
+    expect(new_nico.reload.post_count).to eq(1)
+
+    versions = post.post_versions.order(:version_no)
+    expect(versions.first.tags_json.filter_map { |item| item['external_tag_id'] })
+        .to include(old_nico.id)
+    expect(versions.last.tags_json.filter_map { |item| item['external_tag_id'] })
+        .to include(new_nico.id)
+    expect(versions.last.tags_json.filter_map { |item| item['external_tag_id'] })
+        .not_to include(old_nico.id)
 
     # NEW は active にいる
     post.reload
-    active_names = post.tags.joins(:tag_name).pluck("tag_names.name")
-    expect(active_names).to include("nico:NEW")
-    expect(active_names).not_to include("nico:OLD")
+    expect(post.external_tags).to contain_exactly(new_nico)
   end
 
   def snapshot_tags(post)
@@ -154,6 +161,7 @@ RSpec.describe "nico:sync" do
       url: post.url,
       thumbnail_base: post.thumbnail_base,
       tags: snapshot_tags(post),
+      tags_json: post.snapshot_tags_json,
       parent_post_ids: post.snapshot_parent_post_ids.join(' '),
       original_created_from: post.original_created_from,
       original_created_before: post.original_created_before,
@@ -203,7 +211,7 @@ RSpec.describe "nico:sync" do
     create_post_version_for!(post)
 
     linked = create_tag!('spec_linked', category: 'general')
-    nico = create_tag!('nico:AAA', category: 'nico')
+    nico = create_external_tag!('AAA')
     link_nico_to_tag!(nico, linked)
 
     Tag.bot
@@ -231,7 +239,7 @@ RSpec.describe "nico:sync" do
   end
 
   it '既存 post に差分が無いときは新しい version を作らない' do
-    nico = create_tag!('nico:AAA', category: 'nico')
+    nico = create_external_tag!('AAA')
     no_deerjikist = create_tag!('ニジラー情報不詳', category: 'meta')
 
     post = Post.create!(
@@ -242,7 +250,7 @@ RSpec.describe "nico:sync" do
       original_created_before: Time.iso8601('2026-01-01T03:35:00Z')
     )
 
-    PostTag.create!(post: post, tag: nico)
+    PostExternalTag.create!(post:, external_tag: nico)
     PostTag.create!(post: post, tag: no_deerjikist)
     create_post_version_for!(post)
 
@@ -285,7 +293,7 @@ RSpec.describe "nico:sync" do
       run_rake_task('nico:sync')
     }.to change(NicoTagVersion, :count).by(1)
 
-    nico_tag = Tag.joins(:tag_name).find_by!(tag_names: { name: 'nico:AAA' })
+    nico_tag = ExternalTag.find_by!(platform: :nico, name: 'AAA')
     version = nico_tag.nico_tag_versions.order(:version_no).last
 
