#55

**本番 DB のバックアップをかならずすること**

Reviewed-on: #420
Co-authored-by: miteruzo <miteruzo@naver.com>

backend/app/controllers/application_controller.rb

@@ -92,4 +92,6 @@ class ApplicationController < ActionController::API
 
     value
   end
+
+  def resolve_locale! = Locale.find_by(code: params[:locale]) || Locale.nipponese
 end

backend/app/controllers/materials_controller.rb

@@ -72,6 +72,7 @@ class MaterialsController < ApplicationController
     return head :unauthorized unless current_user
     return head :forbidden unless current_user.gte_member?
 
+    locale = resolve_locale!
     tag_name_raw = params[:tag].to_s.strip
     file = params[:file]
     file_sha256 = MaterialFileSha256.from_upload(file)
@@ -89,7 +90,7 @@ class MaterialsController < ApplicationController
 
     begin
       Material.transaction do
-        tag = resolve_material_tag!(tag_name_raw)
+        tag = resolve_material_tag!(locale, tag_name_raw)
         material = Material.new(tag:, url:,
                                 created_by_user: current_user,
                                 updated_by_user: current_user)
@@ -236,10 +237,8 @@ class MaterialsController < ApplicationController
     nil
   end
 
-  def resolve_material_tag! tag_name_raw
-    tag_name = TagName.find_undiscard_or_create_by!(name: tag_name_raw)
-    tag = tag_name.tag
-    tag || Tag.create!(tag_name:, category: :material)
+  def resolve_material_tag! locale, tag_name_raw
+    Tag.find_or_create_by_tag_name!(locale, tag_name_raw, category: :material)
   end
 
   def material_index_needs_tag_name? filters

backend/app/controllers/nico_tags_controller.rb

@@ -15,17 +15,22 @@ 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)
+    q =
+        ExternalTag
         .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
+               '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
@@ -38,7 +43,7 @@ class NicoTagsController < ApplicationController
     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
 
@@ -46,51 +51,58 @@ 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: }
   end
 
+  def show
+    tag = ExternalTag.find(params[:id])
+
+    render json: external_tag_json(tag)
+  end
+
   def update
     return head :unauthorized unless current_user
     return head :forbidden unless current_user.gte_member?
 
+    locale = resolve_locale!
+
     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
 
     ApplicationRecord.transaction do
-      linked_tags = Tag.normalise_tags!(linked_tag_names, with_tagme: false,
+      linked_tags = Tag.normalise_tags!(locale, 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 +114,21 @@ 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_versions_controller.rb

@@ -2,6 +2,9 @@ class PostVersionsController < ApplicationController
   def index
     post_id = params[:post].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
 
@@ -26,9 +29,14 @@ class PostVersionsController < ApplicationController
                 '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_id
-      q = q.where("JSON_CONTAINS(post_versions.tags_json, JSON_OBJECT('id', #{ tag_id })) " +
-                  "OR JSON_CONTAINS(prev.tags_json, JSON_OBJECT('id', #{ tag_id }))")
+    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
@@ -43,50 +51,19 @@ 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 =
-          normalise_json(row.tags_json)
-          .sort_by { [(case _1.fetch('category')
-                       when 'deerjikist'
-                         0
-                       when 'meme'
-                         1
-                       when 'character'
-                         2
-                       when 'general'
-                         3
-                       when 'material'
-                         4
-                       when 'meta'
-                         5
-                       else
-                         6
-                       end),
-                      _1.fetch('name').downcase] }
-          .map { Post.tag_snapshot_literal(_1) }
-      prev_tags =
-          (normalise_json(row.attributes['prev_tags_json']) || [])
-          .sort_by { [(case _1.fetch('category')
-                       when 'deerjikist'
-                         0
-                       when 'meme'
-                         1
-                       when 'character'
-                         2
-                       when 'general'
-                         3
-                       when 'material'
-                         4
-                       when 'meta'
-                         5
-                       else
-                         6
-                       end),
-                      _1.fetch('name').downcase] }
-          .map { Post.tag_snapshot_literal(_1) }
+      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,
         version_no: row.version_no,
@@ -113,7 +90,41 @@ class PostVersionsController < ApplicationController
     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)
@@ -124,10 +135,7 @@ class PostVersionsController < ApplicationController
             'removed'
           end
 
-      {
-        name:,
-        type:
-      }
+      { name:, type: }
     end
   end
 end

backend/app/controllers/posts_controller.rb

@@ -36,8 +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
 
@@ -49,9 +49,8 @@ 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,
+        .preload(:external_tags, :uploaded_user, :parents, :children,
+                 post_tags: [:sections, { tag: [:deerjikists, :materials,
                                                 { tag_name: :wiki_page }] }])
         .with_attached_thumbnail
 
@@ -103,9 +102,10 @@ class PostsController < ApplicationController
   end
 
   def random
-    post = filtered_posts.preload(:uploaded_user, :parents, :children,
-                                  active_post_tags: [:sections,
-                                                     { tag: [:deerjikists, :materials,
+    post =
+        filtered_posts
+        .preload(:uploaded_user, :parents, :children,
+                 post_tags: [:sections, { tag: [:deerjikists, :materials,
                                                 { tag_name: :wiki_page }] }])
         .with_attached_thumbnail
         .order('RAND()')
@@ -190,8 +190,7 @@ class PostsController < ApplicationController
     post =
         Post
         .includes(:uploaded_user, :parents, :children,
-                  active_post_tags: [:sections,
-                                     { tag: [:deerjikists, :materials,
+                  post_tags: [:sections, { tag: [:deerjikists, :materials,
                                                  { tag_name: :wiki_page }] }])
         .with_attached_thumbnail
         .find_by(id: params[:id])
@@ -214,6 +213,8 @@ class PostsController < ApplicationController
     return head :unauthorized unless current_user
     return head :forbidden unless current_user.gte_member?
 
+    locale = resolve_locale!
+
     preflight = PostCreatePreflight.new(
         attributes: post_create_attributes,
         thumbnail: params[:thumbnail],
@@ -243,7 +244,7 @@ class PostsController < ApplicationController
                                    :post_tag_specs,
                                    :tag_sections,
                                    :normalised_parent_post_ids).symbolize_keys).merge(
-                               thumbnail: params[:thumbnail])).create!
+                               thumbnail: params[:thumbnail])).create!(locale)
 
     post.reload
     render json: PostRepr.base(post), status: :created
@@ -311,6 +312,7 @@ class PostsController < ApplicationController
     base_version_no = parse_base_version_no
     return render_bad_request('base_version_no は必須です．') if !(force) && !(base_version_no)
 
+    locale = Locale.find_by(code: params[:locale].presence) || Locale.nipponese
     title = params[:title].presence
     tag_names = params[:tags].to_s.split
     original_created_from = params[:original_created_from]
@@ -332,7 +334,8 @@ class PostsController < ApplicationController
         base_snapshot = post_snapshot_from_version(base_version)
         current_snapshot = post_snapshot_from_record(post)
       end
-      incoming_snapshot = post_incoming_snapshot(title:,
+      incoming_snapshot = post_incoming_snapshot(locale:,
+                                                 title:,
                                                  original_created_from:,
                                                  original_created_before:,
                                                  tag_names:,
@@ -361,7 +364,7 @@ class PostsController < ApplicationController
             end
           end
 
-      apply_post_snapshot!(post, snapshot_to_apply)
+      apply_post_snapshot!(locale, post, snapshot_to_apply)
     end
 
     return render json: conflict_json, status: :conflict if conflict_json
@@ -384,50 +387,6 @@ 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
@@ -473,8 +432,20 @@ 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|
@@ -502,13 +473,13 @@ 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)
 
@@ -552,7 +523,10 @@ 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
@@ -700,7 +674,7 @@ class PostsController < ApplicationController
 
   def editable_tag_names_from_version version
     version.tags_json
-        .reject { _1.fetch('category') == 'nico' }
+        .select { _1.key?('tag_id') }
         .map { Post.tag_snapshot_literal(_1) }
         .sort
   end
@@ -717,9 +691,7 @@ 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')
@@ -733,12 +705,14 @@ class PostsController < ApplicationController
     end
   end
 
-  def post_incoming_snapshot title:, original_created_from:, original_created_before:,
+  def post_incoming_snapshot locale:, 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: }
+
+    Tag.normalise_tags!(locale, tag_names,
+                        with_tagme: false,
+                        deny_deprecated: true,
+                        with_sections: true) => { tags:, sections: }
 
     tags = Tag.expand_parent_tags(tags).reject(&:deprecated?)
     video_ms = normalise_video_ms(tags, video_ms_param:, duration_param:)
@@ -873,7 +847,7 @@ class PostsController < ApplicationController
     (added_by_current & removed_by_me).present? || (removed_by_current & added_by_me).present?
   end
 
-  def apply_post_snapshot! post, snapshot
+  def apply_post_snapshot! locale, post, snapshot
     PostVersionRecorder.ensure_snapshot!(post, created_by_user: current_user)
 
     post.update!(title: snapshot[:title],
@@ -881,15 +855,12 @@ 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,
+    Tag.normalise_tags!(locale, 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)
+                        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/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)
 
@@ -118,54 +225,77 @@ class TagsController < ApplicationController
       matched_alias_by_tag_name_id[canonical_id] ||= alias_name
     end
 
-    base = Tag.joins(:tag_name)
+    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
-
-    render json: tags.map { |tag|
+    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])
         }
+
+    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
-    tag = Tag.joins(:tag_name)
+    tag =
+        Tag
+        .joins(:tag_name)
         .includes(:tag_name, :materials, tag_name: :wiki_page)
         .find_by(id: params[:id])
-    if tag
-      render json: TagRepr.base(tag)
-    else
+    return render json: TagRepr.base(tag) if tag
+
+    external_tag = ExternalTag.find_by(id: params[:id])
+    return render json: ExternalTagRepr.base(external_tag) if external_tag
+
     head :not_found
   end
-  end
 
   def show_by_name
     name = params[:name].to_s.strip
     return render_bad_request('name は必須です．') if name.blank?
 
-    tag = Tag.joins(:tag_name)
+    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
+    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
@@ -278,15 +408,13 @@ class TagsController < ApplicationController
 
     return unless validate_tag_rename(tag, name)
 
+    locale = resolve_locale!
+
     alias_names = params[:aliases].to_s.split.uniq
     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
 
@@ -308,7 +436,7 @@ class TagsController < ApplicationController
       alias_names.delete(name)
 
       update_aliases!(tag, alias_names)
-      update_parent_tags!(tag, parent_names)
+      update_parent_tags!(locale, tag, parent_names)
 
       tag.reload
 
@@ -334,13 +462,9 @@ 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
 
@@ -547,11 +671,6 @@ 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
@@ -573,7 +692,7 @@ 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?
 
@@ -585,17 +704,14 @@ 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:
@@ -605,11 +721,9 @@ 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)
@@ -640,7 +754,7 @@ 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
 
@@ -655,7 +769,7 @@ 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
 
@@ -664,10 +778,10 @@ class TagsController < ApplicationController
     end
   end
 
-  def update_parent_tags! tag, parent_names
-    parent_tags = Tag.normalise_tags!(parent_names, with_tagme: false,
-                                                    with_no_deerjikist: false,
-                                                    deny_nico: true)
+  def update_parent_tags! locale, tag, parent_names
+    parent_tags = Tag.normalise_tags!(locale, parent_names,
+                                      with_tagme: false,
+                                      with_no_deerjikist: false)
 
     old_parent_tags = tag.parents.to_a
 
@@ -795,4 +909,20 @@ 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/language.rb

@@ -0,0 +1,3 @@
+class Language < ApplicationRecord
+  has_many :languages, class_name: 'Locale', foreign_key: :language_code
+end

backend/app/models/locale.rb

@@ -0,0 +1,24 @@
+class Locale < ApplicationRecord
+  after_create_commit :generate_tag_names!
+
+  belongs_to :language, foreign_key: :language_code, primary_key: :code
+  belongs_to :script, foreign_key: :script_code, primary_key: :code
+
+  def self.nipponese = Locale.find('ja')
+
+  private
+
+  def generate_tag_names!
+    tag_ids = TagName.where(language_code:, primary_flg: true).pluck(:tag_id)
+    Tag.where.not(id: tag_ids).find_each do
+      TagName.create!(tag_id: _1.id,
+                      language_code:,
+                      name: TagName.generate_name(self, _1, _1.name),
+                      script_code:,
+                      primary_flg: true,
+                      # TODO: 公証実装したら書く．
+                      # auto_generated: true,
+                      canonical_id: nil)
+    end
+  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

@@ -55,25 +55,27 @@ class Post < ApplicationRecord
   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,
@@ -90,6 +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
@@ -123,7 +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')
@@ -149,20 +153,27 @@ class Post < ApplicationRecord
   end
 
   def snapshot_tags_json
+    tag_snapshots =
         post_tags
-        .kept
             .joins(tag: :tag_name)
             .includes(:sections, tag: :tag_name)
             .order('tags.id')
-        .map do |pt|
-      { 'id' => pt.tag.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 }
                           } }
-    end
+        }
+
+    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

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

@@ -4,7 +4,7 @@ class PostTagSection < ApplicationRecord
   belongs_to :post
   belongs_to :tag
 
-  belongs_to :post_tag, -> { kept }, foreign_key: [:post_id, :tag_id],
+  belongs_to :post_tag, foreign_key: [:post_id, :tag_id],
                         primary_key: [:post_id, :tag_id],
                         inverse_of: :sections,
                         optional: true

backend/app/models/script.rb

@@ -0,0 +1,3 @@
+class Script < ApplicationRecord
+  ;
+end

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
@@ -54,8 +49,8 @@ class Tag < ApplicationRecord
   has_many :materials
 
   has_many :tag_versions
-  has_many :nico_tag_versions
 
+  has_many :tag_names
   belongs_to :tag_name
   delegate :wiki_page, to: :tag_name
 
@@ -69,17 +64,13 @@ 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,
@@ -105,19 +96,20 @@ class Tag < ApplicationRecord
 
   def has_deerjikists = deerjikists.loaded? ? deerjikists.any? : deerjikists.exists?
 
-  def self.tagme = find_or_create_by_tag_name!('タグ希望', category: :meta)
-  def self.bot = find_or_create_by_tag_name!('bot操作', category: :meta)
-  def self.no_deerjikist = find_or_create_by_tag_name!('ニジラー情報不詳', category: :meta)
-  def self.video = find_or_create_by_tag_name!('動画', category: :meta)
-  def self.niconico = find_or_create_by_tag_name!('ニコニコ', category: :meta)
-  def self.youtube = find_or_create_by_tag_name!('YouTube', category: :meta)
+  def self.tagme = find_or_create_by_tag_name!(Locale.nipponese, 'タグ希望', category: :meta)
+  def self.bot = find_or_create_by_tag_name!(Locale.nipponese, 'bot操作', category: :meta)
+  def self.no_deerjikist =
+      find_or_create_by_tag_name!(Locale.nipponese, 'ニジラー情報不詳', category: :meta)
+  def self.video = find_or_create_by_tag_name!(Locale.nipponese, '動画', category: :meta)
+  def self.niconico = find_or_create_by_tag_name!(Locale.nipponese, 'ニコニコ', category: :meta)
+  def self.youtube = find_or_create_by_tag_name!(Locale.nipponese, 'YouTube', category: :meta)
 
-  def self.normalise_tags! tag_names, with_tagme: true,
+  def self.normalise_tags! locale, 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
 
@@ -145,7 +137,7 @@ class Tag < ApplicationRecord
 
       name = TagName.canonicalise(name).first
 
-      find_or_create_by_tag_name!(name, category: (cat || :general)).tap do |tag|
+      find_or_create_by_tag_name!(locale, name, category: (cat || :general)).tap do |tag|
         if deny_deprecated && tag.deprecated?
           raise DeprecatedTagNormalisationError, [tag.name]
         end
@@ -233,13 +225,25 @@ class Tag < ApplicationRecord
     [left_end_ms, right_end_ms].max
   end
 
-  def self.find_or_create_by_tag_name! name, category:
-    tn = TagName.find_undiscard_or_create_by!(name: name.to_s.strip)
-    tn = tn.canonical if tn.canonical_id?
+  def self.find_or_create_by_tag_name! locale, name, category:
+    language_code = locale.language_code
+    name = name.to_s.strip
 
