#419

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

backend/app/controllers/application_controller.rb

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

backend/app/controllers/materials_controller.rb

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

backend/app/controllers/nico_tags_controller.rb

@@ -15,23 +15,22 @@ class NicoTagsController < ApplicationController
     limit = 1 if limit < 1
 
     post_tag_max_sql =
-        PostTag
+        PostExternalTag
-        .select('tag_id, MAX(created_at) AS max_created_at')
+        .select('external_tag_id, MAX(created_at) AS max_created_at')
-        .group('tag_id')
+        .group('external_tag_id')
         .to_sql
 
-    q = Tag.nico_tags
+    q =
-           .joins(:tag_name)
+        ExternalTag
-           .joins("LEFT JOIN (#{ post_tag_max_sql }) post_tag_max " \
+        .joins("LEFT JOIN (#{ post_tag_max_sql }) post_tag_max " \
-                  'ON post_tag_max.tag_id = tags.id')
+               'ON post_tag_max.external_tag_id = external_tags.id')
-           .includes(:tag_name, tag_name: :wiki_page, linked_tags: { tag_name: :wiki_page })
+    q = q.where('external_tags.name LIKE ?', "%#{ name }%") if name
-    q = q.where('tag_names.name LIKE ?', "%#{ name }%") if name
     if linked_tag
       linked_tag_ids =
-        Tag
+          Tag
-        .joins(:tag_name)
+          .joins(:tag_name)
-        .where('tag_names.name LIKE ?', "%#{ linked_tag }%")
+          .where('tag_names.name LIKE ?', "%#{ linked_tag }%")
-        .pluck(:id)
+          .pluck(:id)
       linked_nico_tag_ids = NicoTagRelation.where(tag_id: linked_tag_ids).pluck(:nico_tag_id)
       q = q.where(id: linked_nico_tag_ids)
     end
@@ -72,8 +71,7 @@ class NicoTagsController < ApplicationController
 
     id = params[:id].to_i
 
-    tag = Tag.find(id)
+    tag = ExternalTag.find(id)
-    return render_bad_request('ニコニコ・タグを指定してください．') unless tag.nico?
 
     linked_tag_names = params[:tags].to_s.split
     linked_tags = nil
@@ -81,9 +79,6 @@ class NicoTagsController < ApplicationController
     ApplicationRecord.transaction do
       linked_tags = Tag.normalise_tags!(linked_tag_names, with_tagme: false,
                                                           with_no_deerjikist: false)
-      if linked_tags.any? { |t| t.nico? }
-        raise Tag::NicoTagNormalisationError
-      end
 
       TagVersioning.record_tag_snapshots!(linked_tags, created_by_user: current_user)
 

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
+    tag_id = params[:tag].presence&.to_i
     page = (params[:page].presence || 1).to_i
     limit = (params[:limit].presence || 20).to_i
 
@@ -10,12 +10,6 @@ class PostVersionsController < ApplicationController
 
     offset = (page - 1) * limit
 
-    tag_name =
-        if tag_id
-          TagName.joins(:tag).find_by(tag: { id: tag_id })
-        end
-    return render json: { versions: [], count: 0 } if tag_id && tag_name.blank?
-
     q = PostVersion.joins(<<~SQL.squish)
           LEFT JOIN
             post_versions prev
@@ -23,17 +17,18 @@ 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',
+        .select('post_versions.*',
-                'prev.thumbnail_base AS prev_thumbnail_base', 'prev.tags AS prev_tags',
+                'prev.title AS prev_title',
+                'prev.url AS prev_url',
+                'prev.thumbnail_base AS prev_thumbnail_base',
+                'prev.tags_json AS prev_tags_json',
                 'prev.video_ms AS prev_video_ms',
                 'prev.original_created_from AS prev_original_created_from',
                 'prev.original_created_before AS prev_original_created_before')
     q = q.where('post_versions.post_id = ?', post_id) if post_id
-    if tag_name
+    if tag_id
-      escaped = ActiveRecord::Base.sanitize_sql_like(tag_name.name)
+      q = q.where("JSON_CONTAINS(post_versions.tags_json, JSON_OBJECT('tag_id', #{ tag_id })) " +
-      q = q.where(("CONCAT(' ', post_versions.tags, ' ') LIKE :kw " +
+                  "OR JSON_CONTAINS(prev.tags_json, JSON_OBJECT('tag_id', #{ tag_id }))")
-                   "OR CONCAT(' ', prev.tags, ' ') LIKE :kw"),
-                  kw: "% #{ escaped } %")
     end
 
     count = q.except(:select, :order, :limit, :offset).count
@@ -52,73 +47,84 @@ class PostVersionsController < ApplicationController
     users_by_id = User.where(id: user_ids).pluck(:id, :name).to_h
 
     rows.map do |row|
-      cur_tags = split_tags(row.tags)
+      cur_tags =
-      prev_tags = split_tags(row.attributes['prev_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) }
 
-      {
+      { post_id: row.post_id,
-        post_id: row.post_id,
         version_no: row.version_no,
         event_type: row.event_type,
-        title: {
+        title: { current: row.title, prev: row.attributes['prev_title'] },
-          current: row.title,
+        url: { current: row.url, prev: row.attributes['prev_url'] },
-          prev: row.attributes['prev_title']
+        thumbnail: { current: nil, prev: nil },
-        },
+        thumbnail_base: { current: row.thumbnail_base,
-        url: {
+                          prev: row.attributes['prev_thumbnail_base'] },
-          current: row.url,
+        video_ms: { current: row.video_ms, prev: row.attributes['prev_video_ms'] },
-          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,
+            current: row.original_created_from&.iso8601,
-          prev: row.attributes['prev_original_created_from']&.iso8601
+            prev: row.attributes['prev_original_created_from']&.iso8601 },
-        },
         original_created_before: {
-          current: row.original_created_before&.iso8601,
+            current: row.original_created_before&.iso8601,
-          prev: row.attributes['prev_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
+            if row.created_by_user_id
-            {
+              { id: row.created_by_user_id,
-              id: row.created_by_user_id,
+                name: users_by_id[row.created_by_user_id] }
-              name: users_by_id[row.created_by_user_id]
+            end }
-            }
-          end
-      }
     end
   end
 
-  def build_version_tags(cur_tags, prev_tags)
+  def build_version_tags cur_tags, prev_tags
     (cur_tags | prev_tags).map do |name|
       type =
-        if cur_tags.include?(name) && prev_tags.include?(name)
+          if cur_tags.include?(name) && prev_tags.include?(name)
-          'context'
+            'context'
-        elsif cur_tags.include?(name)
+          elsif cur_tags.include?(name)
-          'added'
+            'added'
-        else
+          else
-          'removed'
+            'removed'
-        end
+          end
 
-      {
+      { name:, type: }
-        name:,
-        type:
-      }
     end
   end
-
-  def split_tags(tags)
-    tags.to_s.split(/\s+/).reject(&:blank?)
-  end
 end

backend/app/controllers/posts_controller.rb

@@ -36,8 +36,8 @@ class PostsController < ApplicationController
     offset = (page - 1) * limit
 
     pt_max_sql =
-        PostTag
+        PostVersion
-        .select('post_id, MAX(updated_at) AS max_updated_at')
+        .select('post_id, MAX(created_at) AS max_updated_at')
         .group('post_id')
         .to_sql
 
@@ -50,9 +50,8 @@ 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,
-                 active_post_tags: [:sections,
+                 post_tags: [:sections, { tag: [:deerjikists, :materials,
-                                    { tag: [:deerjikists, :materials,
+                                                { tag_name: :wiki_page }] }])
-                                            { tag_name: :wiki_page }] }])
         .with_attached_thumbnail
 
     q = q.where('posts.url LIKE ?', "%#{ url }%") if url
@@ -104,9 +103,8 @@ class PostsController < ApplicationController
 
   def random
     post = filtered_posts.preload(:uploaded_user, :parents, :children,
-                                  active_post_tags: [:sections,
+                                  post_tags: [:sections, { tag: [:deerjikists, :materials,
-                                                     { tag: [:deerjikists, :materials,
+                                                                 { tag_name: :wiki_page }] }])
-                                                             { tag_name: :wiki_page }] }])
                          .with_attached_thumbnail
                          .order('RAND()')
                          .first
@@ -190,9 +188,8 @@ class PostsController < ApplicationController
     post =
         Post
         .includes(:uploaded_user, :parents, :children,
-                  active_post_tags: [:sections,
+                  post_tags: [:sections, { tag: [:deerjikists, :materials,
-                                     { tag: [:deerjikists, :materials,
+                                                 { tag_name: :wiki_page }] }])
-                                             { tag_name: :wiki_page }] }])
         .with_attached_thumbnail
         .find_by(id: params[:id])
     return head :not_found unless post
@@ -384,50 +381,6 @@ class PostsController < ApplicationController
     render_post_form_record_invalid e.record
   end
 