     expect(version.version_no).to eq(1)
@@ -368,4 +376,148 @@ RSpec.describe "nico:sync" do
     expect(versions.second.title).to eq('changed title')
     expect(versions.second.tags).to eq(snapshot_tags(post.reload))
   end
+
+  def create_external_tag!(name)
+    ExternalTag.create!(platform: :nico, name:)
+  end
+
+  def create_nico_sync_post!
+    post = Post.create!(
+      title: 't',
+      url: 'https://www.nicovideo.jp/watch/sm9',
+      uploaded_user: nil
+    )
+
+    PostTag.create!(post:, tag: Tag.no_deerjikist)
+
+    post
+  end
+
+  def run_nico_sync_with_tags! tags
+    stub_python([{
+      'code' => 'sm9',
+      'title' => 't',
+      'tags' => tags,
+      'user' => nil
+    }])
+
+    allow(URI).to receive(:open).and_return(StringIO.new('<html></html>'))
+
+    run_rake_task('nico:sync')
+  end
+
+  it '外部タグだけの変更では bot を付けず，投稿履歴を記録する' do
+    post = create_nico_sync_post!
+    PostVersionRecorder.record!(post:, event_type: :create, created_by_user: nil)
+
+    expect {
+      run_nico_sync_with_tags!(['raw tag[]', 'raw tag[]'])
+    }.to change(PostVersion, :count).by(1)
+        .and change(ExternalTag, :count).by(1)
+        .and change(PostExternalTag, :count).by(1)
+        .and change(NicoTagVersion, :count).by(1)
+        .and change(TagName, :count).by(0)
+
+    external = post.external_tags.sole
+    expect(external.name).to eq('raw tag[]')
+    expect(post.tags.map(&:name)).not_to include('bot操作')
+    expect(post.post_versions.order(:version_no).last.tags_json)
+        .to include('external_tag_id' => external.id)
+
+    expect {
+      run_nico_sync_with_tags!(['raw tag[]'])
+    }.to change(PostVersion, :count).by(0).and change(NicoTagVersion, :count).by(0)
+
+    expect {
+      run_nico_sync_with_tags!([])
+    }.to change(PostVersion, :count).by(1)
+    expect(post.reload.external_tags).to be_empty
+    expect(post.tags.map(&:name)).not_to include('bot操作')
+  end
+
+  it '外部タグが新規記載されたとき，その時点の連携タグを記載する' do
+    post = create_nico_sync_post!
+
+    external_tag = create_external_tag!('AAA')
+    linked_tag = create_tag!('spec_linked', category: :general)
+    link_nico_to_tag!(external_tag, linked_tag)
+
+    run_nico_sync_with_tags!(['AAA'])
+
+    expect(post.reload.external_tags).to include(external_tag)
+    expect(post.tags).to include(linked_tag)
+  end
+
+  it '外部タグに差分がない場合，内外マッピングが変はっても連携タグを再評価しない' do
+    post = create_nico_sync_post!
+
+    external_tag = create_external_tag!('AAA')
+    old_linked_tag = create_tag!('spec_old_linked', category: :general)
+    new_linked_tag = create_tag!('spec_new_linked', category: :general)
+
+    relation = link_nico_to_tag!(external_tag, old_linked_tag)
+
+    run_nico_sync_with_tags!(['AAA'])
+
+    expect(post.reload.tags).to include(old_linked_tag)
+
+    # 人手で連携タグを消除する．
+    PostTag.find_by!(post:, tag: old_linked_tag).destroy!
+
+    # 内外マッピングを変更する．
+    relation.destroy!
+    link_nico_to_tag!(external_tag, new_linked_tag)
+
+    # 外部タグ自体には差分が無い．
+    run_nico_sync_with_tags!(['AAA'])
+
+    post.reload
+
+    expect(post.external_tags).to include(external_tag)
+    expect(post.tags).not_to include(old_linked_tag)
+    expect(post.tags).not_to include(new_linked_tag)
+  end
+
+  it '外部タグの消除では連携タグを消除せず，再記載時にその時点の内外マッピングを適用する' do
+    post = create_nico_sync_post!
+
+    external_tag = create_external_tag!('AAA')
+    old_linked_tag = create_tag!('spec_old_linked', category: :general)
+    new_linked_tag = create_tag!('spec_new_linked', category: :general)
+
+    relation = link_nico_to_tag!(external_tag, old_linked_tag)
+
+    # 外部タグを新規記載する．
+    run_nico_sync_with_tags!(['AAA'])
+
+    post.reload
+    expect(post.external_tags).to include(external_tag)
+    expect(post.tags).to include(old_linked_tag)
+
+    # 外部タグを消除する．
+    run_nico_sync_with_tags!([])
+
+    post.reload
+    expect(post.external_tags).not_to include(external_tag)
+
+    # 外部タグの消除によって連携タグまでは消除されない．
+    expect(post.tags).to include(old_linked_tag)
+
+    # 外部タグが記載されてゐない間に内外マッピングを変更する．
+    relation.destroy!
+    link_nico_to_tag!(external_tag, new_linked_tag)
+
+    # 同じ外部タグを再記載する．
+    run_nico_sync_with_tags!(['AAA'])
+
+    post.reload
+
+    expect(post.external_tags).to include(external_tag)
+
+    # 旧連携タグは自動的には消除されない．
+    expect(post.tags).to include(old_linked_tag)
+
+    # 再記載時点の内外マッピングが新たに適用される．
+    expect(post.tags).to include(new_linked_tag)
+  end
 end