-    Tag.find_undiscard_or_create_by!(tag_name_id: tn.id) do |t|
-      t.category = category
+    tn = TagName.find_or_create_by!(language_code:, name:) do
+      _1.script_code = locale.script_code
+      _1.primary_flg = true
     end
+
+    tag = tn.tag
+    if tag
+      tag.update!(tag_name_id: tn.id) if tag.tag_name_id != tn.id
+      return tag
+    end
+
+    tag = Tag.create!(tag_name: tn, category:)
+    tn.update!(tag:)
+
+    tag
   rescue ActiveRecord::RecordNotUnique
     retry
   end
@@ -259,11 +263,11 @@ 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 +279,10 @@ 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
 
         TagVersioning.record!(target_tag, event_type: :update, created_by_user:)
       end
@@ -293,13 +293,13 @@ 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 +309,9 @@ 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 +352,4 @@ 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

@@ -1,7 +1,5 @@
 class TagName < ApplicationRecord
-  include MyDiscard
-
-  has_one :tag
+  belongs_to :tag, optional: true
   has_one :wiki_page
 
   belongs_to :canonical, class_name: 'TagName', optional: true
@@ -23,6 +21,12 @@ class TagName < ApplicationRecord
     names.map { |name| tns[name]&.canonical&.name || name }.uniq
   end
 
+  def self.generate_name locale, tag, name
+    # TODO: 言語ごとの自動命名ロジック完成したら書く．
+
+    "Tag_##{ tag.id }"
+  end
+
   private
 
   def canonical_must_be_canonical

backend/app/models/tag_name_sanitisation_rule.rb

@@ -10,8 +10,9 @@ class TagNameSanitisationRule < ApplicationRecord
   validate :source_pattern_must_be_regexp
 
   class << self
-    def sanitise(name) =
+    def sanitise(name)
       rules.reduce(name.dup) { |name, (pattern, replacement)| name.gsub(pattern, replacement) }
+    end
 
     def apply!
       TagName.find_each do |tn|
@@ -32,7 +33,7 @@ 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_repr.rb

@@ -87,8 +87,9 @@ module PostRepr
   end
 
   def tag_json post
+    internal_tags =
         post
-        .active_post_tags
+        .post_tags
         .reject { _1.tag.deprecated? }
         .sort_by { _1.tag.name }
         .map { |post_tag|
@@ -96,6 +97,14 @@ module PostRepr
               '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, host: nil

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

@@ -6,7 +6,7 @@ class PostBulkCreator
     @host = host
   end
 
-  def run
+  def run locale
     results = Array.new(@posts.length)
     mutex = Mutex.new
     next_index = 0
@@ -27,7 +27,7 @@ class PostBulkCreator
                 break if index >= @posts.length
 
                 attributes = @posts[index]
-                results[index] = create_row(actor, attributes, index)
+                results[index] = create_row(locale, actor, attributes, index)
               rescue StandardError => e
                 Rails.logger.error(
                     "post_bulk_creator_worker_failure #{ { error: e.class.name,
@@ -61,7 +61,7 @@ class PostBulkCreator
 
   private
 
-  def create_row actor, attributes, index
+  def create_row locale, actor, attributes, index
     preflight =
         PostCreatePreflight.new(
             attributes: attributes,
@@ -76,7 +76,7 @@ class PostBulkCreator
 
     post = PostCreator.new(
         actor: actor,
-        attributes: normalised_attributes(attributes, preflight, index)).create!
+        attributes: normalised_attributes(attributes, preflight, index)).create!(locale)
     result = {
         status: 'created',
         post: { id: post.id } }

backend/app/services/post_create_plan.rb

@@ -20,8 +20,7 @@ class PostCreatePlan
     parent_post_ids = normalise_parent_post_ids
     validate_parent_post_ids!(parent_post_ids)
 
-    {
-        url: @attributes[:url],
+    { url: @attributes[:url],
       title: @attributes[:title].to_s,
       thumbnail_base: @attributes[:thumbnail_base].presence,
       original_created_from: @attributes[:original_created_from].presence,
@@ -50,7 +49,6 @@ class PostCreatePlan
     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::NicoTagNormalisationError if existing_tag&.nico?
       raise Tag::DeprecatedTagNormalisationError, [existing_tag.name] if existing_tag&.deprecated?
 
       direct_tag_specs << {
@@ -139,14 +137,14 @@ class PostCreatePlan
         Tag.expand_parent_tags(existing_snapshot_tags)
            .reject(&:deprecated?)
            .map { |tag|
-             {
-                 name: tag.name,
-                 category: tag.category.to_sym } }
+             { 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({ }) do |spec, merged|
+    specs.each_with_object({ }) { |spec, merged|
       merged[spec[:name]] =
           if merged.key?(spec[:name]) && merged[spec[:name]][:category] != :general
             merged[spec[:name]]
@@ -155,7 +153,7 @@ class PostCreatePlan
                 name: spec[:name],
                 category: spec[:category] }
           end
-    end.values.sort_by { _1[:name] }
+    }.values.sort_by { _1[:name] }
   end
 
   def existing_tags_by_name

backend/app/services/post_creator.rb

@@ -9,7 +9,7 @@ class PostCreator
     @field_warnings = { }
   end
 
-  def create!
+  def create! locale
     thumbnail_attachment = prepare_thumbnail_attachment
     post = Post.new(title: @attributes[:title].presence,
                     url: @attributes[:url],
@@ -25,7 +25,7 @@ class PostCreator
       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.video_ms = planned_video_ms(locale)
       post.save!
       sync_post_tags!(post, post_tags, sections)
       sync_parent_posts!(post, planned_parent_post_ids)
@@ -53,15 +53,15 @@ class PostCreator
 
   def planned_parent_post_ids = planned_create_attributes[:normalised_parent_post_ids]
 
-  def planned_video_ms
-    planned_create_attributes[:video_ms]
+  def planned_video_ms locale
+    planned_create_attributes(locale)[:video_ms]
   end
 
-  def planned_create_attributes
+  def planned_create_attributes locale
     @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 = materialise_tags(locale, @attributes[:snapshot_tag_specs] || [])
+        post_tags = materialise_tags(locale, @attributes[:post_tag_specs] || [])
         {
             snapshot_tags: snapshot_tags,
             post_tags: post_tags,
@@ -92,13 +92,13 @@ class PostCreator
         video_ms: plan[:video_ms] }
   end
 
-  def materialise_tags specs
+  def materialise_tags locale, 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 = Tag.find_or_create_by_tag_name!(locale, name, category:)
       tag.update!(category:) if tag.category.to_sym != category.to_sym
       tags[name] ||= tag
     end.values
@@ -121,9 +121,11 @@ class PostCreator
   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|
@@ -133,10 +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/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
-  end
 
   def self.ensure_snapshot! tag, created_by_user:
-    if tag.nico?
-      return if tag.nico_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
-  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/routes.rb

@@ -1,5 +1,5 @@
 Rails.application.routes.draw do
-  resources :nico_tags, path: 'tags/nico', only: [:index, :update]
+  resources :nico_tags, path: 'tags/nico', only: [:index, :show, :update]
 
   scope 'tags/:parent_id/children', controller: :tag_children do
     post ':child_id', action: :create
@@ -55,7 +55,6 @@ Rails.application.routes.draw do
       get :metadata
       post :bulk
       get :random
-      get :changes
       get :versions, to: 'post_versions#index'
     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/migrate/20260922000000_create_languages.rb

@@ -0,0 +1,20 @@
+class CreateLanguages < ActiveRecord::Migration[8.0]
+  def up
+    create_table :languages, id: { type: :string, limit: 16 }, primary_key: :code do |t|
+      t.string :name, null: false
+      t.datetime :deprecated_at, index: true
+      t.datetime :created_at, null: false
+    end
+
+    execute <<~SQL
+      INSERT INTO
+        languages(code, name, created_at)
+      VALUES
+        ('ja', '日本語', #{ connection.quote Time.current })
+    SQL
+  end
+
+  def down
+    drop_table :languages
+  end
+end

backend/db/migrate/20260922010000_create_scripts.rb

@@ -0,0 +1,20 @@
+class CreateScripts < ActiveRecord::Migration[8.0]
+  def up
+    create_table :scripts, id: { type: 'CHAR(4)' }, primary_key: :code do |t|
+      t.string :name, null: false
+      t.datetime :deprecated_at, index: true
+      t.datetime :created_at, null: false
+    end
+
+    execute <<~SQL
+      INSERT INTO
+        scripts(code, name, created_at)
+      VALUES
+        ('Jpan', '漢字および仮名文字', #{ connection.quote Time.current })
+    SQL
+  end
+
+  def down
+    drop_table :scripts
+  end
+end

backend/db/migrate/20260922020000_create_locales.rb

@@ -0,0 +1,25 @@
+class CreateLocales < ActiveRecord::Migration[8.0]
+  def up
+    create_table :locales, id: { type: :string, limit: 32 }, primary_key: :code do |t|
+      t.string :language_code, limit: 16, null: false, index: true
+      t.column :script_code, 'CHAR(4)', null: false
+      t.string :name, null: false
+      t.datetime :deprecated_at, index: true
+      t.datetime :created_at, null: false
+
+      t.foreign_key :languages, column: :language_code, primary_key: :code
+      t.foreign_key :scripts, column: :script_code, primary_key: :code
+    end
+
+    execute <<~SQL
+      INSERT INTO
+        locales(code, language_code, script_code, name, created_at)
+      VALUES
+        ('ja', 'ja', 'Jpan', '日本語', #{ connection.quote Time.current })
+    SQL
+  end
+
+  def down
+    drop_table :locales
+  end
+end

backend/db/migrate/20260922030000_add_columns_to_tag_names.rb

@@ -0,0 +1,59 @@
+class AddColumnsToTagNames < ActiveRecord::Migration[8.0]
+  def up
+    add_reference :tag_names, :tag, after: :id, foreign_key: true
+    add_column :tag_names, :language_code, :string,
+               limit: 16, null: false, after: :tag_id, default: 'ja'
+    add_column :tag_names, :script_code, 'CHAR(4)',
+               null: false, after: :name, default: 'Jpan'
+    add_column :tag_names, :primary_flg, :boolean,
+               null: false, after: :script_code, default: true
+
+    add_foreign_key :tag_names, :languages, column: :language_code, primary_key: :code
+    add_foreign_key :tag_names, :scripts, column: :script_code, primary_key: :code
+
+    remove_index :tag_names, :name
+    add_index :tag_names, [:language_code, :name], unique: true
+
+    change_column_default :tag_names, :language_code, from: 'ja', to: nil
+    change_column_default :tag_names, :script_code, from: 'Jpan', to: nil
+
+    execute <<~SQL
+      UPDATE
+        tag_names tn
+        LEFT JOIN
+          tags AS t
+          ON
+            t.tag_name_id = COALESCE(tn.canonical_id, tn.id)
+      SET
+        tn.tag_id = t.id
+      , tn.primary_flg = CASE
+                         WHEN tn.canonical_id IS NULL THEN
+                           1
+                         ELSE
+                           0
+                         END
+    SQL
+
+    change_column_default :tag_names, :primary_flg, from: true, to: nil
+
+    add_column :tag_names, :primary_tag_id, :bigint,
+               as: 'CASE WHEN primary_flg THEN tag_id ELSE NULL END'
+    add_index :tag_names, [:primary_tag_id, :language_code], unique: true
+  end
+
+  def down
+    remove_index :tag_names, [:primary_tag_id, :language_code]
+    remove_column :tag_names, :primary_tag_id
+
+    remove_foreign_key :tag_names, column: :language_code
+    remove_foreign_key :tag_names, column: :script_code
+
+    remove_index :tag_names, [:language_code, :name]
+    add_index :tag_names, :name, unique: true
+
+    remove_column :tag_names, :primary_flg
+    remove_column :tag_names, :script_code
+    remove_column :tag_names, :language_code
+    remove_reference :tag_names, :tag, foreign_key: true
+  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_27_123600) do
+ActiveRecord::Schema[8.0].define(version: 2026_09_22_030000) 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_27_123600) 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
@@ -130,6 +138,24 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_27_123600) do
     t.index ["ip_address"], name: "index_ip_addresses_on_ip_address", unique: true
   end
 
+  create_table "languages", primary_key: "code", id: { type: :string, limit: 16 }, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.string "name", null: false
+    t.datetime "deprecated_at"
+    t.datetime "created_at", null: false
+    t.index ["deprecated_at"], name: "index_languages_on_deprecated_at"
+  end
+
+  create_table "locales", primary_key: "code", id: { type: :string, limit: 32 }, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.string "language_code", limit: 16, null: false
+    t.string "script_code", limit: 4, null: false
+    t.string "name", null: false
+    t.datetime "deprecated_at"
+    t.datetime "created_at", null: false
+    t.index ["deprecated_at"], name: "index_locales_on_deprecated_at"
+    t.index ["language_code"], name: "index_locales_on_language_code"
+    t.index ["script_code"], name: "fk_rails_0b74ce96a8"
+  end
+
   create_table "material_export_items", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "material_id", null: false
     t.string "profile", default: "legacy_drive", null: false
@@ -281,6 +307,13 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_27_123600) 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 +344,12 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_27_123600) 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
 
@@ -363,7 +385,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_27_123600) 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\":{\"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}}',`tags_json`)", name: "chk_post_versions_tags_json_schema"
+    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|
@@ -384,6 +406,13 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_27_123600) do
     t.check_constraint "`version_no` > 0", name: "chk_posts_version_no_positive"
   end
 
+  create_table "scripts", primary_key: "code", id: { type: :string, limit: 4 }, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.string "name", null: false
+    t.datetime "deprecated_at"
+    t.datetime "created_at", null: false
+    t.index ["deprecated_at"], name: "index_scripts_on_deprecated_at"
+  end
+
   create_table "settings", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "user_id", null: false
     t.datetime "created_at", null: false
@@ -395,19 +424,6 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_27_123600) 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
@@ -429,14 +445,20 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_27_123600) do
   end
 
   create_table "tag_names", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "tag_id"
+    t.string "language_code", limit: 16, null: false
     t.string "name", null: false
+    t.string "script_code", limit: 4, null: false
+    t.boolean "primary_flg", null: false
     t.bigint "canonical_id"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
-    t.datetime "discarded_at"
+    t.virtual "primary_tag_id", type: :bigint, as: "(case when `primary_flg` then `tag_id` else NULL end)"
     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
+    t.index ["language_code", "name"], name: "index_tag_names_on_language_code_and_name", unique: true
+    t.index ["primary_tag_id", "language_code"], name: "index_tag_names_on_primary_tag_id_and_language_code", unique: true
+    t.index ["script_code"], name: "fk_rails_dd783b3d1c"
+    t.index ["tag_id"], name: "index_tag_names_on_tag_id"
   end
 
   create_table "tag_similarities", primary_key: ["tag_id", "target_tag_id"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
@@ -472,10 +494,8 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_27_123600) 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"
@@ -616,6 +636,19 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_27_123600) 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
@@ -701,6 +734,8 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_27_123600) do
   add_foreign_key "gekanator_question_suggestions", "users"
   add_foreign_key "gekanator_questions", "gekanator_question_suggestions"
   add_foreign_key "gekanator_questions", "users", column: "created_by_id"
+  add_foreign_key "locales", "languages", column: "language_code", primary_key: "code"
+  add_foreign_key "locales", "scripts", column: "script_code", primary_key: "code"
   add_foreign_key "material_export_items", "materials"
   add_foreign_key "material_export_items", "users", column: "created_by_user_id"
   add_foreign_key "material_import_blocks", "users", column: "created_by_user_id"
@@ -709,37 +744,37 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_27_123600) 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"
   add_foreign_key "settings", "users"
   add_foreign_key "tag_implications", "tags"
   add_foreign_key "tag_implications", "tags", column: "parent_tag_id"
+  add_foreign_key "tag_names", "languages", column: "language_code", primary_key: "code"
+  add_foreign_key "tag_names", "scripts", column: "script_code", primary_key: "code"
   add_foreign_key "tag_names", "tag_names", column: "canonical_id"
+  add_foreign_key "tag_names", "tags"
   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 +130,64 @@ 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)
+        name = TagNameSanitisationRule.sanitise("nico:#{ raw }").delete_prefix('nico:')
+        tag = ExternalTag.find_or_create_by!(platform: :nico, name:)
 
-        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/tag_names.rb

@@ -1,5 +1,21 @@
 FactoryBot.define do
   factory :tag_name do
-    name { "tag-#{SecureRandom.hex(4)}" }
+    language_code { 'ja' }
+    script_code { 'Jpan' }
+    primary_flg { true }
+    sequence(:name) { |number| "tag-#{ SecureRandom.hex(4) }-#{ number }" }
+
+    trait :alias do
+      primary_flg { false }
+
+      after(:create) do |tag_name|
+        owner = tag_name.canonical&.tag
+        raise ArgumentError, 'alias requires a canonical tag' unless owner
+
+        # The alias validation still rejects tag_id; its contract is tested separately.
+        tag_name.update_columns(tag_id: owner.id)
+        tag_name.association(:tag).reset
+      end
+    end
   end
 end

backend/spec/factories/tags.rb

@@ -6,17 +6,15 @@ FactoryBot.define do
 
     category { :general }
     post_count { 0 }
+
     association :tag_name
 
     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
+    after(:create) do |tag|
+      tag.tag_name.update!(tag:) if tag.tag_name.tag_id != tag.id
     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/locale_spec.rb

