#354

backend/app/controllers/materials_controller.rb

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

backend/app/controllers/nico_tags_controller.rb

@@ -15,29 +15,30 @@ class NicoTagsController < ApplicationController
     limit = 1 if limit < 1
 
     post_tag_max_sql =
-        PostExternalTag
-        .select('external_tag_id, MAX(created_at) AS max_created_at')
-        .group('external_tag_id')
+        PostTag
+        .select('tag_id, MAX(created_at) AS max_created_at')
+        .group('tag_id')
         .to_sql
 
-    q =
-        ExternalTag
-        .joins("LEFT JOIN (#{ post_tag_max_sql }) post_tag_max " \
-               'ON post_tag_max.external_tag_id = external_tags.id')
-    q = q.where('external_tags.name LIKE ?', "%#{ name }%") if name
+    q = Tag.nico_tags
+           .joins(:tag_name)
+           .joins("LEFT JOIN (#{ post_tag_max_sql }) post_tag_max " \
+                  'ON post_tag_max.tag_id = tags.id')
+           .includes(:tag_name, tag_name: :wiki_page, linked_tags: { tag_name: :wiki_page })
+    q = q.where('tag_names.name LIKE ?', "%#{ name }%") if name
     if linked_tag
       linked_tag_ids =
-          Tag
-          .joins(:tag_name)
-          .where('tag_names.name LIKE ?', "%#{ linked_tag }%")
-          .pluck(:id)
+        Tag
+        .joins(:tag_name)
+        .where('tag_names.name LIKE ?', "%#{ linked_tag }%")
+        .pluck(:id)
       linked_nico_tag_ids = NicoTagRelation.where(tag_id: linked_tag_ids).pluck(:nico_tag_id)
       q = q.where(id: linked_nico_tag_ids)
     end
     if link_status.in?(['linked', 'unlinked'])
       exists_sql =
           'EXISTS (SELECT 1 FROM nico_tag_relations ' \
-          'WHERE nico_tag_relations.nico_tag_id = external_tags.id)'
+          'WHERE nico_tag_relations.nico_tag_id = tags.id)'
       q = link_status == 'linked' ? q.where(exists_sql) : q.where("NOT #{ exists_sql }")
     end
 
@@ -45,21 +46,21 @@ class NicoTagsController < ApplicationController
     sort_sql =
         case order[0]
         when 'name'
-          'external_tags.name'
+          'tag_names.name'
         when 'updated_at'
           'post_tag_max.max_created_at'
         else
-          "external_tags.#{ order[0] }"
+          "tags.#{ order[0] }"
         end
-    tags = q.reselect('external_tags.*',
+    tags = q.reselect('tags.*',
                       Arel.sql('post_tag_max.max_created_at AS recent_post_tag_created_at'))
-            .order(Arel.sql("#{ sort_sql } #{ order[1] }, external_tags.id #{ order[1] }"))
+            .order(Arel.sql("#{ sort_sql } #{ order[1] }, tags.id #{ order[1] }"))
             .limit(limit)
             .offset((page - 1) * limit)
             .to_a
 
     render json: { tags: tags.map { |tag|
-      external_tag_json(tag).merge(
+      TagRepr.base(tag).merge(
           recent_post_tag_created_at: tag.recent_post_tag_created_at,
           linked_tags: tag.linked_tags.map { |lt| TagRepr.base(lt) })
     }, count: }
@@ -71,7 +72,8 @@ class NicoTagsController < ApplicationController
 
     id = params[:id].to_i
 
-    tag = ExternalTag.find(id)
+    tag = Tag.find(id)
+    return render_bad_request('ニコニコ・タグを指定してください．') unless tag.nico?
 
     linked_tag_names = params[:tags].to_s.split
     linked_tags = nil
@@ -79,15 +81,16 @@ class NicoTagsController < ApplicationController
     ApplicationRecord.transaction do
       linked_tags = Tag.normalise_tags!(linked_tag_names, with_tagme: false,
                                                           with_no_deerjikist: false)
+      if linked_tags.any? { |t| t.nico? }
+        raise Tag::NicoTagNormalisationError
+      end
 
       TagVersioning.record_tag_snapshots!(linked_tags, created_by_user: current_user)
 
       tag.linked_tags = linked_tags
       tag.save!
 
