#399

AGENTS.md

@@ -154,14 +154,6 @@ npm run preview
   一つの文字列補間へまとめる、中間変数へ分ける、`format` を使ふ、heredoc を
   使ふ、array 又は Hash を組み立ててから処理する、条件式全体を括弧で囲む、
   method へ抽出する、のいづれかへ書き換へる。
-- Ruby では、一つの文字列を、改行をまたいだ隣接文字列 literal として記述しない。
-- Ruby では、method argument 内でも、複数の文字列 literal を区切りなしで縦に
-  並べない。
-- RSpec の `describe`、`context`、`it` 等の description が長い場合は、意味を
-  保ったまま一行へ収まる文言へ短縮する。
-- 文字列を短縮できない場合は、用途に応じて `format`、heredoc 又は中間変数を
-  検討する。
-- ただし RSpec description では、原則として簡潔な一行の文字列を使ふ。
 - formatter 又は自動修正にも、Ruby の行末バックスラッシュを生成させない。
 - 新規 code だけでなく、今回触れる Ruby code にも行末バックスラッシュを残さない。
 - 例へば class body 内の array は、class body の 2 空白を基準に、更に 4 空白
@@ -259,21 +251,6 @@ result = first_value + \
 
 Bad:
 
-```rb
-it(
-    'returns 409 when stale changes '
-    'do not conflict'
-) do
-```
-
-Good:
-
-```rb
-it 'returns mergeable 409 for stale non-conflicting changes' do
-```
-
-Bad:
-
 ```rb
 records.each {
   do_work(_1) }
@@ -574,32 +551,8 @@ and layout reuse, follow `frontend/AGENTS.md`.
   wording and placement before implementing it.
 - Do not invent replacement copy when removing unrequested wording.
 - Do not create, modify, or run tests unless the user explicitly asks for
-  test work. When the user asks for tests, keep working within the permitted
-  test-file scope and rerun them until they pass or the remaining failure is
-  clearly blocked.
-- Test-only work includes adding, updating, deleting, reorganising, or fixing
-  SyntaxError in tests. During test-only work, do not modify production code.
-- During test-only work, do not change production constants, behaviour, API
-  contracts, validation, routes, authentication, permissions, UI, copy,
-  dependencies, limits, thresholds, defaults, migrations, schema, or
-  environment settings to satisfy tests.
-- Do not make production code match failing tests, mock assumptions, fixtures,
-  snapshots, old expectations, or stale setup. This includes changing
-  production constants merely because a test expects a different value.
-- If test work reveals a production bug, spec mismatch, or missing behaviour,
-  stop without modifying production code and report: the failing test or
-  discovered issue, the related production file, the actual behaviour, the
-  expected behaviour, and why a production change appears necessary.
-- Modify production code for test failures only when the user explicitly asks
-  for that production change. Do not expand a test task into a production task
-  on your own authority.
-- If the user explicitly asks for both production implementation and test
-  updates, implement production code to the confirmed specification first,
-  then add or update tests to verify that specification. Never roll production
-  behaviour back to satisfy old tests.
-- If it is unclear whether the test or the production implementation is stale,
-  or a test cannot be corrected without changing production code, ask the user
-  instead of guessing.
+  test work. When the user asks for tests, keep working and rerun them until
+  they pass or the remaining failure is clearly blocked.
 
 ## Backend rules
 

backend/app/controllers/application_controller.rb

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

backend/app/controllers/materials_controller.rb

@@ -237,7 +237,7 @@ class MaterialsController < ApplicationController
   end
 
   def resolve_material_tag! tag_name_raw
-    tag_name = TagName.find_or_create_by!(name: 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)
   end

backend/app/controllers/post_versions_controller.rb

@@ -1,7 +1,7 @@
 class PostVersionsController < ApplicationController
   def index
     post_id = params[:post].presence
-    tag_id = params[:tag].presence&.to_i
+    tag_id = params[:tag].presence
     page = (params[:page].presence || 1).to_i
     limit = (params[:limit].presence || 20).to_i
 
@@ -10,6 +10,12 @@ class PostVersionsController < ApplicationController
 
     offset = (page - 1) * limit
 
+    tag_name =
+        if tag_id
+          TagName.joins(:tag).find_by(tag: { id: tag_id })
+        end
+    return render json: { versions: [], count: 0 } if tag_id && tag_name.blank?