@@ -0,0 +1,69 @@
+require 'rails_helper'
+
+RSpec.describe Locale, type: :model do
+  def prepare_french_reference!
+    Language.find_or_create_by!(code: 'fr') { _1.name = 'French' }
+    Script.find_or_create_by!(code: 'Latn') { _1.name = 'Latin' }
+  end
+
+  def create_french_locale!
+    prepare_french_reference!
+    described_class.create!(code: 'fr', language_code: 'fr',
+                            script_code: 'Latn', name: 'French')
+  end
+
+  it 'generates a primary name for every existing tag using the name generator' do
+    first = create(:tag, name: 'first_existing_tag')
+    second = create(:tag, name: 'second_existing_tag')
+    allow(TagName).to receive(:generate_name).and_call_original
+    allow(TagName).to receive(:generate_name)
+        .with(kind_of(described_class), first, first.name)
+        .and_return('name_from_generator')
+
+    locale = create_french_locale!
+
+    expect(TagName).to have_received(:generate_name).with(locale, first, first.name)
+    expect(TagName).to have_received(:generate_name).with(locale, second, second.name)
+    expect(first.tag_names.find_by!(language_code: 'fr', primary_flg: true))
+        .to have_attributes(name: 'name_from_generator', tag_id: first.id,
+                            language_code: 'fr', script_code: 'Latn',
+                            primary_flg: true, canonical_id: nil)
+    expect(second.tag_names.find_by!(language_code: 'fr', primary_flg: true))
+        .to have_attributes(tag_id: second.id, language_code: 'fr',
+                            script_code: 'Latn', primary_flg: true, canonical_id: nil)
+    expect(TagName.where(language_code: 'fr', primary_flg: true,
+                         tag_id: [first.id, second.id]).count).to eq(2)
+  end
+
+  it 'does not duplicate an existing primary name in the new language' do
+    prepare_french_reference!
+    tag = create(:tag, name: 'already_named')
+    existing = create(:tag_name, name: 'nom_existant', tag:,
+                                 language_code: 'fr', script_code: 'Latn')
+
+    create_french_locale!
+
+    expect(TagName.where(tag_id: tag.id, language_code: 'fr', primary_flg: true))
+        .to contain_exactly(existing)
+    expect(existing.reload).to have_attributes(
+        tag_id: tag.id, language_code: 'fr', script_code: 'Latn', primary_flg: true)
+  end
+
+  it 'creates a primary name when the language has only an alias' do
+    prepare_french_reference!
+    tag = create(:tag, name: 'alias_only_tag')
+    alias_name = create(:tag_name, :alias, name: 'alias_fr',
+                                          canonical: tag.tag_name,
+                                          language_code: 'fr', script_code: 'Latn')
+
+    create_french_locale!
+
+    expect(alias_name.reload).to have_attributes(
+        canonical_id: tag.tag_name_id, tag_id: tag.id,
+        language_code: 'fr', script_code: 'Latn', primary_flg: false)
+    generated = TagName.find_by!(tag_id: tag.id, language_code: 'fr', primary_flg: true)
+    expect(generated).to have_attributes(tag_id: tag.id, language_code: 'fr',
+                                         script_code: 'Latn', primary_flg: true)
+    expect(TagName.where(tag_id: tag.id, language_code: 'fr', primary_flg: true).count).to eq(1)
+  end
+end

backend/spec/models/material_export_item_spec.rb

@@ -2,7 +2,7 @@ require 'rails_helper'
 
 RSpec.describe MaterialExportItem, type: :model do
   let(:user) { create(:user, :member) }
-  let(:tag) { Tag.create!(tag_name: TagName.create!(name: 'export_item'), category: :material) }
+  let(:tag) { create(:tag, name: 'export_item', category: :material) }
   let(:material) do
     Material.create!(tag:, url: 'https://example.com/material',
                      created_by_user: user, updated_by_user: user)

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!(

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

@@ -1,8 +1,8 @@
 require 'rails_helper'
 
 RSpec.describe PostVersion, type: :model do
-  let!(:tag_name) { TagName.create!(name: 'post_version_spec_tag') }
-  let!(:tag) { Tag.create!(tag_name: tag_name, category: :general) }
+  let!(:tag_name) { create(:tag_name, name: 'post_version_spec_tag') }
+  let!(:tag) { create(:tag, tag_name: tag_name, category: :general) }
 
   let!(:post_record) do
     Post.create!(title: 'spec post', url: 'https://example.com/post-version-spec').tap do |post|

backend/spec/models/tag_name_sanitisation_rule_spec.rb

@@ -34,9 +34,41 @@ RSpec.describe TagNameSanitisationRule, type: :model do
       described_class.create!(priority: 10, source_pattern: '_', replacement: '')
     end
 
+    context 'when only another language has the sanitised name' do
+      include_context 'English locale'
+
+      it 'keeps both names when sanitisation produces a cross-language match' do
+        japanese = create(:tag_name, name: 'foobar')
+        english = create(:tag_name, name: 'temporary',
+                                   language_code: 'en', script_code: 'Latn')
+        english.update_columns(name: 'foo_bar')
+
+        expect { described_class.apply! }.not_to change(TagName, :count)
+
+        expect(english.reload).to have_attributes(name: 'foobar', language_code: 'en')
+        expect(japanese.reload).to have_attributes(name: 'foobar', language_code: 'ja')
+      end
+
+      it 'renames without merging tags or deleting either language identity' do
+        japanese = create(:tag, name: 'foobar')
+        english_name = create(:tag_name, name: 'temporary',
+                                        language_code: 'en', script_code: 'Latn')
+        english = create(:tag, tag_name: english_name)
+        english_name.update_columns(name: 'foo_bar')
+
+        described_class.apply!
+
+        expect(english_name.reload).to have_attributes(
+            name: 'foobar', language_code: 'en', tag_id: english.id)
+        expect(japanese.reload.tag_name).to have_attributes(
+            name: 'foobar', language_code: 'ja', tag_id: japanese.id)
+        expect(english.reload.tag_name_id).to eq(english_name.id)
+      end
+    end
+
     context 'when no conflicting tag_name exists' do
       let!(:tag_name) do
-        TagName.create!(name: 'tmp').tap do |tn|
+        create(:tag_name, name: 'tmp').tap do |tn|
           tn.update_columns(name: 'foo_bar', updated_at: Time.current)
         end
       end
@@ -48,22 +80,22 @@ RSpec.describe TagNameSanitisationRule, type: :model do
     end
 
     context 'when a conflicting canonical tag_name exists' do
-      let!(:existing) { TagName.create!(name: 'foobar') }
+      let!(:existing) { create(:tag_name, name: 'foobar') }
       let!(:source) do
-        TagName.create!(name: 'tmp').tap do |tn|
+        create(:tag_name, name: 'tmp').tap do |tn|
           tn.update_columns(name: 'foo_bar', updated_at: Time.current)
         end
       end
 
       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
 
     context 'when the source tag_name has a tag and the existing one has no tag' do
-      let!(:existing) { TagName.create!(name: 'foobar') }
+      let!(:existing) { create(:tag_name, name: 'foobar') }
       let!(:source_tag) { create(:tag, name: 'tmp', category: :general) }
       let!(:source_tag_name_id) { source_tag.tag_name_id }
 
@@ -75,16 +107,36 @@ 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
+        create(:tag_name, :alias, name: 'foobar', canonical: existing_tag.tag_name)
+      end
+      let!(:source) do
+        create(:tag_name, 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
 
     context 'when both source and existing tag_names have tags' do
-      let!(:existing_tn) { TagName.create!(name: 'foobar') }
-      let!(:existing_tag) { Tag.create!(tag_name: existing_tn, category: :general) }
+      let!(:existing_tn) { create(:tag_name, name: 'foobar') }
+      let!(:existing_tag) { create(:tag, tag_name: existing_tn, category: :general) }
 
-      let!(:source_tn) { TagName.create!(name: 'tmp') }
-      let!(:source_tag) { Tag.create!(tag_name: source_tn, category: :general) }
+      let!(:source_tn) { create(:tag_name, name: 'tmp') }
+      let!(:source_tag) { create(:tag, tag_name: source_tn, category: :general) }
       let!(:source_tag_name_id) { source_tn.id }
 
       before do
@@ -92,13 +144,15 @@ 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_name_spec.rb

@@ -0,0 +1,73 @@
+require 'rails_helper'
+
+RSpec.describe TagName, type: :model do
+  include_context 'English locale'
+
+  it 'allows the same name in different languages' do
+    create(:tag_name, name: 'shared_name')
+    english = build(:tag_name, name: 'shared_name',
+                              language_code: 'en', script_code: 'Latn')
+
+    expect(english).to be_valid
+    expect { english.save! }.to change(described_class, :count).by(1)
+  end
+
+  it 'rejects a duplicate name within the same language' do
+    create(:tag_name, name: 'shared_name')
+    duplicate = build(:tag_name, name: 'shared_name')
+
+    expect(duplicate).to be_invalid
+    expect(duplicate.errors.of_kind?(:name, :taken)).to be(true)
+  end
+
+  it 'creates Japanese factory names without requiring a tag' do
+    name = create(:tag_name)
+
+    expect(name.reload).to have_attributes(
+        language_code: 'ja', script_code: 'Jpan', primary_flg: true, tag_id: nil)
+  end
+
+  it 'persists both sides of the representative tag factory association' do
+    tag = create(:tag)
+
+    expect(tag.reload.tag_name.reload.tag_id).to eq(tag.id)
+    expect(tag.tag_name.tag).to eq(tag)
+  end
+
+  it 'accepts an alias owned by the canonical tag' do
+    tag = create(:tag)
+    alias_name = build(:tag_name, name: 'valid_alias', canonical: tag.tag_name,
+                                 tag:, primary_flg: false)
+
+    expect(alias_name).to have_attributes(
+        canonical_id: tag.tag_name_id, tag_id: tag.id,
+        primary_flg: false, language_code: 'ja')
+    expect(alias_name).to be_valid
+  end
+
+  describe '.canonicalise' do
+    it 'resolves only aliases in the requested language' do
+      japanese = create(:tag, name: 'japanese_canonical')
+      english_name = create(:tag_name, name: 'english_canonical',
+                                      language_code: 'en', script_code: 'Latn')
+      english = create(:tag, tag_name: english_name)
+      japanese_alias = create(:tag_name, :alias, name: 'shared_alias',
+                                                 canonical: japanese.tag_name)
+      english_alias = create(:tag_name, :alias, name: 'temporary_alias',
+                                                canonical: english_name,
+                                                language_code: 'en', script_code: 'Latn')
+      english_alias.update_columns(name: 'shared_alias')
+
+      expect(japanese_alias.reload).to have_attributes(
+          canonical_id: japanese.tag_name_id, tag_id: japanese.id,
+          primary_flg: false, language_code: 'ja')
+      expect(english_alias.reload).to have_attributes(
+          canonical_id: english_name.id, tag_id: english.id,
+          primary_flg: false, language_code: 'en')
+      expect(described_class.canonicalise(locale, ['shared_alias']))
+          .to eq(['english_canonical'])
+      expect(described_class.canonicalise(Locale.nipponese, ['shared_alias']))
+          .to eq(['japanese_canonical'])
+    end
+  end
+end

backend/spec/models/tag_spec.rb

@@ -1,20 +1,73 @@
 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!(Locale.nipponese, [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 'canonicalises aliases in the supplied language only' do
+      Language.find_or_create_by!(code: 'en') { _1.name = 'English' }
+      Script.find_or_create_by!(code: 'Latn') { _1.name = 'Latin' }
+      Locale.insert_all!([
+          { code: 'en', language_code: 'en', script_code: 'Latn',
+            name: 'English', created_at: Time.current }]) unless Locale.exists?(code: 'en')
+      english_locale = Locale.find('en')
+      japanese = create(:tag, name: 'japanese_canonical')
+      english_name = create(:tag_name, name: 'english_canonical',
+                                      language_code: 'en', script_code: 'Latn')
+      english = create(:tag, tag_name: english_name)
+      japanese_alias = create(:tag_name, :alias, name: 'shared_alias',
+                                                 canonical: japanese.tag_name)
+      english_alias = create(:tag_name, :alias, name: 'temporary_alias',
+                                                canonical: english_name,
+                                                language_code: 'en', script_code: 'Latn')
+      english_alias.update_columns(name: 'shared_alias')
+
+      expect(japanese_alias.reload).to have_attributes(
+          canonical_id: japanese.tag_name_id, tag_id: japanese.id,
+          primary_flg: false, language_code: 'ja')
+      expect(english_alias.reload).to have_attributes(
+          canonical_id: english_name.id, tag_id: english.id,
+          primary_flg: false, language_code: 'en')
+      expect(described_class.normalise_tags!(
+          english_locale, ['shared_alias'],
+          with_tagme: false, with_no_deerjikist: false)).to eq([english])
+      expect(described_class.normalise_tags!(
+          Locale.nipponese, ['shared_alias'],
+          with_tagme: false, with_no_deerjikist: false)).to eq([japanese])
+    end
+
     it 'rejects deprecated tags when deny_deprecated is enabled' do
-      tag_name = TagName.create!(name: 'normalise deprecated tag')
-      deprecated_tag = Tag.create!(
+      tag_name = create(:tag_name, name: 'normalise deprecated tag')
+      deprecated_tag = create(:tag,
           tag_name:,
           category: :general,
-        deprecated_at: 1.day.from_now
-      )
+          deprecated_at: 1.day.from_now)
 
       expect {
-        described_class.normalise_tags!(
+        described_class.normalise_tags!(Locale.nipponese,
             [deprecated_tag.name],
-          deny_deprecated: true
-        )
+            deny_deprecated: true)
       }.to raise_error(Tag::DeprecatedTagNormalisationError) { |error|
         expect(error.tag_names).to eq([deprecated_tag.name])
       }
@@ -22,88 +75,79 @@ RSpec.describe Tag, type: :model do
 
     it 'rejects invalid section literals instead of treating them as zero' do
       expect {
-        described_class.normalise_tags!(
+        described_class.normalise_tags!(Locale.nipponese,
             ['normalise_invalid_section[1:aa-2:00]'],
-          with_sections: true
-        )
+            with_sections: true)
       }.to raise_error(Tag::SectionLiteralParseError)
     end
 
     it 'parses open-ended section literals' do
-      result = described_class.normalise_tags!(
+      result = described_class.normalise_tags!(Locale.nipponese,
           ['伊地知ニジカ[1:00-]'],
-        with_sections: true
-      )
+          with_sections: true)
 
       tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
       expect(result.fetch(:sections).fetch(tag.id)).to eq([[60_000, nil]])
     end
 
     it 'parses omitted begin as zero' do
-      result = described_class.normalise_tags!(
+      result = described_class.normalise_tags!(Locale.nipponese,
           ['伊地知ニジカ[-1:00]'],
-        with_sections: true
-      )
+          with_sections: true)
 
       tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
       expect(result.fetch(:sections).fetch(tag.id)).to eq([[0, 60_000]])
     end
 
     it 'treats fully open section literals as plain tags' do
-      result = described_class.normalise_tags!(
+      result = described_class.normalise_tags!(Locale.nipponese,
           ['伊地知ニジカ[-]'],
-        with_sections: true
-      )
+          with_sections: true)
 
       tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
       expect(result.fetch(:sections)[tag.id]).to be_nil
     end
 
     it 'treats [0:00-] as a plain tag' do
-      result = described_class.normalise_tags!(
+      result = described_class.normalise_tags!(Locale.nipponese,
           ['伊地知ニジカ[0:00-]'],
-        with_sections: true
-      )
+          with_sections: true)
 
       tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
       expect(result.fetch(:sections)[tag.id]).to be_nil
     end
 
     it 'expands zero-width sections to one millisecond' do
-      result = described_class.normalise_tags!(
+      result = described_class.normalise_tags!(Locale.nipponese,
           ['伊地知ニジカ[1:00-1:00]'],
-        with_sections: true
-      )
+          with_sections: true)
 
       tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
       expect(result.fetch(:sections).fetch(tag.id)).to eq([[60_000, 60_001]])
     end
 
     it 'swaps reversed section boundaries' do
-      result = described_class.normalise_tags!(
+      result = described_class.normalise_tags!(Locale.nipponese,
           ['伊地知ニジカ[2:00-1:00]'],
-        with_sections: true
-      )
+          with_sections: true)
 
       tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
       expect(result.fetch(:sections).fetch(tag.id)).to eq([[60_000, 120_000]])
     end
 
     it 'merges open-ended sections over later bounded sections' do
-      result = described_class.normalise_tags!(
+      result = described_class.normalise_tags!(Locale.nipponese,
           ['伊地知ニジカ[1:00-][2:00-3:00]'],
-        with_sections: true
-      )
+          with_sections: true)
 
       tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
       expect(result.fetch(:sections).fetch(tag.id)).to eq([[60_000, nil]])
     end
 
     it 'merges adjacent bounded and open-ended sections' do
-      result = described_class.normalise_tags!(
+      result = described_class.normalise_tags!(Locale.nipponese,
           ['伊地知ニジカ[1:00-3:00][3:00-]'],
-        with_sections: true
-      )
+          with_sections: true)
 
       tag = result.fetch(:tags).find { _1.name == '伊地知ニジカ' }
       expect(result.fetch(:sections).fetch(tag.id)).to eq([[60_000, nil]])
@@ -159,17 +203,109 @@ 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
+    context 'with an explicit locale' do
+      include_context 'English locale'
 