frontend/AGENTS.md

@@ -153,6 +153,9 @@ pass or the remaining failure is clearly blocked.
 - When new user-facing wording appears necessary, ask the user for the exact
   wording and placement before implementing it.
 - Do not invent replacement copy when removing unrequested wording.
+- Do not use `タグなし` as user-facing copy for an empty tag state. When the
+  tag state is empty, show no copy. If actual data contains the tag name
+  `タグなし`, treat it as ordinary data and display it normally.
 - When adding dynamic tag colour classes, update `tailwind.config.js` safelist
   if the class cannot be statically detected.
 - Do not introduce new UI libraries or production dependencies without approval.
@@ -599,6 +602,12 @@ offsets, or footer offsets, inspect existing layout components such as
 Do not create a second layout shell before checking whether the current layout
 can be reused or minimally extended.
 
+- Frontend のスマホ／PC表示境界は原則 `md` とする。
+- button stack、footer action、dialogue action は `md` 未満で縦並び、
+  `md` 以上で横並びとする。
+- 同じ画面内で `sm` と `md` を混在させて中間 layout を作らない。
+- 明確に別の responsive 要件がある component だけを例外とする。
+
 ### Delimiter decision table
 
 Use this table before accepting any edited TypeScript or TSX hunk. The table is

frontend/src/App.tsx

@@ -1,8 +1,9 @@
 import { AnimatePresence, LayoutGroup, MotionConfig, motion } from 'framer-motion'
 import { Fragment, useEffect, useMemo, useState } from 'react'
-import { BrowserRouter,
+import { createBrowserRouter,
 	 Navigate,
 	 Route,
+	 RouterProvider,
 	 Routes,
 	 useLocation } from 'react-router-dom'
 
@@ -154,7 +155,7 @@ const PostDetailRoute = ({ user }: { user: User | null }) => {
 }
 
 
-const App: FC = () => {
+const RoutedApp: FC = () => {
   const [user, setUser] = useState<User | null> (null)
   const [status, setStatus] = useState (200)
   const behaviourSettings = useClientBehaviourSettings ()
@@ -253,42 +254,46 @@ const App: FC = () => {
   }
 
   return (
-      <>
-	<RouteBlockerOverlay/>
-	{import.meta.env.DEV && <DevModeWatermark/>}
+      <DialogueProvider>
+	<UnsavedChangesGuardProvider>
+	  <KeyboardShortcutsProvider>
+	  <MotionConfig
+	      reducedMotion={
+		  animationMode === 'normal'
+		  ? 'never'
+		  : animationMode === 'reduced'
+		    ? 'user'
+		    : 'always'
+	      }>
+	    <LayoutWrapper>
+	      <motion.div
+		  layout={animationMode === 'off' ? false : 'position'}
+		  transition={{ layout: appLayoutTransition }}
+		  className="relative flex h-dvh w-full flex-col overflow-y-hidden">
+		<TopNav user={user}/>
+		<RouteTransitionWrapper
+		    animationMode={animationMode}
+		    user={user}
+		    setUser={setUser}/>
+	      </motion.div>
+	    </LayoutWrapper>
+	  </MotionConfig>
 
-	<BrowserRouter>
-	  <DialogueProvider>
-	    <UnsavedChangesGuardProvider>
-	      <KeyboardShortcutsProvider>
-	      <MotionConfig
-		  reducedMotion={
-		      animationMode === 'normal'
-		      ? 'never'
-		      : animationMode === 'reduced'
-			? 'user'
-			: 'always'
-		  }>
-		<LayoutWrapper>
-		  <motion.div
-		      layout={animationMode === 'off' ? false : 'position'}
-		      transition={{ layout: appLayoutTransition }}
-		      className="relative flex flex-col h-dvh w-full overflow-y-hidden">
-		    <TopNav user={user}/>
-		    <RouteTransitionWrapper
-			animationMode={animationMode}
-			user={user}
-			setUser={setUser}/>
-		  </motion.div>
-		</LayoutWrapper>
-	      </MotionConfig>
-
-	      <Toaster/>
-	      </KeyboardShortcutsProvider>
-	    </UnsavedChangesGuardProvider>
-	  </DialogueProvider>
-	</BrowserRouter>
-      </>)
+	  <Toaster/>
+	  </KeyboardShortcutsProvider>
+	</UnsavedChangesGuardProvider>
+      </DialogueProvider>)
 }
 