+
     q = PostVersion.joins(<<~SQL.squish)
           LEFT JOIN
             post_versions prev
@@ -17,18 +23,17 @@ class PostVersionsController < ApplicationController
               prev.post_id = post_versions.post_id
               AND prev.version_no = post_versions.version_no - 1
         SQL
-        .select('post_versions.*',
-                'prev.title AS prev_title',
-                'prev.url AS prev_url',
-                'prev.thumbnail_base AS prev_thumbnail_base',
-                'prev.tags_json AS prev_tags_json',
+        .select('post_versions.*', 'prev.title AS prev_title', 'prev.url AS prev_url',
+                'prev.thumbnail_base AS prev_thumbnail_base', 'prev.tags AS prev_tags',
                 'prev.video_ms AS prev_video_ms',
                 'prev.original_created_from AS prev_original_created_from',
                 'prev.original_created_before AS prev_original_created_before')
     q = q.where('post_versions.post_id = ?', post_id) if post_id
-    if tag_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 tag_name
+      escaped = ActiveRecord::Base.sanitize_sql_like(tag_name.name)
+      q = q.where(("CONCAT(' ', post_versions.tags, ' ') LIKE :kw " +
+                   "OR CONCAT(' ', prev.tags, ' ') LIKE :kw"),
+                  kw: "% #{ escaped } %")
     end
 
     count = q.except(:select, :order, :limit, :offset).count
@@ -47,69 +52,51 @@ class PostVersionsController < ApplicationController
     users_by_id = User.where(id: user_ids).pluck(:id, :name).to_h
 
     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 = split_tags(row.tags)
+      prev_tags = split_tags(row.attributes['prev_tags'])
 
-      { post_id: row.post_id,
+      {
+        post_id: row.post_id,
         version_no: row.version_no,
         event_type: row.event_type,
-        title: { current: row.title, prev: row.attributes['prev_title'] },
-        url: { current: row.url, prev: row.attributes['prev_url'] },
-        thumbnail: { current: nil, prev: nil },
-        thumbnail_base: { current: row.thumbnail_base,
-                          prev: row.attributes['prev_thumbnail_base'] },
-        video_ms: { current: row.video_ms, prev: row.attributes['prev_video_ms'] },
+        title: {
+          current: row.title,
+          prev: row.attributes['prev_title']
+        },
+        url: {
+          current: row.url,
+          prev: row.attributes['prev_url']
+        },
+        thumbnail: {
+          current: nil,
+          prev: nil
+        },
+        thumbnail_base: {
+          current: row.thumbnail_base,
+          prev: row.attributes['prev_thumbnail_base']
+        },
+        video_ms: {
+          current: row.video_ms,
+          prev: row.attributes['prev_video_ms']
+        },
         tags: build_version_tags(cur_tags, prev_tags),
         original_created_from: {
-            current: row.original_created_from&.iso8601,
-            prev: row.attributes['prev_original_created_from']&.iso8601 },
+          current: row.original_created_from&.iso8601,
+          prev: row.attributes['prev_original_created_from']&.iso8601
+        },
         original_created_before: {
-            current: row.original_created_before&.iso8601,
-            prev: row.attributes['prev_original_created_before']&.iso8601 },
+          current: row.original_created_before&.iso8601,
+          prev: row.attributes['prev_original_created_before']&.iso8601
+        },
         created_at: row.created_at.iso8601,
         created_by_user:
-            if row.created_by_user_id
-              { id: row.created_by_user_id,
-                name: users_by_id[row.created_by_user_id] }
-            end }
+          if row.created_by_user_id
+            {
+              id: row.created_by_user_id,
+              name: users_by_id[row.created_by_user_id]
+            }
+          end
+      }
     end
   end
 
@@ -130,4 +117,8 @@ class PostVersionsController < ApplicationController
       }
     end
   end
+
+  def split_tags(tags)
+    tags.to_s.split(/\s+/).reject(&:blank?)
+  end
 end

backend/app/controllers/posts_controller.rb

@@ -36,8 +36,8 @@ class PostsController < ApplicationController
     offset = (page - 1) * limit
 
     pt_max_sql =
-        PostVersion
-        .select('post_id, MAX(created_at) AS max_updated_at')
+        PostTag
+        .select('post_id, MAX(updated_at) AS max_updated_at')
         .group('post_id')
         .to_sql
 
@@ -50,8 +50,9 @@ class PostsController < ApplicationController
         .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,
-                 post_tags: [:sections, { tag: [:deerjikists, :materials,
-                                                { tag_name: :wiki_page }] }])
+                 active_post_tags: [:sections,
+                                    { tag: [:deerjikists, :materials,
+                                            { tag_name: :wiki_page }] }])
         .with_attached_thumbnail
 
     q = q.where('posts.url LIKE ?', "%#{ url }%") if url
@@ -103,8 +104,9 @@ class PostsController < ApplicationController
 
   def random
     post = filtered_posts.preload(:uploaded_user, :parents, :children,
-                                  post_tags: [:sections, { tag: [:deerjikists, :materials,
-                                                                 { tag_name: :wiki_page }] }])
+                                  active_post_tags: [:sections,
+                                                     { tag: [:deerjikists, :materials,
+                                                             { tag_name: :wiki_page }] }])
                          .with_attached_thumbnail
                          .order('RAND()')
                          .first
@@ -188,8 +190,9 @@ class PostsController < ApplicationController
     post =
         Post
         .includes(:uploaded_user, :parents, :children,
-                  post_tags: [:sections, { tag: [:deerjikists, :materials,
-                                                 { tag_name: :wiki_page }] }])
+                  active_post_tags: [:sections,
+                                     { tag: [:deerjikists, :materials,
+                                             { tag_name: :wiki_page }] }])
         .with_attached_thumbnail
         .find_by(id: params[:id])
     return head :not_found unless post
@@ -381,6 +384,50 @@ 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
@@ -455,13 +502,13 @@ class PostsController < ApplicationController
       end
     end
 
-    PostTag.where(post_id: post.id, tag_id: to_remove.to_a).find_each do |pt|
-      pt.destroy!
+    PostTag.where(post_id: post.id, tag_id: to_remove.to_a).kept.find_each do |pt|
+      pt.discard_by!(current_user)
     end
   end
 
   def build_tag_tree_for post
-    post_tags = post.post_tags.reject { |post_tag| post_tag.tag.deprecated? }
+    post_tags = post.active_post_tags.reject { |post_tag| post_tag.tag.deprecated? }
     tags = post_tags.map(&:tag)
     tag_ids = tags.map(&:id)
 
@@ -652,10 +699,7 @@ class PostsController < ApplicationController
   end
 
   def editable_tag_names_from_version version
-    version.tags_json
-        .reject { _1.fetch('category') == 'nico' }
-        .map { Post.tag_snapshot_literal(_1) }
-        .sort
+    version.tags.to_s.split.reject { |name| name.downcase.start_with?('nico:') }.sort
   end
 
   def post_snapshot_from_record post
@@ -670,6 +714,7 @@ 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))

backend/app/controllers/tags_controller.rb

@@ -200,45 +200,14 @@ class TagsController < ApplicationController
              .find_by(id: params[:id])
     return head :not_found unless tag
 
-    rows = normalise_deerjikist_rows(tag)
-    return if performed?