-      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．')
+      it 'creates the representative name with locale attributes and ownership' do
+        tag = described_class.find_or_create_by_tag_name!(
+            locale, 'english_name', category: :character)
+
+        expect(tag.tag_name.reload).to have_attributes(
+            language_code: 'en', script_code: 'Latn',
+            primary_flg: true, tag_id: tag.id)
+        expect(tag.category).to eq('character')
+      end
+
+      it 'creates separate tag identities for the same name in different languages' do
+        japanese = described_class.find_or_create_by_tag_name!(
+            Locale.nipponese, 'same_name', category: :general)
+        english = described_class.find_or_create_by_tag_name!(
+            locale, 'same_name', category: :character)
+
+        expect(english).not_to eq(japanese)
+        expect(english.tag_name.language_code).to eq('en')
+        expect(japanese.tag_name.language_code).to eq('ja')
+        expect(described_class.find_or_create_by_tag_name!(
+            locale, 'same_name', category: :general)).to eq(english)
+        expect(described_class.find_or_create_by_tag_name!(
+            Locale.nipponese, 'same_name', category: :general)).to eq(japanese)
+      end
+
+      it 'preserves the V1 representative when looking up another primary language' do
+        tag = create(:tag, name: '日本語代表名')
+        representative_id = tag.tag_name_id
+        english = create(:tag_name, name: 'english_primary', tag:,
+            language_code: 'en', script_code: 'Latn')
+
+        found = described_class.find_or_create_by_tag_name!(
+            locale, english.name, category: :general)
+
+        expect(found).to eq(tag)
+        expect(tag.reload.tag_name_id).to eq(representative_id)
+        expect(english.reload.tag_id).to eq(tag.id)
+      end
+
+      it 'normalises names using the supplied locale' do
+        tags = described_class.normalise_tags!(
+            locale, ['character:normalised_english'],
+            with_tagme: false, with_no_deerjikist: false)
+
+        expect(tags.length).to eq(1)
+        expect(tags.first.tag_name).to have_attributes(
+            name: 'normalised_english', language_code: 'en', script_code: 'Latn',
+            primary_flg: true, tag_id: tags.first.id)
+      end
+    end
+
+    it 'creates a tag and name with the requested category after stripping whitespace' do
+      tag = nil
+
+      expect {
+        tag = described_class.find_or_create_by_tag_name!(Locale.nipponese,
+            '  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)
+      representative_id = tag.tag_name_id
+      alias_name = create(:tag_name, :alias, name: 'lookup_alias', canonical: tag.tag_name)
+      expect(alias_name.reload).to have_attributes(
+          canonical_id: tag.tag_name_id, tag_id: tag.id,
+          primary_flg: false, language_code: 'ja')
+      found = nil
+
+      expect {
+        found = described_class.find_or_create_by_tag_name!(Locale.nipponese,
+            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')
+      expect(tag.reload.tag_name_id).to eq(representative_id)
+    end
+
+    it 'reuses the canonical tag through another alias' do
+      tag = create(:tag)
+      canonical = tag.tag_name
+      alias_name = create(:tag_name, :alias, name: 'lookup_alias', canonical:)
+      other_alias = create(:tag_name, :alias, name: 'lookup_other_alias', canonical:)
+
+      expect([alias_name.reload, other_alias.reload]).to all(have_attributes(
+          canonical_id: canonical.id, tag_id: tag.id,
+          primary_flg: false, language_code: 'ja'))
+
+      expect {
+        found = described_class.find_or_create_by_tag_name!(Locale.nipponese,
+            alias_name.name, category: :general)
+        expect(found).to eq(tag)
+      }.to change(Tag, :count).by(0).and change(TagName, :count).by(0)
+
+      expect(tag.tag_name).to eq(canonical)
     end
   end
 
@@ -185,18 +321,14 @@ 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 +338,101 @@ 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 = create(:tag_name, :alias, 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 +455,16 @@ 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 +481,32 @@ 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
@@ -365,12 +552,15 @@ 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/gekanator_learning_spec.rb

@@ -898,12 +898,11 @@ RSpec.describe 'Gekanator learning API', type: :request do
 
   describe 'GET /gekanator/questions' do
     it 'omits questions for deprecated tags' do
-      active_tag = Tag.create!(name: 'active_question_tag', category: :general)
-      deprecated_tag = Tag.create!(
+      active_tag = create(:tag, name: 'active_question_tag', category: :general)
+      deprecated_tag = create(:tag,
           name: 'deprecated_question_tag',
           category: :general,
-        deprecated_at: Time.current
-      )
+          deprecated_at: Time.current)
 
       [active_tag, deprecated_tag].each do |question_tag|
         GekanatorQuestion.create!(

backend/spec/requests/gekanator_posts_spec.rb

@@ -4,12 +4,11 @@ require 'rails_helper'
 RSpec.describe 'Gekanator posts API', type: :request do
   describe 'GET /gekanator/posts' do
     it 'omits deprecated tags and returns the stored similarity cosine' do
-      active_tag = Tag.create!(name: 'active tag', category: :general)
-      deprecated_tag = Tag.create!(
+      active_tag = create(:tag, name: 'active tag', category: :general)
+      deprecated_tag = create(:tag,
           name: 'deprecated tag',
           category: :general,
-        deprecated_at: Time.current
-      )
+          deprecated_at: Time.current)
       post_record = Post.create!(title: 'source', url: 'https://example.com/source')
       target_post = Post.create!(title: 'target', url: 'https://example.com/target')
 

backend/spec/requests/locale_write_paths_spec.rb

@@ -0,0 +1,232 @@
+require 'rails_helper'
+
+RSpec.describe 'Locale propagation on write paths', type: :request do
+  include_context 'English locale'
+
+  let(:member) { create(:user, :member) }
+
+  before do
+    sign_in_as(member)
+  end
+
+  def expect_localised_tag(name, expected_locale)
+    tag_name = TagName.find_by!(name:, language_code: expected_locale.language_code)
+    expect(tag_name).to have_attributes(
+        script_code: expected_locale.script_code, primary_flg: true)
+    expect(tag_name.tag_id).to be_present
+    expect(tag_name.tag.tag_name_id).to eq(tag_name.id)
+    tag_name.tag
+  end
+
+  ['en', nil].each do |requested_locale|
+    context "with locale #{ requested_locale.inspect }" do
+      let(:locale_params) { requested_locale ? { locale: requested_locale } : { } }
+      let(:expected_locale) { requested_locale ? locale : Locale.nipponese }
+
+      it 'creates post tags in the requested language or Japanese fallback' do
+        post '/posts', params: locale_params.merge(
+            title: 'Locale post', url: 'https://example.com/locale-post',
+            tags: 'locale_post_tag', parent_post_ids: '')
+
+        expect(response).to have_http_status(:created), response.body
+        tag = expect_localised_tag('locale_post_tag', expected_locale)
+        expect(Post.find(json.fetch('id')).tags).to include(tag)
+      end
+
+      it 'updates post tags in the requested language or Japanese fallback' do
+        record = create(:post)
+        PostVersionRecorder.record!(post: record, event_type: :create, created_by_user: member)
+
+        put "/posts/#{ record.id }", params: locale_params.merge(
+            title: record.title, tags: 'locale_updated_tag', parent_post_ids: '',
+            base_version_no: record.reload.version_no)
+
+        expect(response).to have_http_status(:ok), response.body
+        tag = expect_localised_tag('locale_updated_tag', expected_locale)
+        expect(record.reload.tags).to include(tag)
+      end
+
+      it 'passes the resolved locale to bulk creation' do
+        # Worker propagation is exercised in post_bulk_creator_spec.
+        expect_any_instance_of(PostBulkCreator).to receive(:run)
+            .with(expected_locale).and_return(results: [])
+        manifest = [{ title: 'Bulk locale', url: 'https://example.com/bulk-locale',
+                      tags: 'bulk_locale_tag', parent_post_ids: '' }]
+
+        post '/posts/bulk', params: locale_params.merge(posts: JSON.generate(manifest)),
+                            headers: { 'CONTENT_TYPE' => 'multipart/form-data' }
+
+        expect(response).to have_http_status(:ok), response.body
+      end
+
+      it 'creates materials with tags in the resolved locale' do
+        post '/materials', params: locale_params.merge(
+            tag: 'locale_material', url: 'https://example.com/material')
+
+        expect(response).to have_http_status(:created), response.body
+        tag = expect_localised_tag('locale_material', expected_locale)
+        expect(Material.find(json.fetch('id')).tag).to eq(tag)
+      end
+
+      it 'updates materials with tags in the resolved locale' do
+        tag = create(:tag, category: :material)
+        material = Material.create!(tag:, url: 'https://example.com/material')
+
+        put "/materials/#{ material.id }", params: locale_params.merge(
+            tag: 'locale_material_updated', url: material.url)
+
+        expect(response).to have_http_status(:ok), response.body
+        resolved_tag = expect_localised_tag('locale_material_updated', expected_locale)
+        expect(material.reload.tag).to eq(resolved_tag)
+      end
+
+      it 'links Nico external tags to internal tags in the resolved locale' do
+        external = create(:external_tag)
+        NicoTagVersionRecorder.record!(
+            external_tag: external, event_type: :create, created_by_user: member)
+
+        put "/tags/nico/#{ external.id }", params: locale_params.merge(tags: 'locale_link')
+
+        expect(response).to have_http_status(:ok), response.body
+        tag = expect_localised_tag('locale_link', expected_locale)
+        expect(external.reload.linked_tags).to contain_exactly(tag)
+      end
+
+      it 'creates parent tags in the resolved locale' do
+        tag = create(:tag)
+
+        put "/tags/#{ tag.id }", params: locale_params.merge(
+            name: tag.name, category: tag.category, deprecated: false,
+            aliases: '', parent_tags: 'locale_parent')
+
+        expect(response).to have_http_status(:ok), response.body
+        parent = expect_localised_tag('locale_parent', expected_locale)
+        expect(TagImplication.where(tag:).pluck(:parent_tag_id)).to eq([parent.id])
+      end
+    end
+  end
+
+  context 'with existing names in both languages' do
+    let!(:english_name) do
+      create(:tag_name, name: 'shared_name', language_code: 'en', script_code: 'Latn')
+    end
+    let!(:english_tag) { create(:tag, tag_name: english_name, category: :material) }
+    let!(:japanese_tag) do
+      create(:tag, name: 'temporary_japanese', category: :general).tap do |tag|
+        # Isolate lookup from the separately tested uniqueness validation.
+        tag.tag_name.update_columns(name: 'shared_name')
+      end
+    end
+
+    it 'plans and creates the post using the English identity and category' do
+      post '/posts', params: {
+          locale: 'en', title: 'English identity',
+          url: 'https://example.com/english-identity', tags: 'shared_name', parent_post_ids: '' }
+
+      expect(response).to have_http_status(:created), response.body
+      record = Post.find(json.fetch('id'))
+      expect(record.tags).to include(english_tag)
+      expect(record.tags).not_to include(japanese_tag)
+      expect(english_tag.reload.category).to eq('material')
+      expect(japanese_tag.reload.category).to eq('general')
+    end
+
+    it 'updates the post using the English identity' do
+      record = create(:post)
+      PostVersionRecorder.record!(post: record, event_type: :create, created_by_user: member)
+
+      put "/posts/#{ record.id }", params: {
+          locale: 'en', title: record.title, tags: 'shared_name', parent_post_ids: '',
+          base_version_no: record.reload.version_no }
+
+      expect(response).to have_http_status(:ok), response.body
+      expect(record.reload.tags).to include(english_tag)
+      expect(record.tags).not_to include(japanese_tag)
+    end
+
+    it 'resolves the English material tag' do
+      post '/materials', params: {
+          locale: 'en', tag: 'shared_name', url: 'https://example.com/english-material' }
+
+      expect(response).to have_http_status(:created), response.body
+      expect(Material.find(json.fetch('id')).tag).to eq(english_tag)
+      expect(Material.where(tag: japanese_tag)).to be_empty
+    end
+
+    it 'resolves the English parent tag' do
+      child = create(:tag)
+
+      put "/tags/#{ child.id }", params: {
+          locale: 'en', name: child.name, category: child.category, deprecated: false,
+          aliases: '', parent_tags: 'shared_name' }
+
+      expect(response).to have_http_status(:ok), response.body
+      expect(TagImplication.where(tag: child).pluck(:parent_tag_id)).to eq([english_tag.id])
+    end
+
+    it 'resolves the English internal tag for a Nico link' do
+      external = create(:external_tag)
+      NicoTagVersionRecorder.record!(
+          external_tag: external, event_type: :create, created_by_user: member)
+
+      put "/tags/nico/#{ external.id }", params: { locale: 'en', tags: 'shared_name' }
+
+      expect(response).to have_http_status(:ok), response.body
+      expect(external.reload.linked_tags).to contain_exactly(english_tag)
+    end
+  end
+
+  context 'when renaming or updating aliases in English' do
+    let!(:english_name) do
+      create(:tag_name, name: 'english_original', language_code: 'en', script_code: 'Latn')
+    end
+    let!(:tag) { create(:tag, tag_name: english_name) }
+
+    it 'allows a rename to a name already used in Japanese' do
+      japanese = create(:tag, name: 'rename_target')
+
+      patch "/tags/#{ tag.id }", params: { locale: 'en', name: 'rename_target' }
+
+      expect(response).to have_http_status(:ok), response.body
+      expect(tag.reload.tag_name).to have_attributes(
+          name: 'rename_target', language_code: 'en', tag_id: tag.id)
+      expect(japanese.reload.tag_name).to have_attributes(
+          name: 'rename_target', language_code: 'ja', tag_id: japanese.id)
+    end
+
+    it 'creates aliases in the requested language' do
+      put "/tags/#{ tag.id }", params: {
+          locale: 'en', name: tag.name, category: tag.category, deprecated: false,
+          aliases: 'english_alias', parent_tags: '' }
+
+      expect(response).to have_http_status(:ok), response.body
+      expect(TagName.find_by!(language_code: 'en', name: 'english_alias'))
+          .to have_attributes(
+              script_code: 'Latn', canonical_id: english_name.id,
+              tag_id: tag.id, primary_flg: false, language_code: 'en')
+      expect(tag.reload.tag_name_id).to eq(english_name.id)
+    end
+
+    it 'does not take an alias from another language' do
+      japanese = create(:tag, name: 'japanese_owner')
+      japanese_alias = create(:tag_name, :alias, name: 'shared_alias',
+                                                canonical: japanese.tag_name)
+      expect(japanese_alias.reload).to have_attributes(
+          canonical_id: japanese.tag_name_id, tag_id: japanese.id,
+          primary_flg: false, language_code: 'ja')
+
+      put "/tags/#{ tag.id }", params: {
+          locale: 'en', name: tag.name, category: tag.category, deprecated: false,
+          aliases: 'shared_alias', parent_tags: '' }
+
+      expect(response).to have_http_status(:ok), response.body
+      expect(japanese_alias.reload).to have_attributes(
+          canonical_id: japanese.tag_name_id, tag_id: japanese.id,
+          primary_flg: false, language_code: 'ja')
+      expect(TagName.find_by!(language_code: 'en', name: 'shared_alias'))
+          .to have_attributes(
+              canonical_id: english_name.id, tag_id: tag.id,
+              primary_flg: false, language_code: 'en')
+    end
+  end
+end