+const router = createBrowserRouter ([{
+    path: '*',
+    element: <RoutedApp/> }])
+
+const App: FC = () => (
+    <>
+      <RouteBlockerOverlay/>
+      {import.meta.env.DEV && <DevModeWatermark/>}
+      <RouterProvider router={router}/>
+    </>)
+
 export default App

frontend/src/components/DraggableDroppableTagRow.test.tsx

@@ -13,11 +13,14 @@ vi.mock ('@dnd-kit/core', () => dndKit)
 
 const tag = buildTag ({ id: 7, name: 'ドラッグ元', postCount: 3 })
 
-const renderRow = (activeDndId?: string) => {
+const renderRow = (
+    activeDndId?: string,
+    renderedTag = tag,
+) => {
   renderWithProviders (
       <DraggableDroppableTagRow
           activeDndId={activeDndId}
-          tag={tag}
+          tag={renderedTag}
           nestLevel={2}
           pathKey="cat-general-7"
           suppressClickRef={{ current: false }}/>,
@@ -72,4 +75,26 @@ describe ('DraggableDroppableTagRow', () => {
     renderRow ('tag-node:other')
     expect (tagBody ()).toHaveStyle ({ visibility: 'visible' })
   })
+
+  it ('disables drag and drop for external tags', () => {
+    const external = buildTag ({
+      id:       7,
+      name:     'nico:external',
+      category: 'nico',
+    })
+
+    renderRow (undefined, external)
+
+    expect (dndKit.useDraggable).toHaveBeenCalledWith (
+	expect.objectContaining ({
+	  disabled: true,
+	}),
+    )
+
+    expect (dndKit.useDroppable).toHaveBeenCalledWith (
+	expect.objectContaining ({
+	  disabled: true,
+	}),
+    )
+  })
 })

frontend/src/components/PostEditForm.test.tsx

@@ -78,19 +78,23 @@ describe ('PostEditForm', () => {
 
     render (<PostEditForm post={post} onSave={vi.fn ()}/>)
 
-    expect (screen.getByPlaceholderText ('例: 2 / 2.5 / 1:23')).toHaveValue ('180.5')
+    expect (screen.getByText ('動画時間').parentElement?.querySelector ('input'))
+	.toHaveValue ('180.5')
 
     const tags = screen.getAllByRole ('textbox')[2]
     fireEvent.change (tags, { target: { value: 'general-tag' } })
-    expect (screen.queryByPlaceholderText ('例: 2 / 2.5 / 1:23')).not.toBeInTheDocument ()
+    expect (screen.queryByText ('動画時間')).not.toBeInTheDocument ()
 
     fireEvent.change (tags, {
       target: { value: '動画 general-tag' },
     })
-    expect (screen.getByPlaceholderText ('例: 2 / 2.5 / 1:23')).toHaveValue ('180.5')
+    expect (screen.getByText ('動画時間').parentElement?.querySelector ('input'))
+	.toHaveValue ('180.5')
   })
 
-  it ('shows deduplicated original-created endpoint errors on the shared datetime field', async () => {
+  it (
+      'shows deduplicated original-created endpoint errors on the shared datetime field',
+      async () => {
     const post = buildPost ()
     api.isApiError.mockReturnValue (true)
     postsApi.updatePost.mockRejectedValueOnce ({
@@ -112,5 +116,5 @@ describe ('PostEditForm', () => {
     expect (await screen.findByText ('日時を確認してください．')).toBeInTheDocument ()
     expect (screen.getByText ('終了を確認してください．')).toBeInTheDocument ()
     expect (screen.getAllByText ('日時を確認してください．')).toHaveLength (1)
-  })
+      })
 })

frontend/src/components/PostOriginalCreatedTimeField.test.tsx

@@ -73,7 +73,7 @@ describe ('PostOriginalCreatedTimeField', () => {
             setOriginalCreatedBefore={vi.fn ()}/>,
     )
 