-  def changes
-    id = params[:id].presence
-    tag_id = params[:tag].presence
-    page = (params[:page].presence || 1).to_i
-    limit = (params[:limit].presence || 20).to_i
-
-    page = 1 if page < 1
-    limit = 1 if limit < 1
-
-    offset = (page - 1) * limit
-
-    pts = PostTag.with_discarded
-    pts = pts.where(post_id: id) if id.present?
-    pts = pts.where(tag_id:) if tag_id.present?
-    pts = pts.includes(:post, :created_user, :deleted_user,
-                       tag: [:deerjikists, :materials, { tag_name: :wiki_page }])
-
-    events = []
-    pts.each do |pt|
-      tag = TagRepr.base(pt.tag)
-      post = pt.post
-
-      events << Event.new(
-          post:,
-          tag:,
-          user: pt.created_user && { id: pt.created_user.id, name: pt.created_user.name },
-          change_type: 'add',
-          timestamp: pt.created_at)
-
-      if pt.discarded_at
-        events << Event.new(
-            post:,
-            tag:,
-            user: pt.deleted_user && { id: pt.deleted_user.id, name: pt.deleted_user.name },
-            change_type: 'remove',
-            timestamp: pt.discarded_at)
-      end
-    end
-    events.sort_by!(&:timestamp)
-    events.reverse!
-
-    render json: { changes: (events.slice(offset, limit) || []).as_json, count: events.size }
-  end
-
   private
 
   def filtered_posts
@@ -502,13 +455,13 @@ class PostsController < ApplicationController
       end
     end
 
-    PostTag.where(post_id: post.id, tag_id: to_remove.to_a).kept.find_each do |pt|
+    PostTag.where(post_id: post.id, tag_id: to_remove.to_a).find_each do |pt|
-      pt.discard_by!(current_user)
+      pt.destroy!
     end
   end
 
   def build_tag_tree_for post
-    post_tags = post.active_post_tags.reject { |post_tag| post_tag.tag.deprecated? }
+    post_tags = post.post_tags.reject { |post_tag| post_tag.tag.deprecated? }
     tags = post_tags.map(&:tag)
     tag_ids = tags.map(&:id)
 
@@ -699,7 +652,10 @@ class PostsController < ApplicationController
   end
 
   def editable_tag_names_from_version version
-    version.tags.to_s.split.reject { |name| name.downcase.start_with?('nico:') }.sort
+    version.tags_json
+        .reject { _1.fetch('category') == 'nico' }
+        .map { Post.tag_snapshot_literal(_1) }
+        .sort
   end
 
   def post_snapshot_from_record post
@@ -714,9 +670,7 @@ class PostsController < ApplicationController
   def editable_tag_names_from_post post
     post
         .post_tags
-        .kept
         .joins(tag: :tag_name)
-        .merge(Tag.not_nico)
         .merge(Tag.where(deprecated_at: nil))
         .includes(:sections, tag: :tag_name)
         .order('tag_names.name')
@@ -734,8 +688,7 @@ class PostsController < ApplicationController
                              tag_names:, video_ms_param:, duration_param:, parent_post_ids:
     validate_original_created_values!(original_created_from, original_created_before)
     Tag.normalise_tags!(tag_names, with_tagme: false, deny_deprecated: true,
-                                        with_sections: true) =>
+                                   with_sections: true) => { tags:, sections: }
-        { tags:, sections: }
 
     tags = Tag.expand_parent_tags(tags).reject(&:deprecated?)
     video_ms = normalise_video_ms(tags, video_ms_param:, duration_param:)
@@ -878,15 +831,12 @@ class PostsController < ApplicationController
                  original_created_from: snapshot[:original_created_from],
                  original_created_before: snapshot[:original_created_before])
 
-    Tag.normalise_tags!(snapshot[:tag_names], with_tagme: false,
+    Tag.normalise_tags!(snapshot[:tag_names],
-                                               deny_deprecated: true,
+                        with_tagme: false,
-                                               with_sections: true) =>
+                        deny_deprecated: true,
-        { tags: editable_tags, sections: }
+                        with_sections: true) => { tags:, sections: }
-    TagVersioning.record_tag_snapshots!(editable_tags, created_by_user: current_user)
+    TagVersioning.record_tag_snapshots!(tags, created_by_user: current_user)
 
-    readonly_tags = post.tags.nico.to_a
-
-    tags = readonly_tags + editable_tags
     tags = Tag.expand_parent_tags(tags).reject(&:deprecated?)
 
     post.video_ms = tags.any? { _1.id == Tag.video.id } ? snapshot[:video_ms] : nil

backend/app/controllers/tag_children_controller.rb

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

backend/app/controllers/tags_controller.rb

@@ -100,13 +100,14 @@ class TagsController < ApplicationController
 
   def autocomplete
     q = params[:q].to_s.strip.sub(/\Anot:/i, '')
+    prefix = "#{ ActiveRecord::Base.sanitize_sql_like(q) }%"
 
     with_nico = bool?(:nico, default: true)
     present_only = bool?(:present, default: true)
 
     alias_rows =
         TagName
-        .where('name LIKE ?', "#{ q }%")
+        .where('name LIKE ?', prefix)
         .where.not(canonical_id: nil)
         .pluck(:canonical_id, :name)
 
@@ -118,29 +119,57 @@ class TagsController < ApplicationController
       matched_alias_by_tag_name_id[canonical_id] ||= alias_name
     end
 
-    base = Tag.joins(:tag_name)
+    base =
-              .includes(:tag_name, :materials, tag_name: :wiki_page)
+        Tag
-              .where(deprecated_at: nil)
+        .joins(:tag_name)
+        .includes(:tag_name, :materials, tag_name: :wiki_page)
+        .where(deprecated_at: nil)
     base = base.where('tags.post_count > 0') if present_only
 
-    canonical_hit =
+    canonical_hit = base.where('tag_names.name LIKE ?', prefix)
-        base
-        .where(((with_nico ? '(tags.category = ? AND tag_names.name LIKE ?) OR ' : '') +
-                'tag_names.name LIKE ?'),
-               *(with_nico ? ['nico', "nico:#{ q }%"] : []), "#{ q }%")
 
-    tags =
+    internal_tags =
-        if canonical_ids.present?
+        if with_nico
           canonical_hit.or(base.where(tag_name_id: canonical_ids.uniq))
         else
           canonical_hit
         end
 
-    tags = tags.order(Arel.sql('post_count DESC, tag_names.name')).limit(20).to_a
+    internal_rows =
+        internal_tags
+        .order(Arel.sql('tags.post_count DESC, tag_names.name'))
+        .limit(20)
+        .map { |tag|
+          TagRepr.base(tag).merge(matched_alias: matched_alias_by_tag_name_id[tag.tag_name_id])
+        }
 
-    render json: tags.map { |tag|
+    return render json: internal_rows unless with_nico
-      TagRepr.base(tag).merge(matched_alias: matched_alias_by_tag_name_id[tag.tag_name_id])
+
-    }
+    external_base = ExternalTag.all
+    external_base = external_base.where('post_count > 0') if present_only
+
+    external_rows =
+        external_base
+        .where("CONCAT(platform, ':', name) LIKE ? OR name LIKE ?", prefix, prefix)
+        .order(post_count: :desc, name: :asc)
+        .limit(20)
+        .map { |tag|
+          { 'id' => tag.id,
+            'name' => "#{ tag.platform }:#{ tag.name }",
+            'category' => 'nico',
+            'deprecated_at' => nil,
+            'created_at' => tag.created_at,
+            'updated_at' => tag.created_at,
+            'post_count' => tag.post_count,
+            'matched_alias' => nil }
+        }
+
+    rows =
+        (internal_rows + external_rows)
+        .sort_by { |row| [-row['post_count'], row['name']] }
+        .first(20)
+
+    render json: rows
   end
 
   def show
@@ -200,14 +229,45 @@ class TagsController < ApplicationController
              .find_by(id: params[:id])
     return head :not_found unless tag
 