-      NicoTagVersionRecorder.record!(external_tag: tag,
-                                     event_type: :update,
-                                     created_by_user: current_user)
+      NicoTagVersionRecorder.record!(tag:, event_type: :update, created_by_user: current_user)
     end
 
     render json: tag.linked_tags.map { |t| TagRepr.base(t) }, status: :ok
@@ -99,18 +102,6 @@ class NicoTagsController < ApplicationController
 
   private
 
-  def external_tag_json tag
-    { id: tag.id,
-      name: "#{ tag.platform }:#{ tag.name }",
-      category: 'nico',
-      post_count: tag.post_count,
-      created_at: tag.created_at,
-      updated_at: tag.created_at,
-      deprecated_at: nil,
-      aliases: [],
-      parents: [] }
-  end
-
   def render_nico_tag_form_record_invalid record
     if record.is_a?(TagName) || record.is_a?(Tag)
       render_validation_error fields: { tags: record.errors.full_messages.map { |message|

backend/app/controllers/post_versions_controller.rb

@@ -27,8 +27,8 @@ class PostVersionsController < ApplicationController
                 'prev.original_created_before AS prev_original_created_before')
     q = q.where('post_versions.post_id = ?', post_id) if post_id
     if tag_id
-      q = q.where("JSON_CONTAINS(post_versions.tags_json, JSON_OBJECT('tag_id', #{ tag_id })) " +
-                  "OR JSON_CONTAINS(prev.tags_json, JSON_OBJECT('tag_id', #{ tag_id }))")
+      q = q.where("JSON_CONTAINS(post_versions.tags_json, JSON_OBJECT('id', #{ tag_id })) " +
+                  "OR JSON_CONTAINS(prev.tags_json, JSON_OBJECT('id', #{ tag_id }))")
     end
 
     count = q.except(:select, :order, :limit, :offset).count
@@ -43,19 +43,50 @@ class PostVersionsController < ApplicationController
   private
 
   def serialise_versions rows
-    rows = rows.to_a
     user_ids = rows.map(&:created_by_user_id).compact.uniq
     users_by_id = User.where(id: user_ids).pluck(:id, :name).to_h
-    snapshots = rows.flat_map { |row|
-      [normalise_json(row.tags_json),
-       normalise_json(row.attributes['prev_tags_json']) || []]
-    }
-    external_tag_names = external_tag_names_for(snapshots)
 
     rows.map do |row|
-      cur_tags = snapshot_tag_literals(normalise_json(row.tags_json), external_tag_names)
-      prev_tags = snapshot_tag_literals(
-          normalise_json(row.attributes['prev_tags_json']) || [], external_tag_names)
+      cur_tags =
+          normalise_json(row.tags_json)
+          .sort_by { [(case _1.fetch('category')
+                       when 'deerjikist'
+                         0
+                       when 'meme'
+                         1
+                       when 'character'
+                         2
+                       when 'general'
+                         3
+                       when 'material'
+                         4
+                       when 'meta'
+                         5
+                       else
+                         6
+                       end),
+                      _1.fetch('name').downcase] }
+          .map { Post.tag_snapshot_literal(_1) }
+      prev_tags =
+          (normalise_json(row.attributes['prev_tags_json']) || [])
+          .sort_by { [(case _1.fetch('category')
+                       when 'deerjikist'
+                         0
+                       when 'meme'
+                         1
+                       when 'character'
+                         2
+                       when 'general'
+                         3
+                       when 'material'
+                         4
+                       when 'meta'
+                         5
+                       else
+                         6
+                       end),
+                      _1.fetch('name').downcase] }
+          .map { Post.tag_snapshot_literal(_1) }
 
       { post_id: row.post_id,
         version_no: row.version_no,
@@ -82,52 +113,21 @@ class PostVersionsController < ApplicationController
     end
   end
 