-    const input = screen.getDisplayValue ('2024-01-01T12:34')
+    const input = screen.getByDisplayValue ('2024-01-01T12:34')
     fireEvent.change (input, { target: { value: '2024-01-01T12:35' } })
 
     expect (setFrom).toHaveBeenCalledWith ('2024-01-01T03:35Z')

frontend/src/components/PostOriginalCreatedTimeField.tsx

@@ -24,10 +24,10 @@ const PostOriginalCreatedTimeField: FC<Props> = (
       <FormField label="オリジナルの作成日時" messages={errors}>
 	  {({ describedBy, invalid }) => (
 	      <>
-		<div className="my-1 flex flex-col gap-2 sm:flex-row sm:items-start">
-		  <div className="min-w-0 flex-1">
+		<div className="my-1 flex">
+		  <div className="w-80">
 		    <DateTimeField
-			className="w-full"
+			className="mr-2"
 			disabled={disabled ?? false}
 			aria-describedby={describedBy}
 			aria-invalid={invalid}
@@ -61,10 +61,10 @@ const PostOriginalCreatedTimeField: FC<Props> = (
 		  </div>
 		</div>
 
-		<div className="my-1 flex flex-col gap-2 sm:flex-row sm:items-start">
-		  <div className="min-w-0 flex-1">
+		<div className="my-1 flex">
+		  <div className="w-80">
 		    <DateTimeField
-			className="w-full"
+			className="mr-2"
 			disabled={disabled}
 			aria-describedby={describedBy}
 			aria-invalid={invalid}

frontend/src/components/PrefetchLink.tsx

@@ -6,7 +6,6 @@ import { createPath, useNavigate } from 'react-router-dom'
 import { useOverlayStore } from '@/components/RouteBlockerOverlay'
 import { prefetchForURL } from '@/lib/prefetchers'
 import { useClientBehaviourSettings } from '@/lib/useClientBehaviourSettings'
-import { useUnsavedChangesGuard } from '@/lib/useUnsavedChangesGuard'
 import { cn } from '@/lib/utils'
 
 import type { AnchorHTMLAttributes, MouseEvent, TouchEvent } from 'react'
@@ -36,7 +35,6 @@ export default forwardRef<HTMLAnchorElement, Props> (({
   const navigate = useNavigate ()
   const qc = useQueryClient ()
   const behaviourSettings = useClientBehaviourSettings ()
-  const { confirmDiscardNavigation } = useUnsavedChangesGuard ()
   const linkPreloadMode = behaviourSettings.linkPreload ?? 'intent'
   const path = useMemo (
       () => typeof to === 'string' ? to : createPath (to),
@@ -45,10 +43,6 @@ export default forwardRef<HTMLAnchorElement, Props> (({
   const url = useMemo (() => {
     return (new URL (path, window.location.origin)).toString ()
   }, [path])
-  const nextPathname = useMemo (
-      () => (new URL (path, window.location.origin)).pathname,
-      [path],
-  )
   const setOverlay = useOverlayStore (s => s.setActive)
 
   const doPrefetch = async () => {
@@ -93,13 +87,6 @@ export default forwardRef<HTMLAnchorElement, Props> (({
 
       ev.preventDefault ()
 