backend/spec/requests/materials_spec.rb

@@ -26,10 +26,10 @@ RSpec.describe 'Materials API', type: :request do
 
   describe 'GET /materials' do
     let!(:tag_a) do
-      Tag.create!(tag_name: TagName.create!(name: 'material_index_a'), category: :material)
+      create(:tag, name: 'material_index_a', category: :material)
     end
     let!(:tag_b) do
-      Tag.create!(tag_name: TagName.create!(name: 'material_index_b'), category: :material)
+      create(:tag, name: 'material_index_b', category: :material)
     end
 
     let!(:material_a) do
@@ -116,20 +116,20 @@ RSpec.describe 'Materials API', type: :request do
 
     it 'filters by descendant tags and returns stable parent tag groups' do
       root =
-          Tag.create!(tag_name: TagName.create!(name: 'material_scope_root'),
+          create(:tag, name: 'material_scope_root',
                       category: :material)
       child_b =
-          Tag.create!(tag_name: TagName.create!(name: 'material_scope_b'),
+          create(:tag, name: 'material_scope_b',
                       category: :material)
       child_a =
-          Tag.create!(tag_name: TagName.create!(name: 'material_scope_a'),
+          create(:tag, name: 'material_scope_a',
                       category: :material)
       deprecated =
-          Tag.create!(tag_name: TagName.create!(name: 'material_scope_old'),
+          create(:tag, name: 'material_scope_old',
                       category: :material,
                       deprecated_at: Time.current)
       grandchild =
-          Tag.create!(tag_name: TagName.create!(name: 'material_scope_grandchild'),
+          create(:tag, name: 'material_scope_grandchild',
                       category: :material)
       root_material =
           build_material(tag: root, user: member_user,
@@ -179,7 +179,7 @@ RSpec.describe 'Materials API', type: :request do
 
   describe 'GET /materials/:id' do
     let!(:tag) do
-      Tag.create!(tag_name: TagName.create!(name: 'material_show'), category: :material)
+      create(:tag, name: 'material_show', category: :material)
     end
     let!(:material) do
       build_material(tag:, user: member_user, file: dummy_upload(filename: 'show.png'))
@@ -323,8 +323,8 @@ RSpec.describe 'Materials API', type: :request do
       end
 
       it 'returns 422 when the existing tag is not material/character' do
-        general_tag_name = TagName.create!(name: 'material_create_general_tag')
-        Tag.create!(tag_name: general_tag_name, category: :general)
+        general_tag_name = create(:tag_name, name: 'material_create_general_tag')
+        create(:tag, tag_name: general_tag_name, category: :general)
 
         post '/materials', params: {
           tag: 'material_create_general_tag',
@@ -384,7 +384,7 @@ RSpec.describe 'Materials API', type: :request do
 
   describe 'PUT /materials/:id' do
     let!(:tag) do
-      Tag.create!(tag_name: TagName.create!(name: 'material_update_old'), category: :material)
+      create(:tag, name: 'material_update_old', category: :material)
     end
     let!(:material) do
       build_material(tag:, user: member_user, file: dummy_upload(filename: 'old.png'))
@@ -508,7 +508,7 @@ RSpec.describe 'Materials API', type: :request do
 
       it 'backfills a create tag_version for an existing material tag without history' do
         existing_tag =
-            Tag.create!(tag_name: TagName.create!(name: 'material_update_existing_no_history'),
+            create(:tag, name: 'material_update_existing_no_history',
                         category: :material)
 
         expect(existing_tag.tag_versions).to be_empty
@@ -531,7 +531,7 @@ RSpec.describe 'Materials API', type: :request do
 
       it 'backfills a create tag_version for an existing character tag without history' do
         existing_tag =
-            Tag.create!(tag_name: TagName.create!(name: 'material_update_character_no_history'),
+            create(:tag, name: 'material_update_character_no_history',
                         category: :character)
 
         expect(existing_tag.tag_versions).to be_empty
@@ -651,8 +651,8 @@ RSpec.describe 'Materials API', type: :request do
   end
 
   describe 'GET /materials/download.zip' do
-    let!(:tag_a) { Tag.create!(tag_name: TagName.create!(name: 'zip_a'), category: :material) }
-    let!(:tag_b) { Tag.create!(tag_name: TagName.create!(name: 'zip_b'), category: :material) }
+    let!(:tag_a) { create(:tag, name: 'zip_a', category: :material) }
+    let!(:tag_b) { create(:tag, name: 'zip_b', category: :material) }
     let!(:material_a) do
       build_material(tag: tag_a, user: member_user,
                      file: dummy_upload(filename: 'a.png', body: 'zip-a'))
@@ -701,7 +701,7 @@ RSpec.describe 'Materials API', type: :request do
 
   describe 'GET /materials/versions' do
     let!(:tag) do
-      Tag.create!(tag_name: TagName.create!(name: 'material_history'), category: :material)
+      create(:tag, name: 'material_history', category: :material)
     end
     let!(:material) do
       build_material(tag:, user: member_user, file: dummy_upload(filename: 'history.png'))
@@ -776,7 +776,7 @@ RSpec.describe 'Materials API', type: :request do
 
   describe 'DELETE /materials/:id' do
     let!(:tag) do
-      Tag.create!(tag_name: TagName.create!(name: 'material_destroy'), category: :material)
+      create(:tag, name: 'material_destroy', category: :material)
     end
     let!(:material) do
       build_material(tag:, user: member_user, file: dummy_upload(filename: 'destroy.png'))

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)
@@ -41,37 +65,77 @@ 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)
     end
   end
 
+  describe 'GET /tags/nico/:id' do
+    it 'returns the external tag even when an internal tag has the same id' do
+      internal = create(:tag)
+      external = create(
+          :external_tag,
+          id: internal.id,
+          name: 'external_detail')
+
+      get "/tags/nico/#{ external.id }"
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include(
+          'id' => external.id,
+          'name' => 'nico:external_detail',
+          'category' => 'nico')
+    end
+
+    it 'returns 404 when the external tag does not exist' do
+      internal = create(:tag)
+
+      get "/tags/nico/#{ internal.id }"
+
+      expect(response).to have_http_status(:not_found)
+    end
+  end
+
   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
@@ -85,26 +149,27 @@ 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)
+      linked_a_name = create(:tag_name, name: 'nico_linked_a')
+      linked_a = create(:tag, tag_name: linked_a_name, category: :general)
 
-      linked_b_name = TagName.create!(name: 'nico_linked_b')
-      linked_b = Tag.create!(tag_name: linked_b_name, category: :general)
+      linked_b_name = create(:tag_name, name: 'nico_linked_b')
+      linked_b = create(:tag, 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 +191,26 @@ 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/posts_spec.rb

@@ -17,7 +17,7 @@ RSpec.describe 'Posts API', type: :request do
   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
@@ -93,8 +93,28 @@ RSpec.describe 'Posts API', type: :request do
     count
   end
 
-  let!(:tag_name) { TagName.create!(name: 'spec_tag') }
-  let!(:tag)      { Tag.create!(tag_name: tag_name, category: :general) }
+  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) { create(:tag_name, name: 'spec_tag') }
+  let!(:tag)      { create(:tag, tag_name: tag_name, category: :general) }
 
   let!(:post_record) do
     Post.create!(title: 'spec post', url: 'https://example.com/spec').tap do |p|
@@ -105,11 +125,11 @@ RSpec.describe 'Posts API', type: :request do
   describe 'GET /posts' do
     let!(:user) { create_member_user! }
 
-    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!(:tag_name) { create(:tag_name, name: 'spec_tag') }
+    let!(:tag) { create(:tag, tag_name:, category: :general) }
+    let!(:tag_name2) { create(:tag_name, name: 'unko') }
+    let!(:tag2) { create(:tag, tag_name: tag_name2, category: :deerjikist) }
+    let!(:alias_tag_name) { create(:tag_name, :alias, name: 'manko', canonical: tag_name) }
 
     let!(:hit_post) do
       Post.create!(uploaded_user: user, title: 'hello spec world',
@@ -148,12 +168,15 @@ RSpec.describe 'Posts API', type: :request do
 
     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([])
@@ -162,6 +185,26 @@ RSpec.describe 'Posts API', type: :request do
       ])
     end
 
+    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' }
@@ -210,18 +253,105 @@ RSpec.describe 'Posts API', type: :request do
       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) }
+      let!(:foo_tag_name) { create(:tag_name, name: 'not_spec_foo') }
+      let!(:foo_tag) { create(:tag, tag_name: foo_tag_name, category: :general) }
 
-      let!(:bar_tag_name) { TagName.create!(name: 'not_spec_bar') }
-      let!(:bar_tag) { Tag.create!(tag_name: bar_tag_name, category: :general) }
+      let!(:bar_tag_name) { create(:tag_name, name: 'not_spec_bar') }
+      let!(:bar_tag) { create(:tag, tag_name: bar_tag_name, category: :general) }
 
-      let!(:baz_tag_name) { TagName.create!(name: 'not_spec_baz') }
-      let!(:baz_tag) { Tag.create!(tag_name: baz_tag_name, category: :general) }
+      let!(:baz_tag_name) { create(:tag_name, name: 'not_spec_baz') }
+      let!(:baz_tag) { create(:tag, tag_name: baz_tag_name, category: :general) }
 
       let!(:foo_alias_tag_name) do
-        TagName.create!(name: 'not_spec_foo_alias', canonical: foo_tag_name)
+        create(:tag_name, :alias, name: 'not_spec_foo_alias', canonical: foo_tag_name)
       end
 
       let!(:foo_only_post) do
@@ -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
@@ -553,11 +772,10 @@ RSpec.describe 'Posts API', type: :request do
       end
 
       it 'omits deprecated tags' do
-        deprecated_tag = Tag.create!(
+        deprecated_tag = create(:tag,
             name: 'deprecated_post_tag',
             category: :general,
-          deprecated_at: Time.current
-        )
+            deprecated_at: Time.current)
         PostTag.create!(post: post_record, tag: deprecated_tag)
 
         request
@@ -634,9 +852,9 @@ RSpec.describe 'Posts API', type: :request do
 
         tags =
             15.times.map do |i|
-              tag_name = TagName.create!(name: "show_query_tag_#{ i }")
-              tag = Tag.create!(tag_name:, category: :general)
-              TagName.create!(name: "show_query_alias_#{ i }", canonical: tag_name)
+              tag_name = create(:tag_name, name: "show_query_tag_#{ i }")
+              tag = create(:tag, tag_name:, category: :general)
+              create(:tag_name, :alias, name: "show_query_alias_#{ i }", canonical: tag_name)
               PostTag.create!(post: post_record, tag:)
               tag
             end
@@ -681,6 +899,25 @@ 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
@@ -762,7 +999,7 @@ RSpec.describe 'Posts API', type: :request do
 
   describe 'POST /posts' do
     let(:member) { create(:user, :member) }
-    let!(:alias_tag_name) { TagName.create!(name: 'manko', canonical: tag_name) }
+    let!(:alias_tag_name) { create(:tag_name, :alias, name: 'manko', canonical: tag_name) }
 
     it '401 when not logged in' do
       sign_out
@@ -919,11 +1156,10 @@ RSpec.describe 'Posts API', type: :request do
     end
 
     it 'rejects a deprecated tag specified directly' do
-      Tag.create!(
+      create(:tag,
           name: 'deprecated_direct_tag',
           category: :general,
-        deprecated_at: Time.current
-      )
+          deprecated_at: Time.current)
       sign_in_as(member)
 
       post '/posts', params: post_write_params(
@@ -940,18 +1176,16 @@ RSpec.describe 'Posts API', type: :request do
     end
 
     it 'expands through multiple deprecated parent tags and saves active ancestors' do
-      child = Tag.create!(name: 'active_child', category: :general)
-      deprecated_parent = Tag.create!(
+      child = create(:tag, name: 'active_child', category: :general)
+      deprecated_parent = create(:tag,
           name: 'deprecated_parent',
           category: :general,
-        deprecated_at: Time.current
-      )
-      deprecated_grandparent = Tag.create!(
+          deprecated_at: Time.current)
+      deprecated_grandparent = create(:tag,
           name: 'deprecated_grandparent',
           category: :general,
-        deprecated_at: Time.current
-      )
-      active_grandparent = Tag.create!(name: 'active_grandparent', category: :general)
+          deprecated_at: Time.current)
+      active_grandparent = create(:tag, name: 'active_grandparent', category: :general)
       TagImplication.create!(tag: child, parent_tag: deprecated_parent)
       TagImplication.create!(tag: deprecated_parent, parent_tag: deprecated_grandparent)
       TagImplication.create!(tag: deprecated_grandparent, parent_tag: active_grandparent)
@@ -1162,11 +1396,9 @@ 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
@@ -1386,6 +1618,7 @@ RSpec.describe 'Posts API', type: :request do
         expect(arguments[:thumbnails].keys).to eq([0])
         expect(arguments[:host]).to eq('http://www.example.com')
       end
+      expect(creator).to have_received(:run).with(Locale.nipponese)
     end
 
     it 'rejects malformed manifests as a request-level error' do
@@ -1419,9 +1652,11 @@ 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)
+      tn2 = create(:tag_name, name: 'spec_tag_2')
+      replacement_tag = create(:tag, tag_name: tn2, category: :general)
 
       put "/posts/#{post_record.id}", params: post_update_params(
           post_record,
@@ -1434,14 +1669,45 @@ 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
-      Tag.create!(
+      create(:tag,
           name: 'deprecated_update_tag',
           category: :general,
-        deprecated_at: Time.current
-      )
+          deprecated_at: Time.current)
       sign_in_as(member)
 
       put "/posts/#{ post_record.id }", params: post_update_params(
@@ -1456,11 +1722,9 @@ 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
@@ -1718,7 +1982,8 @@ RSpec.describe 'Posts API', type: :request do
 
         base_version = create_post_version_for!(post_record.reload)
 
-        current_tag = Tag.find_or_create_by_tag_name!('current_added_tag', category: :general)
+        current_tag = Tag.find_or_create_by_tag_name!(
+            Locale.nipponese, 'current_added_tag', category: :general)
         PostTag.create!(post: post_record, tag: current_tag, created_user: member)
 
         PostVersionRecorder.record!(
@@ -1747,7 +2012,8 @@ RSpec.describe 'Posts API', type: :request do
 
         base_version = create_post_version_for!(post_record.reload)
 
-        current_tag = Tag.find_or_create_by_tag_name!('current_merge_tag', category: :general)
+        current_tag = Tag.find_or_create_by_tag_name!(
+            Locale.nipponese, 'current_merge_tag', category: :general)
         PostTag.create!(post: post_record, tag: current_tag, created_user: member)
 
         PostVersionRecorder.record!(
@@ -1777,7 +2043,7 @@ 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,
@@ -1797,14 +2063,14 @@ 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)
 
@@ -1819,17 +2085,26 @@ 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
         sign_in_as(member)
 
         nico_tag = create_nico_tag!('nico:relation_source')
-        linked_tag = Tag.find_or_create_by_tag_name!('relation_linked_tag', category: :general)
+        linked_tag = Tag.find_or_create_by_tag_name!(
+            Locale.nipponese, '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)
@@ -1843,7 +2118,7 @@ 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)
@@ -1879,123 +2154,29 @@ 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
 
@@ -2009,8 +2190,8 @@ RSpec.describe 'Posts API', type: :request do
     let(:oc_from) { Time.zone.local(2019, 12, 31, 0, 0, 0) }
     let(:oc_before) { Time.zone.local(2020, 1, 1, 0, 0, 0) }
 
-    let!(:tag_name2) { TagName.create!(name: 'spec_tag_2') }
-    let!(:tag2) { Tag.create!(tag_name: tag_name2, category: :general) }
+    let!(:tag_name2) { create(:tag_name, name: 'spec_tag_2') }
+    let!(:tag2) { create(:tag, tag_name: tag_name2, category: :general) }
 
     def snapshot_tags(post)
       post.snapshot_tag_names.join(' ')
@@ -2047,7 +2228,7 @@ 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',
@@ -2156,6 +2337,88 @@ 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 }
 
@@ -2286,8 +2549,8 @@ RSpec.describe 'Posts API', type: :request do
       sign_in_as(member)
       base_version = create_post_version_for!(post_record)
 
-      tag_name2 = TagName.create!(name: 'spec_tag_2')
-      Tag.create!(tag_name: tag_name2, category: :general)
+      tag_name2 = create(:tag_name, name: 'spec_tag_2')
+      create(:tag, tag_name: tag_name2, category: :general)
 
       expect do
         put "/posts/#{post_record.id}", params: post_write_params(
@@ -2451,8 +2714,8 @@ RSpec.describe 'Posts API', type: :request do
 
       base_version = create_post_version_for!(post_record.reload)
 