-
     ApplicationRecord.transaction do
-      tag.lock!
-
-      requested_keys = rows.map { |row| [row[:platform], row[:code]] }.uniq
-      row_indexes_by_key = rows_by_key(rows)
-      locked_deerjikists = lock_deerjikists_for_tag_update(tag.id, requested_keys)
-      current_deerjikists = locked_deerjikists.filter { |deerjikist|
-        deerjikist.tag_id == tag.id
-      }
-      requested_deerjikists = locked_deerjikists.filter { |deerjikist|
-        row_indexes_by_key.key?([deerjikist.platform, deerjikist.code])
-      }
-
-      render_deerjikist_conflicts(requested_deerjikists, row_indexes_by_key, tag)
-      raise ActiveRecord::Rollback if performed?
-
-      requested_keys_set = requested_keys.to_set
-      current_deerjikists.each do |deerjikist|
-        key = [deerjikist.platform, deerjikist.code]
-        deerjikist.destroy! unless requested_keys_set.include?(key)
-      end
-
-      existing_keys = requested_deerjikists.to_h { |deerjikist|
-        [[deerjikist.platform, deerjikist.code], true]
-      }
-      requested_keys.each do |platform, code|
-        next if existing_keys[[platform, code]]
-
-        deerjikist = Deerjikist.new(platform:, code:, tag:)
-        row_index = row_indexes_by_key[[platform, code]].first
-        begin
-          render_deerjikist_form_record_invalid(deerjikist, row_index) unless deerjikist.save
-        rescue ActiveRecord::RecordNotUnique
-          conflicts = lock_deerjikists_for_tag_update(tag.id, [[platform, code]])
-          render_deerjikist_conflicts(conflicts, row_indexes_by_key, tag)
-        end
+      tag.deerjikists = []
+      params[:_json].each.with_index do |item, i|
+        platform = item[:platform]
+        code = normalise_deerjikist_code(platform, item[:code])
+        deerjikist = Deerjikist.find_or_initialize_by(platform:, code:)
+        deerjikist.tag = tag
+        render_deerjikist_form_record_invalid(deerjikist, i) unless deerjikist.save
         raise ActiveRecord::Rollback if performed?
       end
     end
@@ -573,7 +542,7 @@ class TagsController < ApplicationController
       return false
     end
 
-    target_tag_name = TagName.find_by(name:)
+    target_tag_name = TagName.with_discarded.find_by(name:)
     return true if target_tag_name.nil?
     return true if target_tag_name.canonical_id?
 
@@ -585,14 +554,17 @@ class TagsController < ApplicationController
     return if name == tag.name
 
     current_tag_name = tag.tag_name
-    target_tag_name = TagName.find_by(name:)
+    target_tag_name = TagName.with_discarded.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:
@@ -602,9 +574,11 @@ 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.where(canonical_id: current_tag_name.id)
+    TagName.with_discarded
+           .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)
@@ -635,7 +609,7 @@ class TagsController < ApplicationController
     end
 
     alias_names.each do |alias_name|
-      alias_tag_name = TagName.find_or_create_by!(name: alias_name)
+      alias_tag_name = TagName.find_undiscard_or_create_by!(name: alias_name)
       affected_tags << alias_tag_name.canonical&.tag
     end
 
@@ -650,7 +624,7 @@ class TagsController < ApplicationController
     end
 
     alias_names.each do |alias_name|
-      alias_tag_name = TagName.find_or_create_by!(name: alias_name)
+      alias_tag_name = TagName.find_undiscard_or_create_by!(name: alias_name)
       alias_tag_name.update!(canonical: tag.tag_name)
     end
 
@@ -679,7 +653,6 @@ class TagsController < ApplicationController
   end
 
   def normalise_deerjikist_code platform, code
-    code = code.to_s
     return code if platform != 'youtube' || code[0] != '@'
 
     url = "https://www.youtube.com/#{ code }"
@@ -696,82 +669,6 @@ class TagsController < ApplicationController
     nil
   end
 
-  def normalise_deerjikist_rows tag
-    rows = []
-
-    params[:_json].each.with_index do |item, index|
-      platform = item[:platform]
-      unless Deerjikist.platforms.key?(platform)
-        render_deerjikist_platform_invalid(index)
-        return rows
-      end
-
-      code = normalise_deerjikist_code(platform, item[:code])
-      deerjikist = Deerjikist.new(platform:, code:, tag:)
-      unless deerjikist.valid?
-        render_deerjikist_form_record_invalid(deerjikist, index)
-        return rows
-      end
-
-      rows << { index:, platform:, code: }
-    end
-
-    rows
-  end
-
-  def lock_deerjikists_for_tag_update tag_id, keys
-    clauses = ['tag_id = ?']
-    values = [tag_id]
-
-    keys.each do |platform, code|
-      clauses << '(platform = ? AND code = ?)'
-      values << platform << code
-    end
-
-    Deerjikist
-      .where(clauses.join(' OR '), *values)
-      .order(:platform, :code)
-      .lock
-      .to_a
-  end
-
-  def render_deerjikist_conflicts deerjikists, row_indexes_by_key, tag
-    conflicts = deerjikists.filter { |deerjikist| deerjikist.tag_id != tag.id }
-    return if conflicts.empty?