-      if (nextPathname !== window.location.pathname)
-	{
-	  const confirmed = await confirmDiscardNavigation ()
-	  if (!(confirmed))
-	    return
-	}
-
       flushSync (() => {
 	setOverlay (true)
       })

frontend/src/components/TagDetailSidebar.test.tsx

@@ -0,0 +1,66 @@
+import { screen } from '@testing-library/react'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import TagDetailSidebar from '@/components/TagDetailSidebar'
+import { setClientTagRelationDisplayMode } from '@/lib/settings'
+import { buildPost, buildTag } from '@/test/factories'
+import { renderWithProviders } from '@/test/render'
+
+import type { ReactNode } from 'react'
+
+vi.mock ('@/components/TagSearch', () => ({
+  default: () => null,
+}))
+
+vi.mock ('@/components/DraggableDroppableTagRow', () => ({
+  default: ({ tag }: { tag: { name: string } }) => (
+      <span>{tag.name}</span>
+  ),
+}))
+
+vi.mock ('@dnd-kit/core', () => ({
+  DndContext: ({ children }: { children: ReactNode }) => <>{children}</>,
+  DragOverlay: ({ children }: { children: ReactNode }) => <>{children}</>,
+  MeasuringStrategy: { Always: 'always' },
+  MouseSensor: vi.fn (),
+  TouchSensor: vi.fn (),
+  pointerWithin: vi.fn (),
+  useDroppable: vi.fn (() => ({
+    setNodeRef: vi.fn (),
+    isOver:     false,
+  })),
+  useSensor: vi.fn (() => ({ })),
+  useSensors: vi.fn (() => []),
+}))
+
+describe ('TagDetailSidebar', () => {
+  beforeEach (() => {
+    localStorage.clear ()
+    vi.clearAllMocks ()
+  })
+
+  it ('keeps internal and external tags with the same numeric id in flat mode', () => {
+    setClientTagRelationDisplayMode ('flat')
+
+    const internal = buildTag ({
+      id:       7,
+      name:     'internal_collision',
+      category: 'general',
+    })
+    const external = buildTag ({
+      id:       7,
+      name:     'nico:external_collision',
+      category: 'nico',
+    })
+
+    renderWithProviders (
+        <TagDetailSidebar
+            post={buildPost ({
+                tags: [internal, external],
+            })}/>,
+    )
+
+    expect (screen.getByText ('internal_collision')).toBeInTheDocument ()
+    expect (screen.getByText ('nico:external_collision')).toBeInTheDocument ()
+  })
+})

frontend/src/components/TagLink.test.tsx

@@ -57,4 +57,31 @@ describe ('TagLink', () => {
     expect (screen.getByText ('正式名')).toBeInTheDocument ()
     expect (screen.queryByRole ('link')).not.toBeInTheDocument ()
   })
+
+  it ('does not show a missing-information marker for external tags', () => {
+    renderWithProviders (
+	<TagLink
+	    tag={buildTag ({
+		id:             7,
+		name:           'nico:external',
+		category:       'nico',
+		hasWiki:        false,
+		materialId:     null,
+		hasDeerjikists: false,
+	    })}
+	    withCount={false}/>,
+    )
+
+    expect (
+	screen.getByRole ('link', { name: 'nico:external' }),
+    ).toBeInTheDocument ()
+
+    expect (
+	screen.queryByRole ('link', { name: '!' }),
+    ).not.toBeInTheDocument ()
+
+    expect (
+	screen.queryByTitle ('nico:external Wiki が存在しません．'),
+    ).not.toBeInTheDocument ()
+  })
 })

frontend/src/components/TagLink.tsx

@@ -4,9 +4,13 @@ import { cn } from '@/lib/utils'
 
 import type { ComponentProps, CSSProperties, FC, HTMLAttributes } from 'react'
 
-import type { Tag } from '@/types'
+import type { Category, Tag } from '@/types'
 
-type CommonProps = { 
+type LightweightTag = {
+    name:     string
+    category: Category }
+
+type FullCommonProps = {
     tag:        Tag
     nestLevel?: number
     truncateOnMobile?: boolean
@@ -14,18 +18,43 @@ type CommonProps = {
     withCount?: boolean }
 
 type PropsWithLink =
-    & CommonProps
+    & FullCommonProps
     & { linkFlg?: true }
     & Partial<ComponentProps<typeof PrefetchLink>>
 
 type PropsWithoutLink =
-    & CommonProps
+    & FullCommonProps
     & { linkFlg: false }
     & Partial<HTMLAttributes<HTMLSpanElement>>
 