-      tag_name2 = TagName.create!(name: 'spec_tag_2')
-      tag2 = Tag.create!(tag_name: tag_name2, category: :general)
+      tag_name2 = create(:tag_name, name: 'spec_tag_2')
+      tag2 = create(:tag, tag_name: tag_name2, category: :general)
 
       expect {
         put "/posts/#{post_record.id}", params: post_write_params(

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/tag_versions_spec.rb

@@ -232,8 +232,8 @@ RSpec.describe 'TagVersions API', type: :request do
     end
 
     it 'does not create tag versions by wiki updates when tag has no versions yet' do
-      wiki_tag_name = TagName.create!(name: 'tag_versions_from_wiki')
-      wiki_tag = Tag.create!(tag_name: wiki_tag_name, category: :general)
+      wiki_tag_name = create(:tag_name, name: 'tag_versions_from_wiki')
+      wiki_tag = create(:tag, tag_name: wiki_tag_name, category: :general)
 
       wiki_page =
           Wiki::Commit.create_content!(

backend/spec/requests/tag_wiki_history_integrity_spec.rb

@@ -8,8 +8,8 @@ RSpec.describe 'Tag and wiki history integrity', type: :request do
   end
 
   def create_tag! name:, category: :general
-    tag_name = TagName.create!(name:)
-    Tag.create!(tag_name:, category:)
+    tag_name = create(:tag_name, name:)
+    create(:tag, tag_name:, category:)
   end
 
   def create_wiki_for_tag! tag:, body: 'wiki body', user: member_user

backend/spec/requests/tags_spec.rb

@@ -2,13 +2,13 @@ require 'cgi'
 require 'rails_helper'
 
 RSpec.describe 'Tags API', type: :request do
-  let!(:tn) { TagName.create!(name: 'spec_tag') }
-  let!(:tag) { Tag.create!(tag_name: tn, category: :general) }
-  let!(:alias_tn) { TagName.create!(name: 'unko', canonical: tn) }
+  let!(:tn) { create(:tag_name, name: 'spec_tag') }
+  let!(:tag) { create(:tag, tag_name: tn, category: :general) }
+  let!(:alias_tn) { create(:tag_name, :alias, name: 'unko', canonical: tn) }
   let!(:post) { Post.create!(url: 'https://example.com/unkounkounko') }
   let!(:post_tag) { PostTag.create!(post:, tag:) }
-  let!(:tn2) { TagName.create!(name: 'unknown') }
-  let!(:tag2) { Tag.create!(tag_name: tn2, category: :general) }
+  let!(:tn2) { create(:tag_name, name: 'unknown') }
+  let!(:tag2) { create(:tag, tag_name: tn2, category: :general) }
 
   def response_tags
     json.fetch('tags')
@@ -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'
 
@@ -67,7 +91,7 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'filters tags by category' do
-      meme = Tag.create!(tag_name: TagName.create!(name: 'meme_only'), category: :meme)
+      meme = create(:tag, name: 'meme_only', category: :meme)
 
       get '/tags', params: { category: 'meme' }
 
@@ -77,12 +101,11 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'filters tags by deprecated state' do
-      deprecated_tag = Tag.create!(
+      deprecated_tag = create(:tag,
           name: 'deprecated_filter',
           category: :general,
-        deprecated_at: 1.day.from_now
-      )
-      active_tag = Tag.create!(name: 'active_filter', category: :general)
+          deprecated_at: 1.day.from_now)
+      active_tag = create(:tag, name: 'active_filter', category: :general)
 
       get '/tags', params: { name: '_filter', deprecated: '1' }
 
@@ -98,9 +121,9 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'filters tags by post_count range' do
-      low = Tag.create!(tag_name: TagName.create!(name: 'pc_low'), category: :general)
-      mid = Tag.create!(tag_name: TagName.create!(name: 'pc_mid'), category: :general)
-      high = Tag.create!(tag_name: TagName.create!(name: 'pc_high'), category: :general)
+      low = create(:tag, name: 'pc_low', category: :general)
+      mid = create(:tag, name: 'pc_mid', category: :general)
+      high = create(:tag, name: 'pc_high', category: :general)
 
       low.update_columns(post_count: 1)
       mid.update_columns(post_count: 3)
@@ -118,8 +141,8 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'filters tags by created_at range' do
-      old_tag = Tag.create!(tag_name: TagName.create!(name: 'created_old'), category: :general)
-      new_tag = Tag.create!(tag_name: TagName.create!(name: 'created_new'), category: :general)
+      old_tag = create(:tag, name: 'created_old', category: :general)
+      new_tag = create(:tag, name: 'created_new', category: :general)
 
       old_time = Time.zone.local(2024, 1, 1, 0, 0, 0)
       new_time = Time.zone.local(2024, 2, 1, 0, 0, 0)
@@ -138,8 +161,8 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'filters tags by updated_at range' do
-      old_tag = Tag.create!(tag_name: TagName.create!(name: 'updated_old'), category: :general)
-      new_tag = Tag.create!(tag_name: TagName.create!(name: 'updated_new'), category: :general)
+      old_tag = create(:tag, name: 'updated_old', category: :general)
+      new_tag = create(:tag, name: 'updated_new', category: :general)
 
       old_time = Time.zone.local(2024, 3, 1, 0, 0, 0)
       new_time = Time.zone.local(2024, 4, 1, 0, 0, 0)
@@ -158,31 +181,90 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'orders tags by custom category order' do
-      Tag.create!(tag_name: TagName.create!(name: 'cat_deerjikist'), category: :deerjikist)
-      Tag.create!(tag_name: TagName.create!(name: 'cat_meme'), category: :meme)
-      Tag.create!(tag_name: TagName.create!(name: 'cat_character'), category: :character)
-      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(:tag, name: 'cat_deerjikist', category: :deerjikist)
+      create(:tag, name: 'cat_meme', category: :meme)
+      create(:tag, name: 'cat_character', category: :character)
+      create(:tag, name: 'cat_general', category: :general)
+      create(:tag, name: 'cat_material', category: :material)
+      create(:tag, name: 'cat_meta', category: :meta)
+      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
       %w[pag_a pag_b pag_c].each do |name|
-        Tag.create!(tag_name: TagName.create!(name:), category: :general)
+        create(:tag, tag_name: create(:tag_name, name:), category: :general)
       end
 
       get '/tags', params: { name: 'pag_', order: 'name:asc', page: 2, limit: 2 }
@@ -193,8 +275,8 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'falls back to default ordering when order is invalid' do
-      low = Tag.create!(tag_name: TagName.create!(name: 'fallback_low'), category: :general)
-      high = Tag.create!(tag_name: TagName.create!(name: 'fallback_high'), category: :general)
+      low = create(:tag, name: 'fallback_low', category: :general)
+      high = create(:tag, name: 'fallback_high', category: :general)
 
       low.update_columns(post_count: 1)
       high.update_columns(post_count: 9)
@@ -207,7 +289,7 @@ RSpec.describe 'Tags API', type: :request do
 
     it 'normalises invalid page and limit' do
       %w[norm_a norm_b].each do |name|
-        Tag.create!(tag_name: TagName.create!(name:), category: :general)
+        create(:tag, tag_name: create(:tag_name, name:), category: :general)
       end
 
       get '/tags', params: { name: 'norm_', order: 'name:asc', page: 0, limit: 0 }
@@ -219,10 +301,9 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'returns aliases and parent tags' do
-      parent_tag = Tag.create!(
-        tag_name: TagName.create!(name: 'index_parent_tag'),
-        category: :meme
-      )
+      parent_tag = create(:tag,
+          tag_name: create(:tag_name, name: 'index_parent_tag'),
+          category: :meme)
       TagImplication.create!(tag:, parent_tag:)
 
       get '/tags', params: { name: 'spec_tag' }
@@ -266,10 +347,9 @@ RSpec.describe 'Tags API', type: :request do
       end
 
       it 'returns aliases and parent tags' do
-        parent_tag = Tag.create!(
-          tag_name: TagName.create!(name: 'show_parent_tag'),
-          category: :character
-        )
+        parent_tag = create(:tag,
+            tag_name: create(:tag_name, name: 'show_parent_tag'),
+            category: :character)
         TagImplication.create!(tag:, parent_tag:)
 
         request
@@ -296,9 +376,127 @@ RSpec.describe 'Tags API', type: :request do
         expect(response).to have_http_status(:not_found)
       end
     end
+
+    it 'falls back to ExternalTag when no internal Tag has the id' do
+      external = create(
+          :external_tag,
+          id: Tag.maximum(:id).to_i + 10_000,
+          name: 'legacy_id_lookup')
+
+      get "/tags/#{ external.id }"
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include(
+          'id' => external.id,
+          'name' => 'nico:legacy_id_lookup',
+          'category' => 'nico')
+    end
+
+    it 'prefers Tag when Tag and ExternalTag have the same id' do
+      internal = create(
+          :tag,
+          tag_name: create(:tag_name, name: 'internal_collision'))
+      create(
+          :external_tag,
+          id: internal.id,
+          name: 'external_collision')
+
+      get "/tags/#{ internal.id }"
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include(
+          'id' => internal.id,
+          'name' => 'internal_collision',
+          'category' => internal.category)
+    end
   end
 
   describe 'GET /tags/autocomplete' do
+    it 'combines internal and external matches without conflating equal ids' do
+      internal = create(:tag, 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') }"
+        create(:tag, 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 = create(:tag, category: :general, name: 'switch_internal', post_count: 1)
+      alias_target = create(:tag, category: :general, name: 'alias_target', post_count: 1)
+      create(:tag_name, :alias, 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"
+        create(:tag, category: :general, name: literal, post_count: 1)
+        alias_target = create(
+            :tag, category: :general, name: 'literal_alias_target', post_count: 1)
+        create(:tag_name, :alias, name: "#{ literal }_alias",
+                                  canonical: alias_target.tag_name)
+        create(:external_tag, name: literal, post_count: 1)
+        create(:tag, 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' }
 
@@ -324,11 +522,10 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'omits deprecated tags' do
-      deprecated_tag = Tag.create!(
+      deprecated_tag = create(:tag,
           name: 'spec_deprecated',
           category: :general,
-        deprecated_at: Time.current
-      )
+          deprecated_at: Time.current)
       deprecated_tag.update_columns(post_count: 1)
 
       get '/tags/autocomplete', params: { q: 'spec_', present: '0' }
@@ -340,6 +537,27 @@ 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 +705,6 @@ 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 +714,6 @@ 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!(
@@ -638,27 +818,27 @@ RSpec.describe 'Tags API', type: :request do
 
   describe 'GET /tags/with-depth' do
     let!(:root_meme) do
-      Tag.create!(tag_name: TagName.create!(name: 'depth_a_root_meme'), category: :meme)
+      create(:tag, name: 'depth_a_root_meme', category: :meme)
     end
 
     let!(:root_material) do
-      Tag.create!(tag_name: TagName.create!(name: 'depth_b_root_material'), category: :material)
+      create(:tag, name: 'depth_b_root_material', category: :material)
     end
 
     let!(:hidden_general_root) do
-      Tag.create!(tag_name: TagName.create!(name: 'depth_hidden_general_root'), category: :general)
+      create(:tag, name: 'depth_hidden_general_root', category: :general)
     end
 
     let!(:child_character) do
-      Tag.create!(tag_name: TagName.create!(name: 'depth_child_character'), category: :character)
+      create(:tag, name: 'depth_child_character', category: :character)
     end
 
     let!(:grandchild_material) do
-      Tag.create!(tag_name: TagName.create!(name: 'depth_grandchild_material'), category: :material)
+      create(:tag, name: 'depth_grandchild_material', category: :material)
     end
 
     let!(:child_general) do
-      Tag.create!(tag_name: TagName.create!(name: 'depth_child_general'), category: :general)
+      create(:tag, name: 'depth_child_general', category: :general)
     end
 
     before do
@@ -701,15 +881,13 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'passes through deprecated tags when finding children' do
-      deprecated_middle = Tag.create!(
+      deprecated_middle = create(:tag,
           name: 'depth_deprecated_middle',
           category: :character,
-        deprecated_at: Time.current
-      )
-      visible_descendant = Tag.create!(
+          deprecated_at: Time.current)
+      visible_descendant = create(:tag,
           name: 'depth_visible_descendant',
-        category: :material
-      )
+          category: :material)
       TagImplication.create!(parent_tag: root_material, tag: deprecated_middle)
       TagImplication.create!(parent_tag: deprecated_middle, tag: visible_descendant)
 
@@ -721,24 +899,20 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'passes through multiple deprecated tags for roots and has_children' do
-      active_child = Tag.create!(
+      active_child = create(:tag,
           name: 'depth_active_child_below_deprecated',
-        category: :character
-      )
-      deprecated_parent = Tag.create!(
+          category: :character)
+      deprecated_parent = create(:tag,
           name: 'depth_deprecated_parent',
           category: :character,
-        deprecated_at: Time.current
-      )
-      deprecated_grandparent = Tag.create!(
+          deprecated_at: Time.current)
+      deprecated_grandparent = create(:tag,
           name: 'depth_deprecated_grandparent',
           category: :material,
-        deprecated_at: Time.current
-      )
-      active_ancestor = Tag.create!(
+          deprecated_at: Time.current)
+      active_ancestor = create(:tag,
           name: 'depth_active_ancestor',
-        category: :meme
-      )
+          category: :meme)
       TagImplication.create!(tag: active_child, parent_tag: deprecated_parent)
       TagImplication.create!(tag: deprecated_parent, parent_tag: deprecated_grandparent)
       TagImplication.create!(tag: deprecated_grandparent, parent_tag: active_ancestor)
@@ -763,15 +937,13 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'treats an active tag with only deprecated ancestors as a root' do
-      active_child = Tag.create!(
+      active_child = create(:tag,
           name: 'depth_root_below_deprecated',
-        category: :character
-      )
-      deprecated_parent = Tag.create!(
+          category: :character)
+      deprecated_parent = create(:tag,
           name: 'depth_root_deprecated_parent',
           category: :material,
-        deprecated_at: Time.current
-      )
+          deprecated_at: Time.current)
       TagImplication.create!(tag: active_child, parent_tag: deprecated_parent)
 
       get '/tags/with-depth'
@@ -785,16 +957,14 @@ RSpec.describe 'Tags API', type: :request do
     end
 
     it 'terminates when deprecated implications contain a cycle' do
-      first = Tag.create!(
+      first = create(:tag,
           name: 'depth_cycle_first',
           category: :character,
-        deprecated_at: Time.current
-      )
-      second = Tag.create!(
+          deprecated_at: Time.current)
+      second = create(:tag,
           name: 'depth_cycle_second',
           category: :material,
-        deprecated_at: Time.current
-      )
+          deprecated_at: Time.current)
       TagImplication.create!(tag: first, parent_tag: root_material)
       TagImplication.create!(tag: second, parent_tag: first)
       now = Time.current
@@ -820,19 +990,19 @@ RSpec.describe 'Tags API', type: :request do
     let!(:material_user) { create_member_user! }
 
     let!(:root_tag) do
-      Tag.create!(tag_name: TagName.create!(name: 'materials_root'), category: :material)
+      create(:tag, name: 'materials_root', category: :material)
     end
 
     let!(:child_a_tag) do
-      Tag.create!(tag_name: TagName.create!(name: 'materials_child_a'), category: :material)
+      create(:tag, name: 'materials_child_a', category: :material)
     end
 
     let!(:child_b_tag) do
-      Tag.create!(tag_name: TagName.create!(name: 'materials_child_b'), category: :character)
+      create(:tag, name: 'materials_child_b', category: :character)
     end
 
     let!(:grandchild_tag) do
-      Tag.create!(tag_name: TagName.create!(name: 'materials_grandchild'), category: :material)
+      create(:tag, name: 'materials_grandchild', category: :material)
     end
 
     let!(:root_material) do
@@ -976,14 +1146,12 @@ RSpec.describe 'Tags API', type: :request do
       end
 
       it 'name, category, aliases, parent tags をまとめて更新できる' do
-        old_parent = Tag.create!(
-          tag_name: TagName.create!(name: 'put_old_parent'),
-          category: :general
-        )
-        kept_parent = Tag.create!(
-          tag_name: TagName.create!(name: 'put_kept_parent'),
-          category: :general
-        )
+        old_parent = create(:tag,
+            tag_name: create(:tag_name, name: 'put_old_parent'),
+            category: :general)
+        kept_parent = create(:tag,
+            tag_name: create(:tag_name, name: 'put_kept_parent'),
+            category: :general)
         TagImplication.create!(tag:, parent_tag: old_parent)
         TagImplication.create!(tag:, parent_tag: kept_parent)
 