+    rows = normalise_deerjikist_rows(tag)
+    return if performed?
+
     ApplicationRecord.transaction do
-      tag.deerjikists = []
+      tag.lock!
-      params[:_json].each.with_index do |item, i|
+
-        platform = item[:platform]
+      requested_keys = rows.map { |row| [row[:platform], row[:code]] }.uniq
-        code = normalise_deerjikist_code(platform, item[:code])
+      row_indexes_by_key = rows_by_key(rows)
-        deerjikist = Deerjikist.find_or_initialize_by(platform:, code:)
+      locked_deerjikists = lock_deerjikists_for_tag_update(tag.id, requested_keys)
-        deerjikist.tag = tag
+      current_deerjikists = locked_deerjikists.filter { |deerjikist|
-        render_deerjikist_form_record_invalid(deerjikist, i) unless deerjikist.save
+        deerjikist.tag_id == tag.id
+      }
+      requested_deerjikists = locked_deerjikists.filter { |deerjikist|
+        row_indexes_by_key.key?([deerjikist.platform, deerjikist.code])
+      }
+
+      render_deerjikist_conflicts(requested_deerjikists, row_indexes_by_key, tag)
+      raise ActiveRecord::Rollback if performed?
+
+      requested_keys_set = requested_keys.to_set
+      current_deerjikists.each do |deerjikist|
+        key = [deerjikist.platform, deerjikist.code]
+        deerjikist.destroy! unless requested_keys_set.include?(key)
+      end
+
+      existing_keys = requested_deerjikists.to_h { |deerjikist|
+        [[deerjikist.platform, deerjikist.code], true]
+      }
+      requested_keys.each do |platform, code|
+        next if existing_keys[[platform, code]]
+
+        deerjikist = Deerjikist.new(platform:, code:, tag:)
+        row_index = row_indexes_by_key[[platform, code]].first
+        begin
+          render_deerjikist_form_record_invalid(deerjikist, row_index) unless deerjikist.save
+        rescue ActiveRecord::RecordNotUnique
+          conflicts = lock_deerjikists_for_tag_update(tag.id, [[platform, code]])
+          render_deerjikist_conflicts(conflicts, row_indexes_by_key, tag)
+        end
         raise ActiveRecord::Rollback if performed?
       end
     end
@@ -251,11 +311,7 @@ class TagsController < ApplicationController
     parent_names = params[:parent_tags].to_s.split.uniq
     deprecated = bool?(:deprecated)
 
-    if tag.nico? && deprecated
+    if category == 'nico'
-      return render_unprocessable_entity 'ニコタグは廃止できません．', field: :deprecated
-    end
-
-    if tag.nico? || category == 'nico'
       return render_unprocessable_entity 'ニコタグは変更できません．', field: :category
     end
 
@@ -303,13 +359,9 @@ class TagsController < ApplicationController
 
     tag = Tag.find(params[:id])
 
-    if tag.nico? && deprecated_given && deprecated
-      return render_unprocessable_entity 'ニコタグは廃止できません．', field: :deprecated
-    end
-
     return unless validate_tag_rename(tag, name)
 
-    if tag.nico? || (category.present? && category == 'nico')
+    if category.present? && category == 'nico'
       return render_unprocessable_entity 'ニコタグは変更できません．', field: :category
     end
 
@@ -516,11 +568,6 @@ class TagsController < ApplicationController
   end
 
   def record_tag_version! tag, event_type:, created_by_user:, name_changed: false, wiki_page: nil
-    if tag.nico?
-      NicoTagVersionRecorder.record!(tag:, event_type:, created_by_user:)
-      return
-    end
-
     TagVersionRecorder.record!(tag:, event_type:, created_by_user:)
 
     return unless name_changed
@@ -542,7 +589,7 @@ class TagsController < ApplicationController
       return false
     end
 
-    target_tag_name = TagName.with_discarded.find_by(name:)
+    target_tag_name = TagName.find_by(name:)
     return true if target_tag_name.nil?
     return true if target_tag_name.canonical_id?
 
@@ -554,17 +601,14 @@ class TagsController < ApplicationController
     return if name == tag.name
 
     current_tag_name = tag.tag_name
-    target_tag_name = TagName.with_discarded.find_by(name:)
+    target_tag_name = TagName.find_by(name:)
 
     if target_tag_name.nil?
       current_tag_name.update!(name:)
       return
     end
 
-    promote_tag_alias!(
+    promote_tag_alias!(tag, current_tag_name:, promoted_tag_name: target_tag_name)
-        tag,
-        current_tag_name:,
-        promoted_tag_name: target_tag_name)
   end
 
   def promote_tag_alias! tag, current_tag_name:, promoted_tag_name:
@@ -574,11 +618,9 @@ class TagsController < ApplicationController
       TagVersioning.ensure_snapshot!(old_owner_tag, created_by_user: current_user)
     end
 
-    promoted_tag_name.undiscard! if promoted_tag_name.discarded?
     promoted_tag_name.update!(canonical: nil)
 
-    TagName.with_discarded
+    TagName.where(canonical_id: current_tag_name.id)
-           .where(canonical_id: current_tag_name.id)
            .where.not(id: promoted_tag_name.id)
            .find_each do |alias_tag_name|
       alias_tag_name.update!(canonical: promoted_tag_name)
@@ -609,7 +651,7 @@ class TagsController < ApplicationController
     end
 
     alias_names.each do |alias_name|
-      alias_tag_name = TagName.find_undiscard_or_create_by!(name: alias_name)
+      alias_tag_name = TagName.find_or_create_by!(name: alias_name)
       affected_tags << alias_tag_name.canonical&.tag
     end
 
@@ -624,7 +666,7 @@ class TagsController < ApplicationController
     end
 
     alias_names.each do |alias_name|
-      alias_tag_name = TagName.find_undiscard_or_create_by!(name: alias_name)
+      alias_tag_name = TagName.find_or_create_by!(name: alias_name)
       alias_tag_name.update!(canonical: tag.tag_name)
     end
 
@@ -635,8 +677,7 @@ class TagsController < ApplicationController
 
   def update_parent_tags! tag, parent_names
     parent_tags = Tag.normalise_tags!(parent_names, with_tagme: false,
-                                                    with_no_deerjikist: false,
+                                                    with_no_deerjikist: false)
-                                                    deny_nico: true)
 
     old_parent_tags = tag.parents.to_a
 
@@ -653,6 +694,7 @@ class TagsController < ApplicationController
   end
 
   def normalise_deerjikist_code platform, code
+    code = code.to_s
     return code if platform != 'youtube' || code[0] != '@'
 
     url = "https://www.youtube.com/#{ code }"
@@ -669,6 +711,82 @@ class TagsController < ApplicationController
     nil
   end
 
+  def normalise_deerjikist_rows tag
+    rows = []
+
+    params[:_json].each.with_index do |item, index|
+      platform = item[:platform]
+      unless Deerjikist.platforms.key?(platform)
+        render_deerjikist_platform_invalid(index)
+        return rows
+      end
+
+      code = normalise_deerjikist_code(platform, item[:code])
+      deerjikist = Deerjikist.new(platform:, code:, tag:)
+      unless deerjikist.valid?
+        render_deerjikist_form_record_invalid(deerjikist, index)
+        return rows
+      end
+
+      rows << { index:, platform:, code: }
+    end
+
+    rows
+  end
+
+  def lock_deerjikists_for_tag_update tag_id, keys
+    clauses = ['tag_id = ?']
+    values = [tag_id]
+
+    keys.each do |platform, code|
+      clauses << '(platform = ? AND code = ?)'
+      values << platform << code
+    end
+
+    Deerjikist
+      .where(clauses.join(' OR '), *values)
+      .order(:platform, :code)
+      .lock
+      .to_a
+  end
+
+  def render_deerjikist_conflicts deerjikists, row_indexes_by_key, tag
+    conflicts = deerjikists.filter { |deerjikist| deerjikist.tag_id != tag.id }
+    return if conflicts.empty?
+
+    tag_names_by_id = Tag
+                      .joins(:tag_name)
+                      .where(id: conflicts.map(&:tag_id).uniq)
+                      .pluck('tags.id', 'tag_names.name')
+                      .to_h
+    fields = { }
+
+    conflicts.each do |deerjikist|
+      message = "この情報は既に「#{ tag_names_by_id[deerjikist.tag_id] }」に紐づいてゐます．"
+      row_indexes_by_key[[deerjikist.platform, deerjikist.code]].each do |index|
+        field = :"deerjikists.#{ index }.code"
+        fields[field] ||= []
+        fields[field] << message
+      end
+    end
+
+    render_validation_error fields:
+  end
+
+  def render_deerjikist_platform_invalid index
+    render_validation_error fields: {
+      :"deerjikists.#{ index }.platform" => ['値が不正です．'],
+    }
+  end
+
+  def rows_by_key rows
+    rows.each_with_object({ }) do |row, result|
+      key = [row[:platform], row[:code]]
+      result[key] ||= []
+      result[key] << row[:index]
+    end
+  end
+
   def render_deerjikist_form_record_invalid deerjikist, index
     fields = { }
 