+type LightweightPropsWithLink =
+    & {
+	tag:              LightweightTag
+	nestLevel?:       number
+	truncateOnMobile?: boolean
+	withWiki:         false
+	withCount:        false
+	linkFlg?:         true }
+    & Partial<ComponentProps<typeof PrefetchLink>>
+
+type LightweightPropsWithoutLink =
+    & {
+	tag:              LightweightTag
+	nestLevel?:       number
+	truncateOnMobile?: boolean
+	withWiki:         false
+	withCount:        false
+	linkFlg:          false }
+    & Partial<HTMLAttributes<HTMLSpanElement>>
+
 type Props =
     | PropsWithLink
     | PropsWithoutLink
+    | LightweightPropsWithLink
+    | LightweightPropsWithoutLink
+
+const isFullTag = (tag: Tag | LightweightTag): tag is Tag =>
+    'id' in tag
 
 
 const TagLink: FC<Props> = ({ tag,
@@ -46,16 +75,18 @@ const TagLink: FC<Props> = ({ tag,
   const spanClass = 'tag-link-colour'
   const linkClass = 'tag-link-colour tag-link-hover-colour'
   const textClass = 'group min-w-0 max-w-full overflow-hidden align-bottom'
-  const rootClass =
-      'inline-flex min-w-0 max-w-full flex-nowrap items-stretch align-baseline gap-x-1 md:items-baseline'
   const markerWrapClass = 'shrink-0 self-start md:self-auto'
   const countClass = 'shrink-0 self-end md:self-auto'
+  const matchedAlias = isFullTag (tag) ? tag.matchedAlias : null
   const textTitle = title
-      ?? (tag.matchedAlias == null ? tag.name : `${ tag.matchedAlias } → ${ tag.name }`)
+      ?? (matchedAlias == null ? tag.name : `${ matchedAlias } → ${ tag.name }`)
 
   return (
-      <span className={rootClass}>
-	{(linkFlg && withWiki) && (
+      <span
+	  className={cn (
+	      'inline-flex min-w-0 max-w-full flex-nowrap items-stretch align-baseline',
+	      'gap-x-1 md:items-baseline')}>
+	{(linkFlg && withWiki && isFullTag (tag)) && (
 	    <span className={markerWrapClass}>
 	      {(tag.materialId != null || tag.hasWiki || tag.hasDeerjikists)
 	       ? (
@@ -118,7 +149,7 @@ const TagLink: FC<Props> = ({ tag,
 		style={{ paddingLeft: `${ (nestLevel - 1) }rem` }}>
 	      ↳
 	    </span>)}
-	{tag.matchedAlias != null && (
+	{matchedAlias != null && (
 	    <>
 	      <span
 		  title={textTitle}
@@ -126,7 +157,7 @@ const TagLink: FC<Props> = ({ tag,
 		  style={colourStyle}
 		  {...props}>
 		<ResponsiveMarqueeText
-		    text={tag.matchedAlias}
+		    text={matchedAlias}
 		    title={textTitle}
 		    truncateOnMobile={truncateOnMobile}/>
 	      </span>
@@ -156,7 +187,7 @@ const TagLink: FC<Props> = ({ tag,
 		  title={textTitle}
 		  truncateOnMobile={truncateOnMobile}/>
 	    </span>)}
-	{withCount && (
+	{(withCount && isFullTag (tag)) && (
 	    <span className={countClass}>{tag.postCount}</span>)}
       </span>)
 }

frontend/src/components/TopNav.test.tsx

@@ -17,21 +17,19 @@ describe ('menuOutline', () => {
     for (const role of ['member', 'admin'] as const)
       {
 	expect (submenuItem (role, '広場', '追加')?.visible).toBe (true)
-	expect (submenuItem (role, '広場', '取込')?.visible).toBe (true)
 	expect (submenuItem (role, '素材', '追加')?.visible).toBe (true)
 	expect (submenuItem (role, 'Wiki', '新規')?.visible).toBe (true)
 	expect (submenuItem (role, 'Wiki', '編輯')?.visible).toBe (true)
       }
 
     expect (submenuItem ('guest', '広場', '追加')?.visible).toBe (false)
-    expect (submenuItem ('guest', '広場', '取込')?.visible).toBe (false)
     expect (submenuItem ('guest', '素材', '追加')?.visible).toBe (false)
     expect (submenuItem ('guest', 'Wiki', '新規')?.visible).toBe (false)
     expect (submenuItem ('guest', 'Wiki', '編輯')?.visible).toBe (false)
   })
 