-  def external_tag_names_for snapshots
-    ids = snapshots.flatten.filter_map { _1['external_tag_id'] }.uniq
-    names = ExternalTag.where(id: ids).pluck(:id, :platform, :name).to_h { |id, platform, name|
-      [id, "#{ platform }:#{ name }"]
-    }
-    missing_ids = ids - names.keys
-
-    NicoTagVersion
-        .where(tag_id: missing_ids)
-        .order(:tag_id, version_no: :desc)
-        .pluck(:tag_id, :name)
-        .each { |id, name| names[id] ||= name }
-
-    names
-  end
-
-  def snapshot_tag_literals snapshots, external_tag_names
-    snapshots
-        .filter_map { |snapshot|
-          if snapshot.key?('tag_id')
-            [tag_category_order(snapshot['category']),
-             Post.tag_snapshot_literal(snapshot)]
-          elsif external_tag_names[snapshot['external_tag_id']]
-            [6, external_tag_names[snapshot['external_tag_id']]]
-          end
-        }
-        .sort_by { |order, name| [order, name.downcase] }
-        .map(&:second)
-  end
-
-  def tag_category_order category
-    ['deerjikist', 'meme', 'character', 'general', 'material', 'meta'].index(category) || 6
-  end
-
-  def build_version_tags cur_tags, prev_tags
+  def build_version_tags(cur_tags, prev_tags)
     (cur_tags | prev_tags).map do |name|
       type =
-          if cur_tags.include?(name) && prev_tags.include?(name)
-            'context'
-          elsif cur_tags.include?(name)
-            'added'
-          else
-            'removed'
-          end
+        if cur_tags.include?(name) && prev_tags.include?(name)
+          'context'
+        elsif cur_tags.include?(name)
+          'added'
+        else
+          'removed'
+        end
 
-      { name:, type: }
+      {
+        name:,
+        type:
+      }
     end
   end
 end

backend/app/controllers/posts_controller.rb

@@ -36,8 +36,8 @@ class PostsController < ApplicationController
     offset = (page - 1) * limit
 
     pt_max_sql =
-        PostVersion
-        .select('post_id, MAX(created_at) AS max_updated_at')
+        PostTag
+        .select('post_id, MAX(updated_at) AS max_updated_at')
         .group('post_id')
         .to_sql
 
@@ -50,8 +50,9 @@ class PostsController < ApplicationController
         .joins("LEFT JOIN (#{ pt_max_sql }) pt_max ON pt_max.post_id = posts.id")
         .reselect('posts.*', Arel.sql("#{ updated_at_all_sql } AS updated_at_all"))
         .preload(:uploaded_user, :parents, :children,
-                 post_tags: [:sections, { tag: [:deerjikists, :materials,
-                                                { tag_name: :wiki_page }] }])
+                 active_post_tags: [:sections,
+                                    { tag: [:deerjikists, :materials,
+                                            { tag_name: :wiki_page }] }])
         .with_attached_thumbnail
 
     q = q.where('posts.url LIKE ?', "%#{ url }%") if url
@@ -103,8 +104,9 @@ class PostsController < ApplicationController
 
   def random
     post = filtered_posts.preload(:uploaded_user, :parents, :children,
-                                  post_tags: [:sections, { tag: [:deerjikists, :materials,
-                                                                 { tag_name: :wiki_page }] }])
+                                  active_post_tags: [:sections,
+                                                     { tag: [:deerjikists, :materials,
+                                                             { tag_name: :wiki_page }] }])
                          .with_attached_thumbnail
                          .order('RAND()')
                          .first
@@ -188,8 +190,9 @@ class PostsController < ApplicationController
     post =
         Post
         .includes(:uploaded_user, :parents, :children,
-                  post_tags: [:sections, { tag: [:deerjikists, :materials,
-                                                 { tag_name: :wiki_page }] }])
+                  active_post_tags: [:sections,
+                                     { tag: [:deerjikists, :materials,
+                                             { tag_name: :wiki_page }] }])
         .with_attached_thumbnail
         .find_by(id: params[:id])
     return head :not_found unless post
@@ -381,6 +384,50 @@ class PostsController < ApplicationController
     render_post_form_record_invalid e.record
   end
 