@@ -1128,14 +1296,12 @@ RSpec.describe 'Tags API', type: :request do
       end
 
       it 'parent tag の snapshot も作成する' do
-        old_parent = Tag.create!(
-          tag_name: TagName.create!(name: 'put_snapshot_old_parent'),
-          category: :general
-        )
-        new_parent = Tag.create!(
-          tag_name: TagName.create!(name: 'put_snapshot_new_parent'),
-          category: :general
-        )
+        old_parent = create(:tag,
+            tag_name: create(:tag_name, name: 'put_snapshot_old_parent'),
+            category: :general)
+        new_parent = create(:tag,
+            tag_name: create(:tag_name, name: 'put_snapshot_new_parent'),
+            category: :general)
         TagImplication.create!(tag:, parent_tag: old_parent)
 
         put "/tags/#{ tag.id }", params: {
@@ -1169,28 +1335,6 @@ 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
@@ -1294,14 +1438,12 @@ RSpec.describe 'Tags API', type: :request do
       end
 
       it '別名を他 tag から奪った場合、奪はれた側の tag version も作成する' do
-        old_owner = Tag.create!(
-          tag_name: TagName.create!(name: 'put_alias_old_owner'),
-          category: :general
-        )
-        stolen_alias = TagName.create!(
+        old_owner = create(:tag,
+            tag_name: create(:tag_name, name: 'put_alias_old_owner'),
+            category: :general)
+        stolen_alias = create(:tag_name, :alias,
             name: 'put_stolen_alias',
-          canonical: old_owner.tag_name
-        )
+            canonical: old_owner.tag_name)
 
         expect(old_owner.tag_name.aliases.map(&:name)).to include('put_stolen_alias')
 
@@ -1332,14 +1474,12 @@ RSpec.describe 'Tags API', type: :request do
       end
 
       it '別 tag の alias 名を rename で奪へる' do
-        old_owner = Tag.create!(
-          tag_name: TagName.create!(name: 'put_alias_collision_owner'),
-          category: :general
-        )
-        stolen_alias = TagName.create!(
+        old_owner = create(:tag,
+            tag_name: create(:tag_name, name: 'put_alias_collision_owner'),
+            category: :general)
+        stolen_alias = create(:tag_name, :alias,
             name: 'put_alias_collision_name',
-          canonical: old_owner.tag_name
-        )
+            canonical: old_owner.tag_name)
         wiki_page =
             Wiki::Commit.create_content!(
                 tag_name: tag.tag_name,
@@ -1355,7 +1495,7 @@ RSpec.describe 'Tags API', type: :request do
           deprecated: '0',
         }
 
-        expect(response).to have_http_status(:ok)
+        expect(response).to have_http_status(:ok), response.body
 
         tag.reload
         old_owner.reload
@@ -1374,10 +1514,9 @@ RSpec.describe 'Tags API', type: :request do
       end
 
       it 'parent_tags に指定すると循環する tag は 422 にする' do
-        child = Tag.create!(
-          tag_name: TagName.create!(name: 'put_cycle_child'),
-          category: :general
-        )
+        child = create(:tag,
+            tag_name: create(:tag_name, name: 'put_cycle_child'),
+            category: :general)
 
         TagImplication.create!(tag: child, parent_tag: tag)
 

backend/spec/requests/wiki_conflict_spec.rb

@@ -10,7 +10,7 @@ RSpec.describe 'Wiki conflict handling', type: :request do
   it 'returns 409 when base_revision_id is stale' do
     page =
         Wiki::Commit.create_content!(
-            tag_name: TagName.create!(name: 'wiki_conflict_request'),
+            tag_name: create(:tag_name, name: 'wiki_conflict_request'),
             body: 'first',
             created_by_user: user,
             message: 'init')

backend/spec/requests/wiki_history_integrity_spec.rb

@@ -10,7 +10,7 @@ RSpec.describe 'Wiki history integrity', type: :request do
 
   def create_wiki_page title:, body: 'body', message: 'init', user: self.user
     Wiki::Commit.create_content!(
-        tag_name: TagName.create!(name: title),
+        tag_name: create(:tag_name, name: title),
         body:,
         created_by_user: user,
         message:)

backend/spec/requests/wiki_spec.rb

@@ -10,7 +10,7 @@ RSpec.describe 'Wiki API', type: :request do
 
   let!(:user) { create_member_user! }
 
-  let!(:tn) { TagName.create!(name: 'spec_wiki_title') }
+  let!(:tn) { create(:tag_name, name: 'spec_wiki_title') }
   let!(:page) do
     Wiki::Commit.create_content!(
         tag_name: tn,
@@ -19,11 +19,10 @@ RSpec.describe 'Wiki API', type: :request do
         message: 'init')
   end
   let!(:tag) do
-    Tag.create!(
+    create(:tag,
         tag_name: tn,
         category: :general,
-      deprecated_at: Time.zone.local(2026, 6, 1)
-    )
+        deprecated_at: Time.zone.local(2026, 6, 1))
   end
 
   describe 'GET /wiki' do
@@ -208,7 +207,7 @@ RSpec.describe 'Wiki API', type: :request do
       { 'X-Transfer-Code' => user.inheritance_code }
     end
 
-    let!(:test_tag_name) { TagName.create!(name: 'TestPage') }
+    let!(:test_tag_name) { create(:tag_name, name: 'TestPage') }
 
     let!(:page) do
       Wiki::Commit.create_content!(
@@ -275,8 +274,8 @@ RSpec.describe 'Wiki API', type: :request do
       end
 
       it 'wiki body だけを変更しても tag version は作成しない' do
-        linked_tag_name = TagName.create!(name: 'wiki_body_only_tag')
-        linked_tag = Tag.create!(tag_name: linked_tag_name, category: :general)
+        linked_tag_name = create(:tag_name, name: 'wiki_body_only_tag')
+        linked_tag = create(:tag, tag_name: linked_tag_name, category: :general)
 
         TagVersionRecorder.record!(
             tag: linked_tag,
@@ -359,13 +358,13 @@ RSpec.describe 'Wiki API', type: :request do
   describe 'GET /wiki/search' do
     before do
       Wiki::Commit.create_content!(
-          tag_name: TagName.create!(name: 'spec_wiki_title_2'),
+          tag_name: create(:tag_name, name: 'spec_wiki_title_2'),
           body: 'search body 2',
           created_by_user: user,
           message: 'init')
 
       Wiki::Commit.create_content!(
-          tag_name: TagName.create!(name: 'unrelated_title'),
+          tag_name: create(:tag_name, name: 'unrelated_title'),
           body: 'unrelated body',
           created_by_user: user,
           message: 'init')
@@ -434,7 +433,7 @@ RSpec.describe 'Wiki API', type: :request do
 
     it 'returns empty array when page has no revisions and filtered by id' do
       # 別ページを作って revision 無し
-      tn2 = TagName.create!(name: 'spec_no_rev')
+      tn2 = create(:tag_name, name: 'spec_no_rev')
       # 異常データ: revision 無し WikiPage を直接作る
       p2 = WikiPage.create!(
           tag_name: tn2,
@@ -514,7 +513,7 @@ RSpec.describe 'Wiki API', type: :request do
 
   describe 'Wiki::Commit.redirect!' do
     it 'raises because redirect revisions are deprecated' do
-      target_tag_name = TagName.create!(name: 'redirect_deprecated_target')
+      target_tag_name = create(:tag_name, name: 'redirect_deprecated_target')
       target =
           Wiki::Commit.create_content!(
               tag_name: target_tag_name,
@@ -535,8 +534,8 @@ RSpec.describe 'Wiki API', type: :request do
   end
 
   it 'wiki title を変更すると対応する tag の version を作成する' do
-    linked_tag_name = TagName.create!(name: 'wiki_linked_tag_for_version')
-    linked_tag = Tag.create!(tag_name: linked_tag_name, category: :general)
+    linked_tag_name = create(:tag_name, name: 'wiki_linked_tag_for_version')
+    linked_tag = create(:tag, tag_name: linked_tag_name, category: :general)
 
     linked_page =
         Wiki::Commit.create_content!(

backend/spec/requests/wiki_title_collision_spec.rb

@@ -9,7 +9,7 @@ RSpec.describe 'Wiki title collision', type: :request do
 
   def create_wiki_page title:, body:
     Wiki::Commit.create_content!(
-        tag_name: TagName.create!(name: title),
+        tag_name: create(:tag_name, name: title),
         body:,
         created_by_user: user,
         message: 'init')

backend/spec/services/material_sync_importer_spec.rb

@@ -2,7 +2,7 @@ require 'rails_helper'
 
 RSpec.describe MaterialSyncImporter do
   let(:user) { create(:user, :member) }
-  let(:tag) { Tag.create!(tag_name: TagName.create!(name: 'sync_tag'), category: :material) }
+  let(:tag) { create(:tag, name: 'sync_tag', category: :material) }
 
   def tempfile_for body
     Tempfile.new(['material-sync-importer', '.png']).tap do |file|

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| create(:tag, 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

@@ -1,6 +1,8 @@
 require 'rails_helper'
 
 RSpec.describe PostBulkCreator do
+  include_context 'English locale'
+
   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) {
@@ -9,6 +11,7 @@ RSpec.describe PostBulkCreator do
     mutex = Mutex.new
     active = 0
     maximum_active = 0
+    creators = []
 
     allow(PostCreatePreflight).to receive(:new) do |attributes:, **|
       preflight = instance_double(PostCreatePreflight)
@@ -28,6 +31,7 @@ RSpec.describe PostBulkCreator do
     end
     allow(PostCreator).to receive(:new) do |attributes:, **|
       creator = instance_double(PostCreator)
+      mutex.synchronize { creators << creator }
       if attributes[:title] == 'broken'
         allow(creator).to receive(:create!).and_raise(StandardError, 'broken')
       else
@@ -45,7 +49,12 @@ RSpec.describe PostBulkCreator do
     results = described_class.new(
         actor:,
         posts:,
-        thumbnails: { }).run.fetch(:results)
+        thumbnails: { }).run(locale).fetch(:results)
+
+    expect(creators.length).to eq(posts.length)
+    creators.each do |creator|
+      expect(creator).to have_received(:create!).with(locale).once
+    end
 
     expect(maximum_active).to eq(2)
     expect(results.length).to eq(posts.length)

backend/spec/services/post_create_plan_spec.rb

@@ -2,7 +2,7 @@ require 'rails_helper'
 
 RSpec.describe PostCreatePlan do
   def create_tag! name, category
-    Tag.create!(name:, category:)
+    create(:tag, name:, category:)
   end
 
   before do
@@ -10,6 +10,56 @@ RSpec.describe PostCreatePlan do
     create_tag!('ニジラー情報不詳', :meta)
   end
 
+  context 'with an explicit locale' do
+    include_context 'English locale'
+
+    it 'resolves the matching language identity and its parents' do
+      english_name = create(:tag_name, name: 'shared_name',
+                                      language_code: 'en', script_code: 'Latn')
+      english = create(:tag, tag_name: english_name, category: :character)
+      japanese = create(:tag, name: 'temporary_japanese', category: :general)
+      # The DB permits this identity; validation has its own contract spec.
+      japanese.tag_name.update_columns(name: 'shared_name')
+      english_parent = create(:tag, name: 'english_parent', category: :material)
+      japanese_parent = create(:tag, name: 'japanese_parent', category: :general)
+      TagImplication.create!(tag: english, parent_tag: english_parent)
+      TagImplication.create!(tag: japanese, parent_tag: japanese_parent)
+
+      plan = described_class.new(attributes: { tags: 'shared_name' }).build!(locale)
+
+      expect(plan[:direct_tag_specs]).to eq(
+          [{ name: 'shared_name', category: :character }])
+      expect(plan[:post_tag_specs]).to include(
+          { name: english_parent.name, category: :material })
+      expect(plan[:post_tag_specs].pluck(:name)).not_to include(japanese_parent.name)
+    end
+
+    it 'resolves aliases within the requested language' do
+      japanese = create(:tag, name: 'japanese_canonical')
+      english_name = create(:tag_name, name: 'english_canonical',
+                                      language_code: 'en', script_code: 'Latn')
+      english = create(:tag, tag_name: english_name, category: :character)
+      japanese_alias = create(:tag_name, :alias, name: 'shared_alias',
+                                                canonical: japanese.tag_name)
+      english_alias = create(:tag_name, :alias, name: 'temporary_alias',
+                                               canonical: english_name,
+                                               language_code: 'en', script_code: 'Latn')
+      english_alias.update_columns(name: 'shared_alias')
+
+      expect(japanese_alias.reload).to have_attributes(
+          canonical_id: japanese.tag_name_id, tag_id: japanese.id,
+          primary_flg: false, language_code: 'ja')
+      expect(english_alias.reload).to have_attributes(
+          canonical_id: english_name.id, tag_id: english.id,
+          primary_flg: false, language_code: 'en')
+
+      plan = described_class.new(attributes: { tags: 'shared_alias' }).build!(locale)
+
+      expect(plan[:direct_tag_specs]).to eq(
+          [{ name: english.name, category: :character }])
+    end
+  end
+
   it 'plans direct and existing default tags without persisting records' do
     counts = [TagName.count, Tag.count]
 
@@ -18,7 +68,7 @@ RSpec.describe PostCreatePlan do
             url: 'https://example.com/post',
             title: 'title',
             tags: 'character:new_character',
-            parent_post_ids: '' }).build!
+            parent_post_ids: '' }).build!(Locale.nipponese)
 
     expect(plan[:tags]).to eq('new_character')
     expect(plan[:direct_tag_specs]).to eq(
@@ -31,7 +81,7 @@ RSpec.describe PostCreatePlan do
 
   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_name, :alias, name: 'にじか', canonical: canonical.tag_name)
     create_tag!('動画', :meta)
 
     plan = described_class.new(
@@ -40,7 +90,7 @@ RSpec.describe PostCreatePlan do
             title: 'video',
             tags: '動画 にじか[0:10-0:20]',
             duration: '1:00',
-            parent_post_ids: '' }).build!
+            parent_post_ids: '' }).build!(Locale.nipponese)
 
     expect(plan[:tags].split).to include('動画', '虹夏[0:10-0:20]')
     expect(plan[:display_tags]).to include(
@@ -50,6 +100,19 @@ RSpec.describe PostCreatePlan do
     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!(Locale.nipponese)
+    }.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]
@@ -60,7 +123,7 @@ RSpec.describe PostCreatePlan do
               url: 'https://example.com/post',
               title: 'title',
               tags: long_name,
-              parent_post_ids: '' }).build!
+              parent_post_ids: '' }).build!(Locale.nipponese)
     }.to raise_error(ActiveRecord::RecordInvalid) { |error|
       expect(error.record.errors[:tags]).not_to be_empty
     }
@@ -74,7 +137,7 @@ RSpec.describe PostCreatePlan do
             title: 'title',
             tags: 'ordinary_tag',
             duration: 'invalid',
-            parent_post_ids: '' }).build!
+            parent_post_ids: '' }).build!(Locale.nipponese)
 
     expect(plan[:duration]).to eq('invalid')
     expect(plan[:video_ms]).to be_nil

backend/spec/services/post_creator_spec.rb

@@ -14,7 +14,6 @@ RSpec.describe PostCreator do
   end
 
   before do
-    allow(Tag).to receive(:normalise_tags!).and_return({ tags: [], sections: {} })
     allow(TagVersioning).to receive(:record_tag_snapshots!)
     allow(Tag).to receive(:expand_parent_tags).and_return([])
     allow(PostVersionRecorder).to receive(:record!)
@@ -34,7 +33,7 @@ RSpec.describe PostCreator do
           url: 'https://example.com/post',
           thumbnail: real_thumbnail_upload,
           thumbnail_base: 'https://example.com/thumb.jpg',
-          tags: '' }).create!
+          tags: '' }).create!(Locale.nipponese)
 
     expect(post.thumbnail).to be_attached
     expect(post.thumbnail_base).to eq('https://example.com/thumb.jpg')
@@ -54,7 +53,7 @@ RSpec.describe PostCreator do
           title: 'title',
           url: 'https://example.com/post',
           thumbnail_base: 'https://example.com/thumb.jpg',
-          tags: '' }).create!
+          tags: '' }).create!(Locale.nipponese)
 