-
-    tag_names_by_id = Tag
-                      .joins(:tag_name)
-                      .where(id: conflicts.map(&:tag_id).uniq)
-                      .pluck('tags.id', 'tag_names.name')
-                      .to_h
-    fields = { }
-
-    conflicts.each do |deerjikist|
-      message = "この情報は既に「#{ tag_names_by_id[deerjikist.tag_id] }」に紐づいてゐます．"
-      row_indexes_by_key[[deerjikist.platform, deerjikist.code]].each do |index|
-        field = :"deerjikists.#{ index }.code"
-        fields[field] ||= []
-        fields[field] << message
-      end
-    end
-
-    render_validation_error fields:
-  end
-
-  def render_deerjikist_platform_invalid index
-    render_validation_error fields: {
-      :"deerjikists.#{ index }.platform" => ['値が不正です．'],
-    }
-  end
-
-  def rows_by_key rows
-    rows.each_with_object({ }) do |row, result|
-      key = [row[:platform], row[:code]]
-      result[key] ||= []
-      result[key] << row[:index]
-    end
-  end
-
   def render_deerjikist_form_record_invalid deerjikist, index
     fields = { }
 

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_or_create_by!(name: title)
+    tag_name = TagName.find_undiscard_or_create_by!(name: title)
 
     page =
         Wiki::Commit.create_content!(

backend/app/models/post.rb

@@ -55,27 +55,25 @@ class Post < ApplicationRecord
   belongs_to :uploaded_user, class_name: 'User', optional: true
 
   has_many :post_tags, dependent: :destroy, inverse_of: :post
-  has_many :tags, through: :post_tags
+  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 :active_tags, -> { where(tags: { deprecated_at: nil }) },
-           through: :post_tags,
-           source: :tag
+           through: :active_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,
@@ -125,6 +123,7 @@ class Post < ApplicationRecord
 
   def snapshot_tag_names
     post_tags
+        .kept
         .joins(tag: :tag_name)
         .includes(:sections, tag: :tag_name)
         .order('tag_names.name')
@@ -138,33 +137,6 @@ class Post < ApplicationRecord
     end
   end
 
-  def self.tag_snapshot_literal tag
-    sections = tag.fetch('sections', []).map do |sec|
-      begin_ms = sec.fetch('begin_ms')
-      end_ms = sec['end_ms']
-
-      "[#{ Post.ms_to_time(begin_ms) }-#{ end_ms ? Post.ms_to_time(end_ms) : '' }]"
-    end
-
-    "#{ tag.fetch('name') }#{ sections.join }"
-  end
-
-  def snapshot_tags_json
-    post_tags
-        .joins(tag: :tag_name)
-        .includes(:sections, tag: :tag_name)
-        .order('tags.id')
-        .map do |pt|
-      { '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
-  end
-
   def self.section_literal section
     end_ms =
         section.end_ms ? Post.ms_to_time(section.end_ms) : ''

backend/app/models/post_tag.rb

@@ -1,7 +1,14 @@
 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],
@@ -11,5 +18,18 @@ class PostTag < ApplicationRecord
 
   validates :post_id, presence: true
   validates :tag_id, presence: true
-  validates :post_id, uniqueness: { scope: :tag_id }
+  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
 end

backend/app/models/post_tag_section.rb

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

backend/app/models/tag.rb

@@ -2,6 +2,8 @@ require 'set'
 
 
 class Tag < ApplicationRecord
+  include MyDiscard
+
   class NicoTagNormalisationError < ArgumentError
     ;
   end
@@ -26,7 +28,9 @@ class Tag < ApplicationRecord
   end
 
   has_many :post_tags, inverse_of: :tag
-  has_many :posts, through: :post_tags
+  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
@@ -230,10 +234,10 @@ class Tag < ApplicationRecord
   end
 
   def self.find_or_create_by_tag_name! name, category:
-    tn = TagName.find_or_create_by!(name: name.to_s.strip)
+    tn = TagName.find_undiscard_or_create_by!(name: name.to_s.strip)
     tn = tn.canonical if tn.canonical_id?
 
-    Tag.find_or_create_by!(tag_name_id: tn.id) do |t|
+    Tag.find_undiscard_or_create_by!(tag_name_id: tn.id) do |t|
       t.category = category
     end
   rescue ActiveRecord::RecordNotUnique
@@ -255,11 +259,11 @@ class Tag < ApplicationRecord
 
         TagVersioning.ensure_snapshot!(source_tag, created_by_user:)
 
-        source_tag.post_tags.find_each do |source_pt|
+        source_tag.post_tags.kept.find_each do |source_pt|
           post_id = source_pt.post_id
           affected_post_ids << post_id
-          source_pt.destroy!
-          unless PostTag.exists?(post_id:, tag: target_tag)
+          source_pt.discard_by!(created_by_user)
+          unless PostTag.kept.exists?(post_id:, tag: target_tag)
             PostTag.create!(post_id:, tag: target_tag)
           end
         end
@@ -271,10 +275,10 @@ class Tag < ApplicationRecord
         end
 
         TagVersioning.record!(source_tag, event_type: :discard, created_by_user:)
-        source_tag.destroy!
+        source_tag.discard!
 
         if source_tag.nico?
-          source_tag_name.destroy!