+  def changes
+    id = params[:id].presence
+    tag_id = params[:tag].presence
+    page = (params[:page].presence || 1).to_i
+    limit = (params[:limit].presence || 20).to_i
+
+    page = 1 if page < 1
+    limit = 1 if limit < 1
+
+    offset = (page - 1) * limit
+
+    pts = PostTag.with_discarded
+    pts = pts.where(post_id: id) if id.present?
+    pts = pts.where(tag_id:) if tag_id.present?
+    pts = pts.includes(:post, :created_user, :deleted_user,
+                       tag: [:deerjikists, :materials, { tag_name: :wiki_page }])
+
+    events = []
+    pts.each do |pt|
+      tag = TagRepr.base(pt.tag)
+      post = pt.post
+
+      events << Event.new(
+          post:,
+          tag:,
+          user: pt.created_user && { id: pt.created_user.id, name: pt.created_user.name },
+          change_type: 'add',
+          timestamp: pt.created_at)
+
+      if pt.discarded_at
+        events << Event.new(
+            post:,
+            tag:,
+            user: pt.deleted_user && { id: pt.deleted_user.id, name: pt.deleted_user.name },
+            change_type: 'remove',
+            timestamp: pt.discarded_at)
+      end
+    end
+    events.sort_by!(&:timestamp)
+    events.reverse!
+
+    render json: { changes: (events.slice(offset, limit) || []).as_json, count: events.size }
+  end
+
   private
 
   def filtered_posts
@@ -455,13 +502,13 @@ class PostsController < ApplicationController
       end
     end
 
-    PostTag.where(post_id: post.id, tag_id: to_remove.to_a).find_each do |pt|
-      pt.destroy!
+    PostTag.where(post_id: post.id, tag_id: to_remove.to_a).kept.find_each do |pt|
+      pt.discard_by!(current_user)
     end
   end
 
   def build_tag_tree_for post
-    post_tags = post.post_tags.reject { |post_tag| post_tag.tag.deprecated? }
+    post_tags = post.active_post_tags.reject { |post_tag| post_tag.tag.deprecated? }
     tags = post_tags.map(&:tag)
     tag_ids = tags.map(&:id)
 
@@ -653,7 +700,7 @@ class PostsController < ApplicationController
 
   def editable_tag_names_from_version version
     version.tags_json
-        .select { _1.key?('tag_id') }
+        .reject { _1.fetch('category') == 'nico' }
         .map { Post.tag_snapshot_literal(_1) }
         .sort
   end
@@ -670,7 +717,9 @@ 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')
@@ -688,7 +737,8 @@ 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) => { tags:, sections: }
+                                        with_sections: true) =>
+        { tags:, sections: }
 
     tags = Tag.expand_parent_tags(tags).reject(&:deprecated?)
     video_ms = normalise_video_ms(tags, video_ms_param:, duration_param:)
@@ -831,12 +881,15 @@ class PostsController < ApplicationController
                  original_created_from: snapshot[:original_created_from],
                  original_created_before: snapshot[:original_created_before])
 
-    Tag.normalise_tags!(snapshot[:tag_names],
-                        with_tagme: false,
-                        deny_deprecated: true,
-                        with_sections: true) => { tags:, sections: }
-    TagVersioning.record_tag_snapshots!(tags, created_by_user: current_user)
+    Tag.normalise_tags!(snapshot[:tag_names], with_tagme: false,
+                                               deny_deprecated: true,
+                                               with_sections: true) =>
+        { tags: editable_tags, sections: }
+    TagVersioning.record_tag_snapshots!(editable_tags, created_by_user: current_user)
 
+    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

@@ -10,6 +10,7 @@ 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)
@@ -32,6 +33,7 @@ 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