     expect(post.thumbnail_base).to eq('https://example.com/thumb.jpg')
     expect(post.thumbnail).to be_attached
@@ -72,7 +71,34 @@ RSpec.describe PostCreator do
           tags: '' })
 
     post_count = Post.count
-    expect { creator.create! }.to raise_error(Post::RemoteThumbnailFetchFailed)
+    expect { creator.create!(Locale.nipponese) }
+        .to raise_error(Post::RemoteThumbnailFetchFailed)
     expect(Post.count).to eq(post_count)
   end
+
+  context 'with an explicit locale' do
+    include_context 'English locale'
+
+    [false, true].each do |planned|
+      it "passes locale to tag creation with planned attributes: #{ planned }" do
+        attributes = {
+            title: 'Locale propagation',
+            url: 'https://example.com/locale-post',
+            tags: 'character:locale_character' }
+        if planned
+          specs = [{ name: 'locale_character', category: :character }]
+          attributes.merge!(snapshot_tag_specs: specs, post_tag_specs: specs)
+        end
+        allow(Tag).to receive(:find_or_create_by_tag_name!).and_call_original
+
+        post = described_class.new(actor:, attributes:).create!(locale)
+
+        expect(Tag).to have_received(:find_or_create_by_tag_name!)
+            .with(locale, 'locale_character', category: :character).at_least(:once)
+        tag_name = TagName.find_by!(language_code: 'en', name: 'locale_character')
+        expect(tag_name).to have_attributes(script_code: 'Latn', primary_flg: true)
+        expect(post.tags).to include(tag_name.tag)
+      end
+    end
+  end
 end

backend/spec/services/post_import_previewer_spec.rb

@@ -64,7 +64,7 @@ RSpec.describe PostImportPreviewer do
     end
 
     it 'applies metadata to automatic fields and recognises metadata tags' do
-      Tag.create!(name: 'known-tag', category: :general)
+      create(:tag, name: 'known-tag', category: :general)
       allow(PostMetadataFetcher).to receive(:fetch).and_return(
         title: 'metadata title',
         thumbnail_base: 'https://example.com/thumb.jpg',

backend/spec/services/wiki/commit_integrity_spec.rb

@@ -6,7 +6,7 @@ RSpec.describe Wiki::Commit do
 
   def create_page title:, body: 'initial body'
     described_class.create_content!(
-        tag_name: TagName.create!(name: title),
+        tag_name: create(:tag_name, name: title),
         body:,
         created_by_user: user,
         message: 'init')
@@ -16,7 +16,7 @@ RSpec.describe Wiki::Commit do
     it 'creates page, revision, and version with normalised body' do
       expect {
         described_class.create_content!(
-            tag_name: TagName.create!(name: 'commit_integrity_create'),
+            tag_name: create(:tag_name, name: 'commit_integrity_create'),
             body: "a\r\nb\r\n\r\n",
             created_by_user: user,
             message: 'init')
@@ -36,7 +36,7 @@ RSpec.describe Wiki::Commit do
     end
 
     it 'rejects body that becomes blank after normalisation' do
-      tag_name = TagName.create!(name: 'commit_integrity_blank')
+      tag_name = create(:tag_name, name: 'commit_integrity_blank')
 
       expect {
         described_class.create_content!(
@@ -76,8 +76,8 @@ RSpec.describe Wiki::Commit do
     end
 
     it 'does not record tag_version on body-only wiki update' do
-      tag_name = TagName.create!(name: 'commit_integrity_linked_tag')
-      tag = Tag.create!(tag_name:, category: :general)
+      tag_name = create(:tag_name, name: 'commit_integrity_linked_tag')
+      tag = create(:tag, tag_name:, category: :general)
 
       page =
           described_class.create_content!(

backend/spec/services/wiki/commit_spec.rb

@@ -4,7 +4,7 @@ RSpec.describe Wiki::Commit do
   let(:user) { create_member_user! }
 
   def create_page(title: 'commit_spec_page', body: 'initial body')
-    tag_name = TagName.create!(name: title)
+    tag_name = create(:tag_name, name: title)
 
     Wiki::Commit.create_content!(
         tag_name:,
@@ -80,8 +80,8 @@ RSpec.describe Wiki::Commit do
     end
 
     it 'does not record tag version when corresponding tag has no versions' do
-      tag_name = TagName.create!(name: 'commit_linked_tag_without_versions')
-      tag = Tag.create!(tag_name:, category: :general)
+      tag_name = create(:tag_name, name: 'commit_linked_tag_without_versions')
+      tag = create(:tag, tag_name:, category: :general)
 
       page =
           described_class.create_content!(
@@ -107,8 +107,8 @@ RSpec.describe Wiki::Commit do
     end
 
     it 'does not record tag version when corresponding tag has no versions' do
-      tag_name = TagName.create!(name: 'commit_linked_tag_without_versions')
-      tag = Tag.create!(tag_name:, category: :general)
+      tag_name = create(:tag_name, name: 'commit_linked_tag_without_versions')
+      tag = create(:tag, tag_name:, category: :general)
 
       page =
           described_class.create_content!(

backend/spec/services/wiki_version_recorder_spec.rb

@@ -5,7 +5,7 @@ RSpec.describe WikiVersionRecorder do
 
   def create_page title:, body: 'body'
     Wiki::Commit.create_content!(
-        tag_name: TagName.create!(name: title),
+        tag_name: create(:tag_name, name: title),
         body:,
         created_by_user: user,
         message: 'init')

backend/spec/services/youtube/sync_spec.rb

@@ -172,7 +172,8 @@ RSpec.describe Youtube::Sync do
       Tag.video
       Tag.no_deerjikist
 
-      deerjikist_tag = Tag.find_or_create_by_tag_name!('テスト投稿者', category: :deerjikist)
+      deerjikist_tag = Tag.find_or_create_by_tag_name!(
+          Locale.nipponese, 'テスト投稿者', category: :deerjikist)
       Deerjikist.create!(
         platform: 'youtube',
         code: 'UC_MAPPED',
@@ -226,7 +227,8 @@ RSpec.describe Youtube::Sync do
       )
       PostTag.create!(post:, tag: Tag.no_deerjikist)
 
-      deerjikist_tag = Tag.find_or_create_by_tag_name!('後から判明した投稿者', category: :deerjikist)
+      deerjikist_tag = Tag.find_or_create_by_tag_name!(
+          Locale.nipponese, '後から判明した投稿者', category: :deerjikist)
       Deerjikist.create!(
         platform: 'youtube',
         code: 'UC_MAPPED_LATER',
@@ -268,6 +270,10 @@ 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/support/locale_context.rb

@@ -0,0 +1,13 @@
+RSpec.shared_context 'English locale' do
+  let!(:locale) do
+    Language.find_or_create_by!(code: 'en') { _1.name = 'English' }
+    Script.find_or_create_by!(code: 'Latn') { _1.name = 'Latin' }
+    unless Locale.exists?(code: 'en')
+      # Reference data only: do not generate names for unrelated existing tags.
+      Locale.insert_all!([
+          { code: 'en', language_code: 'en', script_code: 'Latn',
+            name: 'English', created_at: Time.current }])
+    end
+    Locale.find('en')
+  end
+end

backend/spec/tasks/nico_sync_spec.rb

@@ -8,14 +8,27 @@ RSpec.describe 'nico:sync' do
   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!(Locale.nipponese, name, category:)
   end
 
   def link_nico_to_tag!(nico_tag, tag)
     NicoTagRelation.create!(nico_tag_id: nico_tag.id, tag_id: tag.id)
   end
 
+  def create_nico_sanitisation_rules!
+    TagNameSanitisationRule.create!(priority: 20,
+                                    source_pattern: '\\?',
+                                    replacement: '_')
+
+    TagNameSanitisationRule.create!(priority: 40,
+                                    source_pattern: '_$',
+                                    replacement: '')
+
+    TagNameSanitisationRule.create!(priority: 45,
+                                    source_pattern: '^([^:]+\\:)?_',
+                                    replacement: '\\1')
+  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
     # 既存 post（正規表現で拾われるURL）
     post = Post.create!(
@@ -30,7 +43,7 @@ RSpec.describe 'nico:sync' do
 
     # 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')
+    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）
@@ -54,7 +67,7 @@ RSpec.describe 'nico:sync' do
     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(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'))
@@ -108,7 +121,7 @@ RSpec.describe 'nico:sync' do
     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',
@@ -116,12 +129,12 @@ RSpec.describe 'nico:sync' do
     )
 
     # 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
@@ -132,15 +145,21 @@ RSpec.describe 'nico:sync' do
 
     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)
@@ -206,7 +225,7 @@ 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
@@ -234,7 +253,7 @@ 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!(
@@ -245,7 +264,7 @@ 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)
 
@@ -288,7 +307,7 @@ 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)
@@ -371,4 +390,203 @@ 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 'サニタイズ後の既存外部タグを再利用し、その連携タグを記載する' do
+    post = create_nico_sync_post!
+
+    create_nico_sanitisation_rules!
+
+    external_tag = create_external_tag!('AAA')
+    linked_tag = create_tag!('spec_linked', category: :general)
+    link_nico_to_tag!(external_tag, linked_tag)
+
+    expect {
+      run_nico_sync_with_tags!(['AAA?'])
+    }.not_to change(ExternalTag, :count)
+
+    post.reload
+
+    expect(post.external_tags).to contain_exactly(external_tag)
+    expect(post.tags).to include(linked_tag)
+    expect(ExternalTag.exists?(platform: :nico, name: 'AAA?')).to be(false)
+  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
+
+  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)
+
+    create_nico_sanitisation_rules!
+
+    expect {
+      run_nico_sync_with_tags!(['AAA?', 'AAA?'])
+    }.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.reload.external_tags.sole
+
+    expect(external).to have_attributes(
+        platform: 'nico',
+        name: 'AAA')
+    expect(ExternalTag.exists?(platform: :nico, name: 'AAA?')).to be(false)
+
+    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!(['AAA?'])
+    }.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 'nico: prefix を含む従来の規則で外部タグ名をサニタイズする' do
+    post = create_nico_sync_post!
+    create_nico_sanitisation_rules!
+
+    run_nico_sync_with_tags!(['foo:_bar'])
+
+    expect(post.reload.external_tags.sole)
+        .to have_attributes(platform: 'nico', name: 'foo:_bar')
+  end
 end

backend/spec/tasks/post_similarity_calc_spec.rb

@@ -6,10 +6,10 @@ RSpec.describe 'post_similarity:calc' do
 
   it 'calculates similarities from active tags only' do
     # 必要最低限のデータ
-    t1 = Tag.create!(name: "t1")
-    t2 = Tag.create!(name: "t2")
-    t3 = Tag.create!(name: "t3")
-    deprecated_tag = Tag.create!(name: 'deprecated', deprecated_at: Time.current)
+    t1 = create(:tag, name: 't1')
+    t2 = create(:tag, name: 't2')
+    t3 = create(:tag, name: 't3')
+    deprecated_tag = create(:tag, name: 'deprecated', deprecated_at: Time.current)
 
     p1 = Post.create!(url: "https://example.com/1")
     p2 = Post.create!(url: "https://example.com/2")

backend/spec/tasks/tag_similarity_calc_spec.rb

@@ -6,10 +6,10 @@ RSpec.describe 'tag_similarity:calc' do
 
   it 'calculates similarities for active tags only' do
     # 必要最低限のデータ
-    t1 = Tag.create!(name: "t1")
-    t2 = Tag.create!(name: "t2")
-    t3 = Tag.create!(name: "t3")
-    deprecated_tag = Tag.create!(name: 'deprecated', deprecated_at: Time.current)
+    t1 = create(:tag, name: 't1')
+    t2 = create(:tag, name: 't2')
+    t3 = create(:tag, name: 't3')
+    deprecated_tag = create(:tag, name: 'deprecated', deprecated_at: Time.current)
 
     p1 = Post.create!(url: "https://example.com/1")
     p2 = Post.create!(url: "https://example.com/2")

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/DraggableDroppableTagRow.tsx

@@ -38,6 +38,7 @@ const DraggableDroppableTagRow: FC<Props> = ({
       { normal: { duration: .2, ease: 'easeOut' as const } },
   )
   const dndId = `tag-node:${ pathKey }`
+  const dndDisabled = tag.category === 'nico'
 
   const downPosRef = useRef<{ x: number; y: number } | null> (null)
   const armedRef = useRef (false)
@@ -63,6 +64,7 @@ const DraggableDroppableTagRow: FC<Props> = ({
 	  listeners,
 	  setNodeRef: setDragRef,
 	  transform } = useDraggable ({ id:       dndId,
+					disabled: dndDisabled,
 					data:  { kind:   'tag',
 						 dndId,
 						 tagId:  tag.id,
@@ -71,6 +73,7 @@ const DraggableDroppableTagRow: FC<Props> = ({
 
   const { setNodeRef: setDropRef, isOver: over } = useDroppable ({
       id:       dndId,
+      disabled: dndDisabled,
       data:     { kind: 'tag', tagId: tag.id } })
 
   const activeDragging = activeDndId === dndId

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/TagDetailSidebar.tsx

@@ -150,16 +150,17 @@ const buildFlatTagByCategory = (
     byCategory: TagByCategory,
 ): TagByCategory => {
   const tagsTmp = { } as TagByCategory
-  const seen = new Set<number> ()
+  const seen = new Set<string> ()
 
   for (const category of CATEGORIES)
     tagsTmp[category] = []
 
   const visit = (tag: TagWithSections) => {
-    if (seen.has (tag.id))
+    const key = `${ tag.category }:${ tag.id }`
+    if (seen.has (key))
       return
 
-    seen.add (tag.id)
+    seen.add (key)
     tagsTmp[tag.category].push ({ ...tag, children: [] })
 
     for (const child of tag.children ?? [])

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

@@ -86,7 +86,7 @@ const TagLink: FC<Props> = ({ tag,
 	  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)) && (
+	{(linkFlg && withWiki && isFullTag (tag) && tag.category !== 'nico') && (
 	    <span className={markerWrapClass}>
 	      {(tag.materialId != null || tag.hasWiki || tag.hasDeerjikists)
 	       ? (

frontend/src/components/common/PageTitle.tsx

@@ -1,10 +1,8 @@
-import React from 'react'
-
 import { cn } from '@/lib/utils'
 
-import type { FC } from 'react'
+import type { ComponentProps, FC } from 'react'
 
-type Props = { children: React.ReactNode; className?: string }
+type Props = ComponentProps<'h1'>
 
 
 const PageTitle: FC<Props> = ({ children, className, ...rest }) => (

frontend/src/lib/posts.test.ts

@@ -116,4 +116,25 @@ describe ('posts API functions', () => {
         { params: { page: 2, limit: 50 } },
     )
   })
+
+  it ('maps an explicit external tag history filter to external_tag', async () => {
+    api.apiGet.mockResolvedValueOnce ({ versions: [], count: 0 })
+
+    await fetchPostChanges ({
+      externalTag: '7',
+      page:        2,
+      limit:       50,
+    })
+
+    expect (api.apiGet).toHaveBeenCalledWith (
+	'/posts/versions',
+	{
+	  params: {
+	    external_tag: '7',
+	    page:         2,
+	    limit:        50,
+	  },
+	},
+    )
+  })
 })

frontend/src/lib/posts.ts

@@ -28,15 +28,18 @@ export const fetchPost = async (id: string): Promise<Post> => await apiGet (`/po
 
 
 export const fetchPostChanges = async (
-    { post, tag, page, limit }: {
+    { post, tag, externalTag, page, limit }: {
 	post?:        string
 	tag?:         string
+	externalTag?: string
 	page:         number
 	limit:        number }): Promise<{
     versions: PostVersion[]
     count:    number }> =>
 	await apiGet ('/posts/versions', { params: { ...(post && { post }),
 						     ...(tag && { tag }),
+						     ...(externalTag && {
+						       external_tag: externalTag }),
 						     page, limit } })
 
 

frontend/src/lib/prefetchers.test.ts

@@ -137,4 +137,19 @@ describe ('prefetchForURL', () => {
     expect (tagsApi.fetchTags).not.toHaveBeenCalled ()
     expect (wikiApi.fetchWikiPages).not.toHaveBeenCalled ()
   })
+
+  it ('prefetches external tag post history without treating it as an internal tag', async () => {
+    await prefetchForURL (
+	qc (),
+	'http://localhost/posts/changes?external_tag=12&page=2&limit=50',
+    )
+
+    expect (postsApi.fetchPostChanges).toHaveBeenCalledWith ({
+      externalTag: '12',
+      page:        2,
+      limit:       50,
+    })
+
+    expect (tagsApi.fetchTag).not.toHaveBeenCalled ()
+  })
 })