+          source_tag_name.discard!
         else
           source_tag_name.update_columns(canonical_id: target_tag.tag_name_id,
                                          updated_at: Time.current)
@@ -289,13 +293,13 @@ class Tag < ApplicationRecord
       end
 
       # 投稿件数を再集計
-      target_tag.update_columns(post_count: PostTag.where(tag: target_tag).count)
+      target_tag.update_columns(post_count: PostTag.kept.where(tag: target_tag).count)
     end
 
     target_tag.reload
   end
 
-  def snapshot_aliases = tag_name.aliases.order(:name).pluck(:name)
+  def snapshot_aliases = tag_name.aliases.kept.order(:name).pluck(:name)
 
   def snapshot_parent_tag_ids = parents.order(:id).pluck(:id)
 

backend/app/models/tag_name.rb

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

backend/app/models/tag_name_sanitisation_rule.rb

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

backend/app/representations/post_repr.rb

@@ -88,14 +88,14 @@ module PostRepr
 
   def tag_json post
     post
-        .post_tags
+        .active_post_tags
         .reject { _1.tag.deprecated? }
         .sort_by { _1.tag.name }
-        .map do |post_tag|
+        .map { |post_tag|
       TagRepr.inline(post_tag.tag).merge(
           'children' => [],
           'sections' => post_tag.sections.as_json(only: [:begin_ms, :end_ms]))
-    end
+    }
   end
 
   def thumbnail_url post, host: nil

backend/app/services/post_creator.rb

@@ -121,11 +121,9 @@ 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|
@@ -135,9 +133,10 @@ class PostCreator
                                end_ms:)
       end
     end
-
     PostTag.where(post_id: post.id,
-                  tag_id: (current_ids - desired_ids).to_a).destroy_all
+                  tag_id: (current_ids - desired_ids).to_a).kept.find_each do |post_tag|
+      post_tag.discard_by!(@actor)
+    end
   end
 
   def sync_parent_posts! post, ids

backend/app/services/post_version_recorder.rb

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

backend/app/services/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.where(post_id: post.id).pluck(:tag_id).to_set
+      current_tag_ids ||= PostTag.kept.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).find_each do |pt|
-        pt.destroy!
+      PostTag.where(post_id: post.id, tag_id: to_remove.to_a).kept.find_each do |pt|
+        pt.discard_by!(nil)
       end
     end
 

backend/config/routes.rb

@@ -55,6 +55,7 @@ Rails.application.routes.draw do
       get :metadata
       post :bulk
       get :random
+      get :changes
       get :versions, to: 'post_versions#index'
     end
 

backend/db/migrate/20260727123600_add_tags_json_to_post_versions.rb

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

backend/db/migrate/20260921000000_delete_inactive_records_from_post_tags.rb

@@ -1,53 +0,0 @@
-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

@@ -1,11 +0,0 @@
-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

@@ -1,46 +0,0 @@
-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

@@ -1,18 +0,0 @@
-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/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_09_21_030000) do
+ActiveRecord::Schema[8.0].define(version: 2026_07_13_000000) 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
@@ -311,12 +311,23 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
     t.check_constraint "`begin_ms` >= 0", name: "chk_post_tag_sections_begin_ms_natural"
   end
 
-  create_table "post_tags", primary_key: ["post_id", "tag_id"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+  create_table "post_tags", 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
 
@@ -338,7 +349,6 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
     t.string "url", limit: 768, null: false
     t.string "thumbnail_base", limit: 2000
     t.text "tags", null: false
-    t.json "tags_json", null: false
     t.text "parent_post_ids", null: false
     t.datetime "original_created_from"
     t.datetime "original_created_before"
@@ -352,7 +362,6 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) 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"
   end
 
   create_table "posts", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
@@ -384,6 +393,19 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) 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
@@ -409,7 +431,9 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
     t.bigint "canonical_id"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.datetime "discarded_at"
     t.index ["canonical_id"], name: "index_tag_names_on_canonical_id"
+    t.index ["discarded_at"], name: "index_tag_names_on_discarded_at"
     t.index ["name"], name: "index_tag_names_on_name", unique: true
   end
 
@@ -446,8 +470,10 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) 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"
@@ -588,19 +614,6 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) 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
@@ -694,6 +707,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) 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"
@@ -702,15 +716,18 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
   add_foreign_key "materials", "users", column: "updated_by_user_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_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", "post_tags", column: ["post_id", "tag_id"], primary_key: ["post_id", "tag_id"], on_delete: :cascade
+  add_foreign_key "post_tag_sections", "posts"
+  add_foreign_key "post_tag_sections", "tags"
   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"
@@ -720,6 +737,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
   add_foreign_key "tag_names", "tag_names", column: "canonical_id"
   add_foreign_key "tag_similarities", "tags"
   add_foreign_key "tag_similarities", "tags", column: "target_tag_id"
+  add_foreign_key "tag_versions", "tags"
   add_foreign_key "tag_versions", "users", column: "created_by_user_id"
   add_foreign_key "tags", "tag_names"
   add_foreign_key "theatre_comments", "theatres"

backend/lib/tasks/sync_nico.rake

@@ -16,7 +16,7 @@ namespace :nico do
     end
 
     def sync_post_tags! post, desired_tag_ids, current_tag_ids: nil
-      current_tag_ids ||= PostTag.where(post_id: post.id).pluck(:tag_id).to_set
+      current_tag_ids ||= PostTag.kept.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
@@ -30,8 +30,8 @@ namespace :nico do
         end
       end
 
-      PostTag.where(post_id: post.id, tag_id: to_remove.to_a).find_each do |pt|
-        pt.destroy!