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

backend/app/controllers/wiki_pages_controller.rb

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

backend/app/models/external_tag.rb

@@ -1,20 +0,0 @@
-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

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

backend/app/models/nico_tag_version.rb

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

backend/app/models/post.rb

@@ -55,27 +55,25 @@ class Post < ApplicationRecord
   belongs_to :uploaded_user, class_name: 'User', optional: true
 
   has_many :post_tags, dependent: :destroy, inverse_of: :post
-  has_many :tags, through: :post_tags
+  has_many :active_post_tags, -> { kept }, class_name: 'PostTag', inverse_of: :post
+  has_many :post_tags_with_discarded, -> { with_discarded }, class_name: 'PostTag'
+  has_many :tags, through: :active_post_tags
   has_many :active_tags, -> { where(tags: { deprecated_at: nil }) },
-           through: :post_tags,
-           source: :tag
+           through: :active_post_tags, source: :tag
 
   has_many :user_post_views, dependent: :delete_all
   has_many :post_similarities, dependent: :delete_all
   has_many :post_versions
-
   has_many :gekanator_guessed_games,
            class_name: 'GekanatorGame',
            foreign_key: :guessed_post_id,
            dependent: :delete_all,
            inverse_of: :guessed_post
-
   has_many :gekanator_correct_games,
            class_name: 'GekanatorGame',
            foreign_key: :correct_post_id,
            dependent: :delete_all,
            inverse_of: :correct_post
-
   has_many :gekanator_question_examples, dependent: :delete_all
 
   has_many :parent_post_implications,
@@ -92,9 +90,6 @@ class Post < ApplicationRecord
            inverse_of: :parent_post
   has_many :children, through: :child_post_implications, source: :post
 
-  has_many :post_external_tags, dependent: :destroy
-  has_many :external_tags, through: :post_external_tags
-
   has_one_attached :thumbnail
 
   attribute :version_no, :integer, default: 1
@@ -128,6 +123,7 @@ class Post < ApplicationRecord
 
   def snapshot_tag_names
     post_tags
+        .kept
         .joins(tag: :tag_name)
         .includes(:sections, tag: :tag_name)
         .order('tag_names.name')
@@ -153,27 +149,20 @@ class Post < ApplicationRecord
   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
+    post_tags
+        .kept
+        .joins(tag: :tag_name)
+        .includes(:sections, tag: :tag_name)
+        .order('tags.id')
+        .map do |pt|
+      { 'id' => pt.tag.id,
+        'version_no' => pt.tag.version_no,
+        'name' => pt.tag.name,
+        'category' => pt.tag.category,
+        'sections' => pt.sections.sort_by(&:begin_ms).map {
+          { 'begin_ms' => _1.begin_ms, 'end_ms' => _1.end_ms }
+        } }
+    end
   end
 
   def self.section_literal section

backend/app/models/post_external_tag.rb

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

backend/app/models/post_tag.rb

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

backend/app/models/post_tag_section.rb

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

backend/app/models/tag.rb

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

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

backend/app/models/tag_name_sanitisation_rule.rb

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

backend/app/representations/post_repr.rb

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

backend/app/services/nico_tag_version_recorder.rb

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

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

backend/app/services/post_creator.rb

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

backend/app/services/tag_versioning.rb

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

@@ -47,14 +47,10 @@ 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"
@@ -64,7 +60,7 @@ class VersionRecorder
       raise "#{ version_class.name } create event already exists"
     end
 
-    return if !(latest) || !(tracks_version_no_on_record?)
+    return unless latest
 
     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.where(post_id: post.id).pluck(:tag_id).to_set
+      current_tag_ids ||= PostTag.kept.where(post_id: post.id).pluck(:tag_id).to_set
       desired_tag_ids = desired_tag_ids.compact.to_set
 
       to_add = desired_tag_ids - current_tag_ids
@@ -117,8 +117,8 @@ module Youtube
         end
       end
 
-      PostTag.where(post_id: post.id, tag_id: to_remove.to_a).find_each do |pt|
-        pt.destroy!
+      PostTag.where(post_id: post.id, tag_id: to_remove.to_a).kept.find_each do |pt|
+        pt.discard_by!(nil)
       end
     end
 

backend/config/routes.rb

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

backend/db/migrate/20260921000000_delete_inactive_records_from_post_tags.rb

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

backend/db/migrate/20260921010000_add_foreign_key_on_post_id_and_tag_id_in_post_tag_sections.rb