backend/app/controllers/wiki_pages_controller.rb

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

backend/app/models/external_tag.rb

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

backend/app/models/nico_tag_relation.rb

@@ -1,6 +1,9 @@
 class NicoTagRelation < ApplicationRecord
-  belongs_to :nico_tag, class_name: 'Tag'
+  belongs_to :nico_tag,
-  belongs_to :tag, class_name: 'Tag'
+             class_name: 'ExternalTag',
+             foreign_key: :nico_tag_id,
+             inverse_of: :nico_tag_relations
+  belongs_to :tag, class_name: 'Tag', foreign_key: :tag_id
 
   validates :nico_tag_id, presence: true
   validates :tag_id, presence: true

backend/app/models/nico_tag_version.rb

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

backend/app/models/post.rb

@@ -55,25 +55,27 @@ class Post < ApplicationRecord
   belongs_to :uploaded_user, class_name: 'User', optional: true
 
   has_many :post_tags, dependent: :destroy, inverse_of: :post
-  has_many :active_post_tags, -> { kept }, class_name: 'PostTag', inverse_of: :post
+  has_many :tags, through: :post_tags
-  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: :active_post_tags, source: :tag
+           through: :post_tags,
+           source: :tag
 
   has_many :user_post_views, dependent: :delete_all
   has_many :post_similarities, dependent: :delete_all
   has_many :post_versions
+
   has_many :gekanator_guessed_games,
            class_name: 'GekanatorGame',
            foreign_key: :guessed_post_id,
            dependent: :delete_all,
            inverse_of: :guessed_post
+
   has_many :gekanator_correct_games,
            class_name: 'GekanatorGame',
            foreign_key: :correct_post_id,
            dependent: :delete_all,
            inverse_of: :correct_post
+
   has_many :gekanator_question_examples, dependent: :delete_all
 
   has_many :parent_post_implications,
@@ -90,6 +92,9 @@ class Post < ApplicationRecord
            inverse_of: :parent_post
   has_many :children, through: :child_post_implications, source: :post
 
+  has_many :post_external_tags, dependent: :delete_all
+  has_many :external_tags, through: :post_external_tags
+
   has_one_attached :thumbnail
 
   attribute :version_no, :integer, default: 1
@@ -123,7 +128,6 @@ class Post < ApplicationRecord
 
   def snapshot_tag_names
     post_tags
-        .kept
         .joins(tag: :tag_name)
         .includes(:sections, tag: :tag_name)
         .order('tag_names.name')
@@ -137,6 +141,41 @@ class Post < ApplicationRecord
     end
   end
 
+  def self.tag_snapshot_literal tag
+    sections = tag.fetch('sections', []).map do |sec|
+      begin_ms = sec.fetch('begin_ms')
+      end_ms = sec['end_ms']
+
+      "[#{ Post.ms_to_time(begin_ms) }-#{ end_ms ? Post.ms_to_time(end_ms) : '' }]"
+    end
+
+    "#{ tag.fetch('name') }#{ sections.join }"
+  end
+
+  def snapshot_tags_json
+    tag_snapshots =
+        post_tags
+            .joins(tag: :tag_name)
+            .includes(:sections, tag: :tag_name)
+            .order('tags.id')
+            .map { |pt|
+          { 'tag_id' => pt.tag.id,
+            'version_no' => pt.tag.version_no,
+            'name' => pt.tag.name,
+            'category' => pt.tag.category,
+            'sections' => pt.sections.sort_by(&:begin_ms).map {
+                            { 'begin_ms' => _1.begin_ms, 'end_ms' => _1.end_ms }
+                          } }
+        }
+
+    external_tag_snapshots =
+        post_external_tags.order(:external_tag_id).map {
+          { 'external_tag_id' => _1.external_tag_id }
+        }
+
+    tag_snapshots + external_tag_snapshots
+  end
+
   def self.section_literal section
     end_ms =
         section.end_ms ? Post.ms_to_time(section.end_ms) : ''

backend/app/models/post_external_tag.rb

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

backend/app/models/post_tag.rb

@@ -1,14 +1,7 @@
 class PostTag < ApplicationRecord
-  include Discard::Model
-
-  before_destroy do
-    raise ActiveRecord::ReadOnlyRecord, '消さないでください．'
-  end
-
   belongs_to :post
   belongs_to :tag, counter_cache: :post_count
   belongs_to :created_user, class_name: 'User', optional: true
-  belongs_to :deleted_user, class_name: 'User', optional: true
 
   has_many :sections, -> { order(:begin_ms) }, class_name: 'PostTagSection',
                                                foreign_key: [:post_id, :tag_id],
@@ -18,18 +11,5 @@ class PostTag < ApplicationRecord
 
   validates :post_id, presence: true
   validates :tag_id, presence: true
-  validates :post_id, uniqueness: {
+  validates :post_id, uniqueness: { scope: :tag_id }
-      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, -> { kept }, foreign_key: [:post_id, :tag_id],
+  belongs_to :post_tag, foreign_key: [:post_id, :tag_id],
-                                     primary_key: [:post_id, :tag_id],
+                        primary_key: [:post_id, :tag_id],
-                                     inverse_of: :sections,
+                        inverse_of: :sections,
-                                     optional: true
+                        optional: true
 
   validates :post_id, presence: true
   validates :tag_id, presence: true

backend/app/models/tag.rb

@@ -2,8 +2,6 @@ require 'set'
 
 
 class Tag < ApplicationRecord
-  include MyDiscard
-
   class NicoTagNormalisationError < ArgumentError
     ;
   end
@@ -28,15 +26,12 @@ class Tag < ApplicationRecord
   end
 
   has_many :post_tags, inverse_of: :tag
-  has_many :active_post_tags, -> { kept }, class_name: 'PostTag', inverse_of: :tag
+  has_many :posts, through: :post_tags
-  has_many :post_tags_with_discarded, -> { with_discarded }, class_name: 'PostTag'
-  has_many :posts, through: :active_post_tags
-
-  has_many :nico_tag_relations, foreign_key: :nico_tag_id, dependent: :destroy
-  has_many :linked_tags, through: :nico_tag_relations, source: :tag
 
   has_many :reversed_nico_tag_relations,
-           class_name: 'NicoTagRelation', foreign_key: :tag_id, dependent: :destroy
+           class_name: 'NicoTagRelation',
+           foreign_key: :nico_tag_id,
+           dependent: :destroy
   has_many :linked_nico_tags, through: :reversed_nico_tag_relations, source: :nico_tag
 
   has_many :tag_implications, foreign_key: :parent_tag_id, dependent: :destroy
@@ -54,7 +49,6 @@ class Tag < ApplicationRecord
   has_many :materials
 
   has_many :tag_versions
-  has_many :nico_tag_versions
 
   belongs_to :tag_name
   delegate :wiki_page, to: :tag_name
@@ -69,17 +63,15 @@ class Tag < ApplicationRecord
                   character:   'character',
                   general:     'general',
                   material:    'material',
-                  nico:        'nico',
                   meta:        'meta'
 
   validates :category, presence: true, inclusion: { in: Tag.categories.keys }
 
-  validate :nico_tag_name_must_start_with_nico
+  validate :tag_name_mustnt_start_with_nico
   validate :tag_name_must_be_canonical
   validate :category_must_be_deerjikist_with_deerjikists
-  validate :nico_tags_cannot_be_deprecated
 