+      PostTag.where(post_id: post.id, tag_id: to_remove.to_a).kept.find_each do |pt|
+        pt.discard_by!(nil)
       end
     end
 

backend/spec/db/delete_discarded_tags_spec.rb

@@ -1,123 +0,0 @@
-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/models/post_tag_spec.rb

@@ -1,73 +1,5 @@
-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,
@@ -80,25 +12,18 @@ RSpec.describe PostTag, type: :model do
     end
 
     it 'does not load sections for another tag on the same post' do
-      post_tag = create(:post_tag)
-      post = post_tag.post
+      post = create(:post)
+      tag = create(:tag)
       other_tag = create(: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)
-
+      post_tag = create(:post_tag, post:, tag:)
       create(:post_tag_section,
              post:,
              tag: other_tag,
              begin_ms: 1000,
              end_ms: 2000)
 
-      expect(post_tag.reload.sections).to contain_exactly(own_section)
+      expect(post_tag.sections).to be_empty
     end
 
     it 'allows open-ended sections' do

backend/spec/models/post_version_spec.rb

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

backend/spec/models/tag_name_sanitisation_rule_spec.rb

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

backend/spec/models/tag_spec.rb

@@ -173,47 +173,6 @@ RSpec.describe Tag, type: :model do
     end
   end
 
-  describe '.find_or_create_by_tag_name!' do
-    it 'creates a tag and name with the requested category after stripping whitespace' do
-      tag = nil
-
-      expect {
-        tag = described_class.find_or_create_by_tag_name!(
-            '  lookup_new  ', category: :character)
-      }.to change(Tag, :count).by(1).and change(TagName, :count).by(1)
-
-      expect(tag.name).to eq('lookup_new')
-      expect(tag.category).to eq('character')
-    end
-
-    it 'reuses the canonical tag for an alias without changing its category' do
-      tag = create(:tag, category: :character)
-      alias_name = TagName.create!(name: 'lookup_alias', canonical: tag.tag_name)
-      found = nil
-
-      expect {
-        found = described_class.find_or_create_by_tag_name!(
-            alias_name.name, category: :general)
-      }.to change(Tag, :count).by(0).and change(TagName, :count).by(0)
-
-      expect(found).to eq(tag)
-      expect(found.category).to eq('character')
-    end
-
-    it 'creates a tag for an existing canonical name reached through an alias' do
-      canonical = create(:tag_name)
-      alias_name = TagName.create!(name: 'lookup_alias', canonical:)
-      tag = nil
-
-      expect {
-        tag = described_class.find_or_create_by_tag_name!(
-            alias_name.name, category: :general)
-      }.to change(Tag, :count).by(1).and change(TagName, :count).by(0)
-
-      expect(tag.tag_name).to eq(canonical)
-    end
-  end
-
   describe '.merge_tags!' do
     let!(:target_tag) { create(:tag, category: :general) }
     let!(:source_tag) { create(:tag, category: :general) }
@@ -226,14 +185,18 @@ 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 'deletes the source tag, moves its post link, and keeps its name as an alias' do
+      it 'discards the source post_tag, creates an active target post_tag, discards the source tag, and aliases the source tag_name' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        target_link = PostTag.find_by(post: post_record, 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)
 
-        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_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(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
         expect(target_tag.reload.post_count).to eq(1)
       end
@@ -243,101 +206,38 @@ 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 '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)
-
+      it 'discards the source post_tag, keeps one active target post_tag, discards the source tag, and aliases the source tag_name' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        target_links = PostTag.where(post: post_record, tag: target_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)
 
-        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(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(Tag.unscoped.exists?(source_tag.id)).to be(false)
+        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(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
         expect(target_tag.reload.post_count).to eq(1)
       end
     end
 
-    it 'keeps source history and records the new target alias after deleting the source' do
-      user = create_member_user!
-      source_name = source_tag.name
-      source_alias = TagName.create!(name: 'merge_alias', canonical: source_tag_name)
-      TagVersioning.ensure_snapshot!(source_tag, created_by_user: user)
-      original_version = source_tag.tag_versions.first
-
-      described_class.merge_tags!(target_tag, [source_tag], created_by_user: user)
-
-      versions = TagVersion.where(tag_id: source_tag.id).order(:version_no)
-      expect(versions.pluck(:version_no, :event_type))
-          .to eq([[1, 'create'], [2, 'discard']])
-      expect(versions.first).to eq(original_version)
-      expect(versions.last).to have_attributes(
-          name: source_name, aliases: source_alias.name, created_by_user: user)
-      expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
-
-      target_versions = target_tag.tag_versions.order(:version_no)
-      expect(target_versions.pluck(:event_type)).to eq(['create', 'update'])
-      expect(target_versions.last.aliases.split).to eq([source_name])
-    end
-
-    it 'deletes source relationships while preserving unrelated relationships' do
-      parent = create(:tag)
-      child = create(:tag)
-      nico_tag = create(:tag, :nico)
-      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])
 
-        target_link = PostTag.find_by(post: post_record, 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)
 
         expect(Tag.find(target_tag.id)).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(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(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
         expect(target_tag.reload.post_count).to eq(1)
       end
@@ -360,16 +260,18 @@ RSpec.describe Tag, type: :model do
         )
       end
 
-      it 'still merges and keeps the source name as an alias without validating it' do
+      it 'still merges, but discards the source tag_name instead of aliasing it' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        target_link = PostTag.find_by(post: post_record, 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)
+        discarded_source_tag_name = TagName.with_discarded.find(source_tag_name.id)
 
-        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
-        expect(target_link).to be_present
+        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(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
+        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
         expect(target_tag.reload.post_count).to eq(1)
       end
     end
@@ -386,72 +288,36 @@ RSpec.describe Tag, type: :model do
             message: 'init')
       end
 
-      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)
-
+      it 'rolls back the transaction' do
         expect {
-          described_class.merge_tags!(target_tag, [earlier_source, source_tag])
+          described_class.merge_tags!(target_tag, [source_tag])
         }.to raise_error(ActiveRecord::RecordInvalid)
 
-        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(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(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) do
-        create(:tag, category: :nico, tag_name: create(:tag_name, name: 'nico:foo'))
-      end
-      let!(:source_tag) do
-        create(:tag, category: :nico, tag_name: create(:tag_name, name: 'nico:bar'))
-      end
+      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 'deletes the source tag and name instead of keeping an alias' do
+      it 'discards the source tag_name instead of aliasing it' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
-        expect(TagName.unscoped.exists?(source_tag_name_id)).to be(false)
+        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
-
-      it 'keeps nico history while deleting source links and allows recreating the name' do
-        linked_tag = create(:tag)
-        NicoTagRelation.create!(nico_tag: source_tag, tag: linked_tag)
-        kept_relation = NicoTagRelation.create!(nico_tag: target_tag, tag: linked_tag)
-        user = create_member_user!