-  it ('uses /posts/new as the import entrypoint', () => {
-    expect (submenuItem ('member', '広場', '取込')?.to).toBe ('/posts/new')
+  it ('uses /posts/new as the post creation entrypoint', () => {
+    expect (submenuItem ('member', '広場', '追加')?.to).toBe ('/posts/new')
   })
 
   it ('keeps material suppression admin-only', () => {

frontend/src/components/dialogues/DialogueProvider.test.tsx

@@ -80,7 +80,9 @@ describe ('DialogueProvider', () => {
     expect (dialogue).toHaveClass ('max-h-[calc(100dvh-1rem)]', 'flex-col', 'max-w-3xl')
     await waitFor (() => expect (screen.getByRole ('button', { name: '左操作' }))
 	.toBeInTheDocument ())
-    expect (screen.getByRole ('button', { name: '左操作' })).toHaveClass ('w-full', 'sm:w-auto')
+    expect (screen.getByRole ('button', { name: '左操作' })).toHaveClass (
+	'w-full',
+	'md:w-auto')
     fireEvent.click (screen.getByRole ('button', { name: '左操作' }))
 
     await waitFor (() => expect (action).toHaveBeenCalledTimes (1))

frontend/src/components/dialogues/DialogueProvider.tsx

@@ -215,12 +215,12 @@ const DialogueProvider: FC<Props> = ({ children }) => {
 
 		       <DialogFooter
 			   className="shrink-0 flex-col gap-2 pt-4
-				      sm:flex-row sm:justify-between sm:gap-0 sm:space-x-0">
-			 <div className="flex w-full flex-col gap-2 sm:w-auto sm:flex-row">
+				      md:flex-row md:justify-between md:gap-0 md:space-x-0">
+			 <div className="flex w-full flex-col gap-2 md:w-auto md:flex-row">
 			   {startActions.map (action => (
 			       <Button
 				   key={action.label}
-				   className="w-full sm:w-auto"
+				   className="w-full md:w-auto"
 				   variant={action.variant === 'danger'
 				       ? 'destructive'
 				       : 'default'}
@@ -232,9 +232,9 @@ const DialogueProvider: FC<Props> = ({ children }) => {
 			       </Button>))}
 			 </div>
 
-			 <div className="flex w-full flex-col gap-2 sm:w-auto sm:flex-row">
+			 <div className="flex w-full flex-col gap-2 md:w-auto md:flex-row">
 			   <Button
-			       className="w-full sm:w-auto"
+			       className="w-full md:w-auto"
 			       variant="outline"
 			       onClick={() => closeRequest (active.id)}
 			       disabled={pendingIds.includes (active.id)
@@ -245,7 +245,7 @@ const DialogueProvider: FC<Props> = ({ children }) => {
 			   {endActions.map (action => (
 			       <Button
 				   key={action.label}
-				   className="w-full sm:w-auto"
+				   className="w-full md:w-auto"
 				   variant={action.variant === 'danger'
 				       ? 'destructive'
 				       : 'default'}

frontend/src/components/posts/PostCreationDataFields.tsx

@@ -17,13 +17,15 @@ type Props = {
     type?:       string
     placeholder?: string }
   thumbnailField: ReactNode
-  core:           PostCoreDataFieldsProps }
+  core:           PostCoreDataFieldsProps
+  extraFields?:   ReactNode }
 
 
 const PostCreationDataFields: FC<Props> = (
     { url,
       thumbnailField,
-      core },
+      core,
+      extraFields },
 ) => (
     <>
       <PostTextField
@@ -39,6 +41,8 @@ const PostCreationDataFields: FC<Props> = (
       {thumbnailField}
 
       <PostCoreDataFields {...core}/>
+
+      {extraFields}
     </>)
 
 export default PostCreationDataFields