-  scope :nico_tags, -> { nico }
+  def self.nico_tags = ExternalTag.where(platform: :nico)
 
   CATEGORY_PREFIXES = {
       'general:'    => :general,
@@ -114,10 +106,9 @@ class Tag < ApplicationRecord
 
   def self.normalise_tags! tag_names, with_tagme: true,
                                       with_no_deerjikist: true,
-                                      deny_nico: true,
                                       deny_deprecated: false,
                                       with_sections: false
-    if deny_nico && tag_names.any? { |n| n.downcase.start_with?('nico:') }
+    if tag_names.any? { |n| n.downcase.start_with?('nico:') }
       raise NicoTagNormalisationError
     end
 
@@ -234,10 +225,10 @@ class Tag < ApplicationRecord
   end
 
   def self.find_or_create_by_tag_name! name, category:
-    tn = TagName.find_undiscard_or_create_by!(name: name.to_s.strip)
+    tn = TagName.find_or_create_by!(name: name.to_s.strip)
     tn = tn.canonical if tn.canonical_id?
 
-    Tag.find_undiscard_or_create_by!(tag_name_id: tn.id) do |t|
+    Tag.find_or_create_by!(tag_name_id: tn.id) do |t|
       t.category = category
     end
   rescue ActiveRecord::RecordNotUnique
@@ -259,11 +250,11 @@ class Tag < ApplicationRecord
 
         TagVersioning.ensure_snapshot!(source_tag, created_by_user:)
 
-        source_tag.post_tags.kept.find_each do |source_pt|
+        source_tag.post_tags.find_each do |source_pt|
           post_id = source_pt.post_id
           affected_post_ids << post_id
-          source_pt.discard_by!(created_by_user)
+          source_pt.destroy!
-          unless PostTag.kept.exists?(post_id:, tag: target_tag)
+          unless PostTag.exists?(post_id:, tag: target_tag)
             PostTag.create!(post_id:, tag: target_tag)
           end
         end
@@ -275,14 +266,10 @@ class Tag < ApplicationRecord
         end
 
         TagVersioning.record!(source_tag, event_type: :discard, created_by_user:)
-        source_tag.discard!
+        source_tag.destroy!
 
-        if source_tag.nico?
+        source_tag_name.update_columns(canonical_id: target_tag.tag_name_id,
-          source_tag_name.discard!
+                                       updated_at: Time.current)
-        else
-          source_tag_name.update_columns(canonical_id: target_tag.tag_name_id,
-                                         updated_at: Time.current)
-        end
 
         TagVersioning.record!(target_tag, event_type: :update, created_by_user:)
       end
@@ -293,13 +280,13 @@ class Tag < ApplicationRecord
       end
 
       # 投稿件数を再集計
-      target_tag.update_columns(post_count: PostTag.kept.where(tag: target_tag).count)
+      target_tag.update_columns(post_count: PostTag.where(tag: target_tag).count)
     end
 
     target_tag.reload
   end
 
-  def snapshot_aliases = tag_name.aliases.kept.order(:name).pluck(:name)
+  def snapshot_aliases = tag_name.aliases.order(:name).pluck(:name)
 
   def snapshot_parent_tag_ids = parents.order(:id).pluck(:id)
 
@@ -309,11 +296,9 @@ class Tag < ApplicationRecord
 
   private
 
-  def nico_tag_name_must_start_with_nico
+  def tag_name_mustnt_start_with_nico
-    n = name.to_s
+    if name.to_s.downcase.start_with?('nico:')
-    if ((nico? && !(n.downcase.start_with?('nico:'))) || 
+      errors.add :name, 'タグの命名規則に反してゐます．'
-        (!(nico?) && n.downcase.start_with?('nico:')))
-      errors.add :name, 'ニコニコ・タグの命名規則に反してゐます．'
     end
   end
 
@@ -354,10 +339,4 @@ class Tag < ApplicationRecord
 
     total_s * 1_000 + match[:ms].to_s.ljust(3, '0')[0, 3].to_i
   end
-
-  def nico_tags_cannot_be_deprecated
-    if nico? && deprecated_at.present?
-      errors.add :deprecated_at, 'ニコタグは廃止できません．'
-    end
-  end
 end

backend/app/models/tag_name.rb

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

backend/app/models/tag_name_sanitisation_rule.rb

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

backend/app/representations/post_repr.rb

@@ -88,14 +88,14 @@ module PostRepr
 
   def tag_json post
     post
-        .active_post_tags
+        .post_tags
         .reject { _1.tag.deprecated? }
         .sort_by { _1.tag.name }
-        .map { |post_tag|
+        .map do |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/nico_tag_version_recorder.rb

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

backend/app/services/post_create_plan.rb

@@ -20,23 +20,22 @@ class PostCreatePlan
     parent_post_ids = normalise_parent_post_ids
     validate_parent_post_ids!(parent_post_ids)
 
-    {
+    { url: @attributes[:url],
-        url: @attributes[:url],
+      title: @attributes[:title].to_s,
-        title: @attributes[:title].to_s,
+      thumbnail_base: @attributes[:thumbnail_base].presence,
-        thumbnail_base: @attributes[:thumbnail_base].presence,
+      original_created_from: @attributes[:original_created_from].presence,
-        original_created_from: @attributes[:original_created_from].presence,
+      original_created_before: @attributes[:original_created_before].presence,
-        original_created_before: @attributes[:original_created_before].presence,
+      tags: serialised_tags(direct_tag_specs, tag_sections),
-        tags: serialised_tags(direct_tag_specs, tag_sections),
+      display_tags: display_tags(direct_tag_specs, tag_sections),
-        display_tags: display_tags(direct_tag_specs, tag_sections),
+      duration: @attributes[:duration].to_s,
-        duration: @attributes[:duration].to_s,
+      video_ms: video_ms,
-        video_ms: video_ms,
+      parent_post_ids: parent_post_ids.join(' '),
-        parent_post_ids: parent_post_ids.join(' '),
+      direct_tag_specs: direct_tag_specs,
-        direct_tag_specs: direct_tag_specs,
+      default_tag_specs: default_tag_specs,
-        default_tag_specs: default_tag_specs,
+      snapshot_tag_specs: snapshot_tag_specs,
-        snapshot_tag_specs: snapshot_tag_specs,
+      post_tag_specs: post_tag_specs,
-        post_tag_specs: post_tag_specs,
+      tag_sections: tag_sections,
-        tag_sections: tag_sections,
+      normalised_parent_post_ids: parent_post_ids }
-        normalised_parent_post_ids: parent_post_ids }
   end
 
   private
@@ -50,7 +49,6 @@ class PostCreatePlan
     tag_names.each do |raw_name|
       tag_name, category, sections = parse_raw_tag_name(raw_name)
       existing_tag = existing_tags_by_name[tag_name]
-      raise Tag::NicoTagNormalisationError if existing_tag&.nico?
       raise Tag::DeprecatedTagNormalisationError, [existing_tag.name] if existing_tag&.deprecated?
 