-        source_name = source_tag.name
-
-        described_class.merge_tags!(target_tag, [source_tag], created_by_user: user)
-
-        expect(NicoTagRelation.all).to contain_exactly(kept_relation)
-        versions = NicoTagVersion.where(tag_id: source_tag.id).order(:version_no)
-        expect(versions.pluck(:version_no, :event_type))
-            .to eq([[1, 'create'], [2, 'discard']])
-        expect(versions.last).to have_attributes(
-            name: source_name, linked_tags: linked_tag.name, created_by_user: user)
-
-        recreated = described_class.find_or_create_by_tag_name!(source_name, category: :nico)
-
-        expect(recreated.id).not_to eq(source_tag.id)
-        expect(recreated.tag_name_id).not_to eq(source_tag_name_id)
-        expect(recreated.nico_tag_versions).to be_empty
-        expect(versions.reload.size).to eq(2)
-      end
     end
 
     def snapshot_tags(post)
@@ -467,7 +333,6 @@ RSpec.describe Tag, type: :model do
         url: post.url,
         thumbnail_base: post.thumbnail_base,
         tags: snapshot_tags(post),
-        tags_json: post.snapshot_tags_json,
         parent_post_ids: post.snapshot_parent_post_ids.join(' '),
         original_created_from: post.original_created_from,
         original_created_before: post.original_created_before,
@@ -499,15 +364,12 @@ 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('id') }).to eq([target_tag.id])
-        expect(affected_versions.first.tags_json.map { |item| item.fetch('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 post_tags' do
+    context 'when the source tag has no active post_tags' do
       let!(:another_post) do
         Post.create!(url: 'https://example.com/posts/3', title: 'another post')
       end

backend/spec/requests/posts_spec.rb

@@ -55,7 +55,6 @@ RSpec.describe 'Posts API', type: :request do
             thumbnail_base: post.thumbnail_base,
             video_ms: post.video_ms,
             tags: post.snapshot_tag_names.join(' '),
-            tags_json: post.snapshot_tags_json,
             parent_post_ids: post.snapshot_parent_post_ids.join(' '),
             original_created_from: post.original_created_from,
             original_created_before: post.original_created_before,
@@ -148,15 +147,12 @@ 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([])
@@ -165,26 +161,6 @@ 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' }
@@ -483,71 +459,6 @@ 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 > ?`
@@ -1252,7 +1163,9 @@ RSpec.describe 'Posts API', type: :request do
 
     context 'when nico tag already exists in tags' do
       before do
-        Tag.find_or_create_by_tag_name!('nico:nico_tag', category: :nico)
+        Tag.find_undiscard_or_create_by!(
+            tag_name: TagName.find_undiscard_or_create_by!(name: 'nico:nico_tag'),
+            category: :nico)
       end
 
       it 'returns 422 with tag field errors' do
@@ -1505,11 +1418,9 @@ 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')
-      replacement_tag = Tag.create!(tag_name: tn2, category: :general)
+      Tag.create!(tag_name: tn2, category: :general)
 
       put "/posts/#{post_record.id}", params: post_update_params(
           post_record,
@@ -1522,38 +1433,6 @@ 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('id' => tag.id,
-                           'sections' => [{ 'begin_ms' => 1000, 'end_ms' => 2000 }]))
-      expect(versions.last.tags_json.map { |item| item.fetch('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('id') }
-      end
-      expect(snapshots.map { |ids| ids.include?(tag.id) }).to eq([true, false, true])
     end
 
     it 'rejects a deprecated tag specified directly' do
@@ -1578,7 +1457,9 @@ RSpec.describe 'Posts API', type: :request do
 
     context 'when nico tag already exists in tags' do
       before do
-        Tag.find_or_create_by_tag_name!('nico:nico_tag', category: :nico)
+        Tag.find_undiscard_or_create_by!(
+            tag_name: TagName.find_undiscard_or_create_by!(name: 'nico:nico_tag'),
+            category: :nico)
       end
 
       it 'returns 422 with tag field errors' do
@@ -1831,7 +1712,10 @@ RSpec.describe 'Posts API', type: :request do
         expect(post_record.reload.title).to eq('updated by other user')
       end
 
-      it 'returns mergeable 409 for stale non-conflicting tag changes without merge' do
+      it(
+          'returns 409 with mergeable true when stale tag changes '
+          'do not conflict but merge is not requested'
+      ) do
         sign_in_as(member)
 
         base_version = create_post_version_for!(post_record.reload)
@@ -1997,29 +1881,123 @@ RSpec.describe 'Posts API', type: :request do
       expect(response).to have_http_status(:not_found)
     end
 
-    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)
-
+    it '200 and returns viewed boolean' do
       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
-    it 'returns 404 for the retired history endpoint' do
-      get '/posts/changes'
+    let(:member) { create(:user, :member) }
 
-      expect(response).to have_http_status(:not_found)
+    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)
     end
   end
 
@@ -2049,7 +2027,6 @@ RSpec.describe 'Posts API', type: :request do
         url: post.url,
         thumbnail_base: post.thumbnail_base,
         tags: snapshot_tags(post),
-        tags_json: post.snapshot_tags_json,
         parent_post_ids: post.snapshot_parent_post_ids.join(' '),
         original_created_from: post.original_created_from,
         original_created_before: post.original_created_before,
@@ -2071,7 +2048,7 @@ RSpec.describe 'Posts API', type: :request do
     end
 
     let!(:v2) do
-      post_record.post_tags.find_by!(tag: tag).destroy!