       direct_tag_specs << {
@@ -139,14 +137,14 @@ class PostCreatePlan
         Tag.expand_parent_tags(existing_snapshot_tags)
            .reject(&:deprecated?)
            .map { |tag|
-             {
+             { name: tag.name,
-                 name: tag.name,
+               category: tag.category.to_sym }
-                 category: tag.category.to_sym } }
+        }
     merge_tag_specs(snapshot_tag_specs + expanded_parent_specs)
   end
 
   def merge_tag_specs specs
-    specs.each_with_object({ }) do |spec, merged|
+    specs.each_with_object({ }) { |spec, merged|
       merged[spec[:name]] =
           if merged.key?(spec[:name]) && merged[spec[:name]][:category] != :general
             merged[spec[:name]]
@@ -155,7 +153,7 @@ class PostCreatePlan
                 name: spec[:name],
                 category: spec[:category] }
           end
-    end.values.sort_by { _1[:name] }
+    }.values.sort_by { _1[:name] }
   end
 
   def existing_tags_by_name

backend/app/services/post_creator.rb

@@ -121,9 +121,11 @@ class PostCreator
   def sync_post_tags! post, desired_tags, sections
     desired_ids = desired_tags.map(&:id).to_set
     current_ids = post.tags.pluck(:id).to_set
+
     Tag.where(id: desired_ids - current_ids).find_each do |tag|
       PostTag.create_or_find_by!(post:, tag:, created_user: @actor)
     end
+
     PostTagSection.where(post_id: post.id).destroy_all
     sections.each do |tag_id, ranges|
       ranges.each do |begin_ms, end_ms|
@@ -133,10 +135,9 @@ class PostCreator
                                end_ms:)
       end
     end
+
     PostTag.where(post_id: post.id,
-                  tag_id: (current_ids - desired_ids).to_a).kept.find_each do |post_tag|
+                  tag_id: (current_ids - desired_ids).to_a).destroy_all
-      post_tag.discard_by!(@actor)
-    end
   end
 
   def sync_parent_posts! post, ids

backend/app/services/post_version_recorder.rb

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

backend/app/services/tag_versioning.rb

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

backend/app/services/version_recorder.rb

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

backend/app/services/youtube/sync.rb

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

backend/config/routes.rb

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

backend/db/migrate/20260727123600_add_tags_json_to_post_versions.rb

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

backend/db/migrate/20260921000000_delete_inactive_records_from_post_tags.rb

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

backend/db/migrate/20260921010000_add_foreign_key_on_post_id_and_tag_id_in_post_tag_sections.rb

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

backend/db/migrate/20260921020000_delete_discarded_records_from_tags.rb

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

backend/db/migrate/20260921030000_delete_discarded_records_from_tag_names.rb

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

backend/db/migrate/20260921040000_create_external_tags.rb

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

backend/db/migrate/20260921050000_create_post_external_tags.rb

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

backend/db/migrate/20260921060000_change_foreign_key_on_nico_tag_relations.rb

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

backend/db/migrate/20260921230000_migrate_external_tags.rb

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

backend/db/schema.rb

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

backend/lib/tasks/sync_nico.rake

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

backend/spec/db/delete_discarded_tags_spec.rb

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

backend/spec/models/post_tag_spec.rb

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

backend/spec/models/post_version_spec.rb

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

backend/spec/models/tag_name_sanitisation_rule_spec.rb

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

backend/spec/models/tag_spec.rb

@@ -173,6 +173,47 @@ 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) }
@@ -185,18 +226,14 @@ RSpec.describe Tag, type: :model do
     context 'when merging a simple source tag' do
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
 
-      it 'discards the source post_tag, creates an active target post_tag, discards the source tag, and aliases the source tag_name' do
+      it 'deletes the source tag, moves its post link, and keeps its name as an alias' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        source_pt = PostTag.with_discarded.find(source_post_tag.id)
+        target_link = PostTag.find_by(post: post_record, tag: target_tag)
-        active_target = PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)
 
-        expect(source_pt.discarded_at).to be_present
+        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
-        expect(source_pt.tag_id).to eq(source_tag.id)
+        expect(target_link).to be_present
-        expect(active_target).to be_present
+        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
@@ -206,38 +243,101 @@ RSpec.describe Tag, type: :model do
       let!(:target_post_tag) { PostTag.create!(post: post_record, tag: target_tag) }
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
 
-      it 'discards the source post_tag, keeps one active target post_tag, discards the source tag, and aliases the source tag_name' do
+      it 'deletes the source link and preserves the existing target link' do
+        create(:post_tag_section, post: post_record, tag: source_tag,
+                                  begin_ms: 1000, end_ms: 2000)
+        target_section = create(:post_tag_section, post: post_record,
+                                                   tag: target_tag,
+                                                   begin_ms: 3000, end_ms: nil)
+
         described_class.merge_tags!(target_tag, [source_tag])
 
-        source_pt = PostTag.with_discarded.find(source_post_tag.id)
+        target_links = PostTag.where(post: post_record, tag: target_tag)
-        active = PostTag.kept.where(post_id: post_record.id, tag_id: target_tag.id)
 
-        expect(source_pt.discarded_at).to be_present
+        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
-        expect(source_pt.tag_id).to eq(source_tag.id)
+        expect(target_links).to contain_exactly(target_post_tag)
-        expect(active.count).to eq(1)
+        expect(PostTagSection.where(post: post_record, tag: source_tag)).to be_empty
-        expect(active.first.id).to eq(target_post_tag.id)
+        expect(target_post_tag.reload.sections).to contain_exactly(target_section)
 
-        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
+        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
-        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])
 
-        source_pt = PostTag.with_discarded.find(source_post_tag.id)
+        target_link = PostTag.find_by(post: post_record, tag: target_tag)
-        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.with_discarded.find(source_tag.id)).to be_discarded
+        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
-        expect(source_pt.discarded_at).to be_present
+        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
-        expect(source_pt.tag_id).to eq(source_tag.id)
+        expect(target_link).to be_present
-        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
@@ -260,18 +360,16 @@ RSpec.describe Tag, type: :model do
         )
       end
 
-      it 'still merges, but discards the source tag_name instead of aliasing it' do
+      it 'still merges and keeps the source name as an alias without validating it' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        source_pt = PostTag.with_discarded.find(source_post_tag.id)
+        target_link = PostTag.find_by(post: post_record, tag: target_tag)
-        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(source_pt.discarded_at).to be_present
+        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
-        expect(source_pt.tag_id).to eq(source_tag.id)
+        expect(target_link).to be_present
-        expect(active_target).to be_present
 
-        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
+        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
+        expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
         expect(target_tag.reload.post_count).to eq(1)
       end
     end
@@ -288,36 +386,72 @@ RSpec.describe Tag, type: :model do
             message: 'init')
       end
 
-      it 'rolls back the transaction' do
+      it 'rolls back earlier deletions, links, and history when a later source has a wiki' do
+        earlier_source = create(:tag)
+        earlier_name = earlier_source.tag_name
+        source_section = create(:post_tag_section, post: post_record,
+                                                   tag: source_tag,
+                                                   begin_ms: 1000, end_ms: 2000)
+
         expect {
-          described_class.merge_tags!(target_tag, [source_tag])
+          described_class.merge_tags!(target_tag, [earlier_source, source_tag])
         }.to raise_error(ActiveRecord::RecordInvalid)
 
-        expect(Tag.with_discarded.find(source_tag.id)).not_to be_discarded
+        expect(Tag.unscoped.exists?(earlier_source.id)).to be(true)
-        expect(TagName.with_discarded.find(source_tag_name.id)).not_to be_discarded
+        expect(earlier_name.reload.canonical_id).to be_nil
-        expect(PostTag.kept.find(source_post_tag.id).tag_id).to eq(source_tag.id)
+        expect(TagVersion.where(tag_id: [earlier_source.id, source_tag.id, target_tag.id]))
-        expect(PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)).to be_nil
+            .to be_empty
+        expect(Tag.unscoped.exists?(source_tag.id)).to be(true)
+        expect(TagName.unscoped.exists?(source_tag_name.id)).to be(true)
+        expect(source_post_tag.reload.tag_id).to eq(source_tag.id)
+        expect(source_post_tag.sections).to contain_exactly(source_section)
+        expect(PostTag.find_by(post: post_record, tag: target_tag)).to be_nil
+        expect(source_tag.reload.post_count).to eq(1)
         expect(source_tag_name.reload.canonical_id).to be_nil
         expect(target_tag.reload.post_count).to eq(0)
       end
     end
 
     context 'when merging a nico source tag' do
-      let!(:target_tag) { create(:tag, category: :nico, name: 'nico:foo') }
+      let!(:target_tag) do
-      let!(:source_tag) { create(:tag, category: :nico, name: 'nico:bar') }
+        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!(:source_tag_name_id) { source_tag.tag_name_id }
 
-      it 'discards the source tag_name instead of aliasing it' do
+      it 'deletes the source tag and name instead of keeping an alias' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        discarded_source_tag = Tag.with_discarded.find(source_tag.id)
+        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
-        discarded_source_tag_name = TagName.with_discarded.find(source_tag_name_id)
+        expect(TagName.unscoped.exists?(source_tag_name_id)).to be(false)
-
-        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)
@@ -333,6 +467,7 @@ RSpec.describe Tag, type: :model do
         url: post.url,
         thumbnail_base: post.thumbnail_base,
         tags: snapshot_tags(post),
+        tags_json: post.snapshot_tags_json,
         parent_post_ids: post.snapshot_parent_post_ids.join(' '),
         original_created_from: post.original_created_from,
         original_created_before: post.original_created_before,
@@ -364,12 +499,15 @@ RSpec.describe Tag, type: :model do
         expect(latest.event_type).to eq('update')
         expect(latest.created_by_user).to be_nil
         expect(latest.tags).to eq(snapshot_tags(post_record.reload))
+        expect(latest.tags_json.map { |item| item.fetch('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 active post_tags' do
+    context 'when the source tag has no post_tags' do
       let!(:another_post) do
         Post.create!(url: 'https://example.com/posts/3', title: 'another post')
       end

backend/spec/requests/posts_spec.rb

@@ -55,6 +55,7 @@ RSpec.describe 'Posts API', type: :request do
             thumbnail_base: post.thumbnail_base,
             video_ms: post.video_ms,
             tags: post.snapshot_tag_names.join(' '),
+            tags_json: post.snapshot_tags_json,
             parent_post_ids: post.snapshot_parent_post_ids.join(' '),
             original_created_from: post.original_created_from,
             original_created_before: post.original_created_before,
@@ -147,12 +148,15 @@ RSpec.describe 'Posts API', type: :request do
 
     it 'keeps children and sections keys in non-detail tag responses' do
       PostTagSection.create!(post: hit_post, tag:, begin_ms: 1_000, end_ms: nil)
+      deprecated_tag = create(:tag, deprecated_at: Time.current)
+      create(:post_tag, post: hit_post, tag: deprecated_tag)
 
       get '/posts'
 
       expect(response).to have_http_status(:ok)
 
       hit_json = json.fetch('posts').find { |post| post['id'] == hit_post.id }
+      expect(hit_json.fetch('tags').map { |item| item.fetch('id') }).to eq([tag.id])
       tag_json = hit_json.fetch('tags').find { |item| item['name'] == 'spec_tag' }
 
       expect(tag_json.fetch('children')).to eq([])
@@ -161,6 +165,26 @@ RSpec.describe 'Posts API', type: :request do
       ])
     end
 
+    it 'preloads tag details and sections as the number of posts grows' do
+      5.times do
+        link = create(:post_tag, post: create(:post, uploaded_user: user))
+        create(:post_tag_section, post: link.post, tag: link.tag,
+                                  begin_ms: 1000, end_ms: 2000)
+      end
+      get '/posts', params: { limit: 1 }
+
+      one_post_queries = count_sql_queries do
+        get '/posts', params: { limit: 1 }
+      end
+      many_post_queries = count_sql_queries do
+        get '/posts', params: { limit: 20 }
+      end
+
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('posts').size).to eq(8)
+      expect(many_post_queries).to be <= one_post_queries
+    end
+
     context 'when q is provided' do
       it 'filters posts by q (hit case)' do
         get '/posts', params: { tags: 'spec_tag' }
@@ -459,6 +483,71 @@ RSpec.describe 'Posts API', type: :request do
       end
     end
 
+    context 'when update times include version history' do
+      let(:t0) { Time.zone.parse('2020-01-01 12:00:00') }
+      let(:t1) { t0 + 1.day }
+      let(:t2) { t0 + 2.days }
+      let(:t3) { t0 + 3.days }
+      let!(:history_post) do
+        create(:post, url: 'https://example.com/version-time/history',
+                      created_at: t0, updated_at: t0)
+      end
+      let!(:plain_post) do
+        create(:post, url: 'https://example.com/version-time/plain',
+                      created_at: t1, updated_at: t1)
+      end
+      let!(:newer_post) do
+        create(:post, url: 'https://example.com/version-time/newer',
+                      created_at: t0, updated_at: t3)
+      end
+
+      before do
+        link = create(:post_tag, post: history_post, tag:)
+        travel_to(t0) do
+          PostVersionRecorder.record!(post: history_post,
+                                      event_type: :create, created_by_user: nil)
+          PostVersionRecorder.record!(post: newer_post,
+                                      event_type: :create, created_by_user: nil)
+        end
+        travel_to(t2) do
+          link.destroy!
+          PostVersionRecorder.record!(post: history_post,
+                                      event_type: :update, created_by_user: nil)
+        end
+        create(:post_tag, post: plain_post, tag:, created_at: t3)
+      end
+
+      ['asc', 'desc'].each do |direction|
+        it "sorts by the later of post update and latest version time (#{ direction })" do
+          get '/posts', params: { url: '/version-time/', order: "updated_at:#{ direction }" }
+
+          expect(response).to have_http_status(:ok)
+          expected_ids = [plain_post.id, history_post.id, newer_post.id]
+          expected_ids.reverse! if direction == 'desc'
+          expect(json.fetch('posts').map { |item| item.fetch('id') }).to eq(expected_ids)
+          expect(json.fetch('count')).to eq(3)
+
+          times = json.fetch('posts').to_h do |item|
+            [item.fetch('id'), Time.zone.parse(item.fetch('updated_at'))]
+          end
+          expect(times).to eq({ plain_post.id => t1,
+                               history_post.id => t2,
+                               newer_post.id => t3 })
+          expect(history_post.reload.updated_at).to eq(t0)
+        end
+      end
+
+      it 'filters inclusively by the latest version time after a tag is deleted' do
+        get '/posts', params: { url: '/version-time/',
+                               updated_from: t2.iso8601,
+                               updated_to: t2.iso8601 }
+
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch('posts').map { |item| item.fetch('id') }).to eq([history_post.id])
+        expect(json.fetch('count')).to eq(1)
+      end
+    end
+
     context 'when original_created_from/original_created_to are provided' do
       # 注意: controller の現状ロジックに合わせてる
       # original_created_from は `original_created_before > ?`
@@ -1163,9 +1252,7 @@ RSpec.describe 'Posts API', type: :request do
 
     context 'when nico tag already exists in tags' do
       before do
-        Tag.find_undiscard_or_create_by!(
+        Tag.find_or_create_by_tag_name!('nico:nico_tag', category: :nico)
-            tag_name: TagName.find_undiscard_or_create_by!(name: 'nico:nico_tag'),
-            category: :nico)
       end
 
       it 'returns 422 with tag field errors' do
@@ -1418,9 +1505,11 @@ RSpec.describe 'Posts API', type: :request do
 
     it '200 and updates title + resync tags when member' do
       sign_in_as(member)
+      create(:post_tag_section, post: post_record, tag:,
+                                begin_ms: 1000, end_ms: 2000)
 
       tn2 = TagName.create!(name: 'spec_tag_2')
-      Tag.create!(tag_name: tn2, category: :general)
+      replacement_tag = Tag.create!(tag_name: tn2, category: :general)
 
       put "/posts/#{post_record.id}", params: post_update_params(
           post_record,
@@ -1433,6 +1522,38 @@ RSpec.describe 'Posts API', type: :request do
 
       names = json['tags'].map { |n| n['name'] }
       expect(names).to include('spec_tag_2')
+      expect(names).not_to include('spec_tag')
+      expect(PostTag.exists?(post: post_record, tag:)).to be(false)
+      expect(PostTagSection.exists?(post: post_record, tag:)).to be(false)
+      expect(tag.reload.post_count).to eq(0)
+      expect(replacement_tag.reload.post_count).to eq(1)
+
+      versions = post_record.post_versions.order(:version_no)
+      expect(versions.first.tags_json).to include(
+          a_hash_including('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
@@ -1457,9 +1578,7 @@ RSpec.describe 'Posts API', type: :request do
 
     context 'when nico tag already exists in tags' do
       before do
-        Tag.find_undiscard_or_create_by!(
+        Tag.find_or_create_by_tag_name!('nico:nico_tag', category: :nico)
-            tag_name: TagName.find_undiscard_or_create_by!(name: 'nico:nico_tag'),
-            category: :nico)
       end
 
       it 'returns 422 with tag field errors' do
@@ -1878,123 +1997,29 @@ RSpec.describe 'Posts API', type: :request do
       expect(response).to have_http_status(:not_found)
     end
 