+      post_record.post_tags.kept.find_by!(tag: tag).discard_by!(member)
       PostTag.create!(post: post_record, tag: tag2, created_user: member)
       post_record.update!(
         title: 'updated spec post',

backend/spec/requests/tags_deerjikists_spec.rb

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

backend/spec/requests/wiki_body_search_pending_spec.rb

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

backend/spec/requests/wiki_restore_pending_spec.rb

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

backend/spec/services/youtube/sync_spec.rb

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

backend/spec/tasks/nico_sync_spec.rb

@@ -8,7 +8,8 @@ RSpec.describe 'nico:sync' do
   end
 
   def create_tag!(name, category:)
-    Tag.find_or_create_by_tag_name!(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 }
   end
 
   def link_nico_to_tag!(nico_tag, tag)
@@ -107,7 +108,7 @@ RSpec.describe 'nico:sync' do
     expect(calls).to eq(2)
   end
 
-  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
+  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
     post = Post.create!(
       title: 'old',
       url: 'https://www.nicovideo.jp/watch/sm9',
@@ -116,8 +117,8 @@ 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')
-    PostTag.create!(post:, tag: old_nico)
-    create_post_version_for!(post)
+    old_pt = PostTag.create!(post: post, tag: old_nico)
+    expect(old_pt.discarded_at).to be_nil
 
     # 今回は NEW のみ欲しい
     new_nico = create_tag!('nico:NEW', category: 'nico')
@@ -131,17 +132,9 @@ RSpec.describe 'nico:sync' do
 
     run_rake_task('nico:sync')
 
-    expect(PostTag.exists?(post:, 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.map { |item| item.fetch('id') })
-        .to include(old_nico.id)
-    expect(versions.last.tags_json.map { |item| item.fetch('id') })
-        .to include(new_nico.id)
-    expect(versions.last.tags_json.map { |item| item.fetch('id') })
-        .not_to include(old_nico.id)
+    # 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
 
     # NEW は active にいる
     post.reload
@@ -163,7 +156,6 @@ RSpec.describe 'nico:sync' do
       url: post.url,
       thumbnail_base: post.thumbnail_base,
       tags: snapshot_tags(post),
-      tags_json: post.snapshot_tags_json,
       parent_post_ids: post.snapshot_parent_post_ids.join(' '),
       original_created_from: post.original_created_from,
       original_created_before: post.original_created_before,

frontend/src/lib/postNewQueryState.test.ts

@@ -29,15 +29,15 @@ describe ('post new review URL state', () => {
 	.toEqual (['one', 'two'])
   })
 
-  it ('allows at most a 6 143 byte request target', () => {
+  it ('allows at most a 4095-byte request target', () => {
     const baseUrl = 'https://example.com/'
     const baseLength = postNewReviewPathByteLength ([baseUrl])
-    const allowed = `${ baseUrl }${ 'a'.repeat (6_143 - baseLength) }`
+    const allowed = `${ baseUrl }${ 'a'.repeat (4_095 - baseLength) }`
     const denied = `${ allowed }a`
 
-    expect (postNewReviewPathByteLength ([allowed])).toBe (6_143)
+    expect (postNewReviewPathByteLength ([allowed])).toBe (4_095)
     expect (isPostNewReviewPathWithinLimit ([allowed])).toBe (true)
-    expect (postNewReviewPathByteLength ([denied])).toBe (6_144)
+    expect (postNewReviewPathByteLength ([denied])).toBe (4_096)
     expect (isPostNewReviewPathWithinLimit ([denied])).toBe (false)
   })
 })

frontend/src/lib/postNewQueryState.ts

@@ -1,5 +1,5 @@
 const POST_NEW_REVIEW_PATH_PREFIX = '/posts/new?urls='
-const MAX_POST_NEW_REVIEW_TARGET_BYTES = 6_144
+const MAX_POST_NEW_REVIEW_TARGET_BYTES = 4_096
 
 const textEncoder = new TextEncoder ()
 

frontend/src/pages/posts/PostImportSourcePage.test.tsx

@@ -60,7 +60,7 @@ describe ('PostImportSourcePage', () => {
     const input = screen.getByRole ('textbox', { name: '' })
 
     fireEvent.change (input, {
-      target: { value: `https://example.com/${ 'a'.repeat (6_200) }` } })
+      target: { value: `https://example.com/${ 'a'.repeat (4_100) }` } })
 
     expect (screen.getByRole ('button', { name: '次へ' })).toBeDisabled ()
   })

frontend/src/test/coverage.todo.test.ts

@@ -0,0 +1,10 @@
+import { describe, it } from 'vitest'
+
+describe ('pending high-level browser coverage', () => {
+  it.todo ('adds MSW-backed API boundary tests in a follow-up issue')
+  it.todo ('covers TheatreDetailPage with timer polling, comment posting, and next-post updates')
+  it.todo ('covers NicoTagListPage linking and pagination against realistic API payloads')
+  it.todo ('covers TagDetailSidebar drag/drop parent-child editing with pointer-event fidelity')
+  it.todo ('covers TopNav desktop and mobile menu flows as browser-level integration tests')
+  it.todo ('covers full App bootstrap for user creation, user verification, and 503 handling')
+})