-    it '200 and returns viewed boolean' do
+    it 'returns viewed state and current tags with their sections' do
+      create(:post_tag_section, post: post_record, tag:,
+                                begin_ms: 1000, end_ms: nil)
+      deprecated_tag = create(:tag, deprecated_at: Time.current)
+      create(:post_tag, post: post_record, tag: deprecated_tag)
+
       get '/posts/random'
+
       expect(response).to have_http_status(:ok)
       expect(json).to have_key('viewed')
       expect([true, false]).to include(json['viewed'])
+      expect(json.fetch('tags')).to contain_exactly(
+          a_hash_including('id' => tag.id,
+                           'children' => [],
+                           'sections' => [{ 'begin_ms' => 1000, 'end_ms' => nil }]))
     end
   end
 
   describe 'GET /posts/changes' do
-    let(:member) { create(:user, :member) }
+    it 'returns 404 for the retired history endpoint' do
+      get '/posts/changes'
 
-    it 'returns add/remove events (history) for a post' do
+      expect(response).to have_http_status(:not_found)
-      # 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
 
@@ -2024,6 +2049,7 @@ RSpec.describe 'Posts API', type: :request do
         url: post.url,
         thumbnail_base: post.thumbnail_base,
         tags: snapshot_tags(post),
+        tags_json: post.snapshot_tags_json,
         parent_post_ids: post.snapshot_parent_post_ids.join(' '),
         original_created_from: post.original_created_from,
         original_created_before: post.original_created_before,
@@ -2045,7 +2071,7 @@ RSpec.describe 'Posts API', type: :request do
     end
 
     let!(:v2) do
-      post_record.post_tags.kept.find_by!(tag: tag).discard_by!(member)
+      post_record.post_tags.find_by!(tag: tag).destroy!
       PostTag.create!(post: post_record, tag: tag2, created_user: member)
       post_record.update!(
         title: 'updated spec post',

backend/spec/requests/tags_deerjikists_spec.rb

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

backend/spec/services/youtube/sync_spec.rb

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

backend/spec/tasks/nico_sync_spec.rb

@@ -8,8 +8,7 @@ RSpec.describe 'nico:sync' do
   end
 
   def create_tag!(name, category:)
-    tn = TagName.find_undiscard_or_create_by!(name: name.to_s.strip)
+    Tag.find_or_create_by_tag_name!(name, category:)
-    Tag.find_undiscard_or_create_by!(tag_name_id: tn.id) { |t| t.category = category }
   end
 
   def link_nico_to_tag!(nico_tag, tag)
@@ -108,7 +107,7 @@ RSpec.describe 'nico:sync' do
     expect(calls).to eq(2)
   end
 
-  it '既存 post にあった古い nico tag は active から外され、履歴として discard される' do
+  it '古い nico tag の関連を物理削除し、変更前後の履歴を version に残す' do
     post = Post.create!(
       title: 'old',
       url: 'https://www.nicovideo.jp/watch/sm9',
@@ -117,8 +116,8 @@ 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')
-    old_pt = PostTag.create!(post: post, tag: old_nico)
+    PostTag.create!(post:, tag: old_nico)
-    expect(old_pt.discarded_at).to be_nil
+    create_post_version_for!(post)
 
     # 今回は NEW のみ欲しい
     new_nico = create_tag!('nico:NEW', category: 'nico')
@@ -132,9 +131,17 @@ RSpec.describe 'nico:sync' do
 
     run_rake_task('nico:sync')
 
-    # OLD は active から外れる（discarded_at が入る）
+    expect(PostTag.exists?(post:, tag: old_nico)).to be(false)
-    old_pts = PostTag.where(post_id: post.id, tag_id: old_nico.id).order(:id).to_a
+    expect(old_nico.reload.post_count).to eq(0)
-    expect(old_pts.last.discarded_at).to be_present
+    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)
 
     # NEW は active にいる
     post.reload
@@ -156,6 +163,7 @@ RSpec.describe 'nico:sync' do
       url: post.url,
       thumbnail_base: post.thumbnail_base,
       tags: snapshot_tags(post),
+      tags_json: post.snapshot_tags_json,
       parent_post_ids: post.snapshot_parent_post_ids.join(' '),
       original_created_from: post.original_created_from,
       original_created_before: post.original_created_before,
@@ -370,4 +378,119 @@ RSpec.describe 'nico:sync' do
     expect(versions.second.title).to eq('changed title')
     expect(versions.second.tags).to eq(snapshot_tags(post.reload))
   end
+
+  def create_external_tag!(name)
+    ExternalTag.create!(platform: :nico, name:)
+  end
+
+  def create_nico_sync_post!
+    post = Post.create!(
+      title: 't',
+      url: 'https://www.nicovideo.jp/watch/sm9',
+      uploaded_user: nil
+    )
+
+    PostTag.create!(post:, tag: Tag.no_deerjikist)
+
+    post
+  end
+
+  def run_nico_sync_with_tags! tags
+    stub_python([{
+      'code' => 'sm9',
+      'title' => 't',
+      'tags' => tags,
+      'user' => nil
+    }])
+
+    allow(URI).to receive(:open).and_return(StringIO.new('<html></html>'))
+
+    run_rake_task('nico:sync')
+  end
+
+  it '外部タグが新規記載されたとき，その時点の連携タグを記載する' do
+    post = create_nico_sync_post!
+
+    external_tag = create_external_tag!('AAA')
+    linked_tag = create_tag!('spec_linked', category: :general)
+    link_nico_to_tag!(external_tag, linked_tag)
+
+    run_nico_sync_with_tags!(['AAA'])
+
+    expect(post.reload.external_tags).to include(external_tag)
+    expect(post.tags).to include(linked_tag)
+  end
+
+  it '外部タグに差分がない場合，内外マッピングが変はっても連携タグを再評価しない' do
+    post = create_nico_sync_post!
+
+    external_tag = create_external_tag!('AAA')
+    old_linked_tag = create_tag!('spec_old_linked', category: :general)
+    new_linked_tag = create_tag!('spec_new_linked', category: :general)
+
+    relation = link_nico_to_tag!(external_tag, old_linked_tag)
+
+    run_nico_sync_with_tags!(['AAA'])
+
+    expect(post.reload.tags).to include(old_linked_tag)
+
+    # 人手で連携タグを消除する．
+    PostTag.find_by!(post:, tag: old_linked_tag).destroy!
+
+    # 内外マッピングを変更する．
+    relation.destroy!
+    link_nico_to_tag!(external_tag, new_linked_tag)
+
+    # 外部タグ自体には差分が無い．
+    run_nico_sync_with_tags!(['AAA'])
+
+    post.reload
+
+    expect(post.external_tags).to include(external_tag)
+    expect(post.tags).not_to include(old_linked_tag)
+    expect(post.tags).not_to include(new_linked_tag)
+  end
+
+  it '外部タグの消除では連携タグを消除せず，再記載時にその時点の内外マッピングを適用する' do
+    post = create_nico_sync_post!
+
+    external_tag = create_external_tag!('AAA')
+    old_linked_tag = create_tag!('spec_old_linked', category: :general)
+    new_linked_tag = create_tag!('spec_new_linked', category: :general)
+
+    relation = link_nico_to_tag!(external_tag, old_linked_tag)
+
+    # 外部タグを新規記載する．
+    run_nico_sync_with_tags!(['AAA'])
+
+    post.reload
+    expect(post.external_tags).to include(external_tag)
+    expect(post.tags).to include(old_linked_tag)
+
+    # 外部タグを消除する．
+    run_nico_sync_with_tags!([])
+
+    post.reload
+    expect(post.external_tags).not_to include(external_tag)
+
+    # 外部タグの消除によって連携タグまでは消除されない．
+    expect(post.tags).to include(old_linked_tag)
+
+    # 外部タグが記載されてゐない間に内外マッピングを変更する．
+    relation.destroy!
+    link_nico_to_tag!(external_tag, new_linked_tag)
+
+    # 同じ外部タグを再記載する．
+    run_nico_sync_with_tags!(['AAA'])
+
+    post.reload
+
+    expect(post.external_tags).to include(external_tag)
+
+    # 旧連携タグは自動的には消除されない．
+    expect(post.tags).to include(old_linked_tag)
+
+    # 再記載時点の内外マッピングが新たに適用される．
+    expect(post.tags).to include(new_linked_tag)
+  end
 end