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

backend/db/migrate/20260921020000_delete_discarded_records_from_tags.rb

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

backend/db/migrate/20260921030000_delete_discarded_records_from_tag_names.rb

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

backend/db/migrate/20260921040000_create_external_tags.rb

@@ -1,54 +0,0 @@
-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

@@ -1,29 +0,0 @@
-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

@@ -1,6 +0,0 @@
-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

@@ -1,199 +0,0 @@
-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_09_21_230000) do
+ActiveRecord::Schema[8.0].define(version: 2026_07_27_123600) 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
@@ -48,14 +48,6 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_230000) 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
@@ -289,13 +281,6 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_230000) 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
@@ -326,12 +311,23 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_230000) do
     t.check_constraint "`begin_ms` >= 0", name: "chk_post_tag_sections_begin_ms_natural"
   end
 
-  create_table "post_tags", primary_key: ["post_id", "tag_id"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+  create_table "post_tags", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "post_id", null: false
     t.bigint "tag_id", null: false
     t.bigint "created_user_id"
+    t.bigint "deleted_user_id"
     t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.datetime "discarded_at"
+    t.virtual "is_active", type: :boolean, as: "(`discarded_at` is null)", stored: true
+    t.virtual "active_unique_key", type: :string, as: "(case when (`discarded_at` is null) then concat(`post_id`,_utf8mb4':',`tag_id`) else NULL end)", stored: true
+    t.index ["active_unique_key"], name: "idx_post_tags_active_unique", unique: true
     t.index ["created_user_id"], name: "index_post_tags_on_created_user_id"
+    t.index ["deleted_user_id"], name: "index_post_tags_on_deleted_user_id"
+    t.index ["discarded_at"], name: "index_post_tags_on_discarded_at"
+    t.index ["post_id", "discarded_at"], name: "index_post_tags_on_post_id_and_discarded_at"
+    t.index ["post_id"], name: "index_post_tags_on_post_id"
+    t.index ["tag_id", "discarded_at"], name: "index_post_tags_on_tag_id_and_discarded_at"
     t.index ["tag_id"], name: "index_post_tags_on_tag_id"
   end
 
@@ -367,7 +363,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_230000) 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"
+    t.check_constraint "json_schema_valid(_utf8mb4'{\"type\":\"array\",\"items\":{\"type\":\"object\",\"properties\":{\"id\":{\"type\":\"integer\",\"minimum\":1},\"version_no\":{\"type\":\"integer\",\"minimum\":1},\"name\":{\"type\":\"string\",\"minLength\":1},\"category\":{\"type\":\"string\",\"enum\":[\"deerjikist\",\"meme\",\"character\",\"general\",\"material\",\"meta\",\"nico\"]},\"sections\":{\"type\":\"array\",\"items\":{\"type\":\"object\",\"properties\":{\"begin_ms\":{\"type\":\"integer\",\"minimum\":0},\"end_ms\":{\"type\":[\"integer\",\"null\"],\"minimum\":0}},\"required\":[\"begin_ms\",\"end_ms\"],\"additionalProperties\":false}}},\"required\":[\"id\",\"version_no\",\"name\",\"category\",\"sections\"],\"additionalProperties\":false}}',`tags_json`)", name: "chk_post_versions_tags_json_schema"
   end
 
   create_table "posts", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
@@ -399,6 +395,19 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_230000) 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
@@ -424,7 +433,9 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_230000) 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
 
@@ -461,8 +472,10 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_230000) 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"
@@ -603,19 +616,6 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_230000) 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
@@ -709,25 +709,27 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_230000) 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", "post_tags", column: ["post_id", "tag_id"], primary_key: ["post_id", "tag_id"], on_delete: :cascade
+  add_foreign_key "post_tag_sections", "posts"
+  add_foreign_key "post_tag_sections", "tags"
   add_foreign_key "post_tags", "posts"
   add_foreign_key "post_tags", "tags"
   add_foreign_key "post_tags", "users", column: "created_user_id"
+  add_foreign_key "post_tags", "users", column: "deleted_user_id"
   add_foreign_key "post_versions", "posts"
   add_foreign_key "post_versions", "users", column: "created_by_user_id"
   add_foreign_key "posts", "users", column: "uploaded_user_id"
@@ -737,6 +739,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_230000) do
   add_foreign_key "tag_names", "tag_names", column: "canonical_id"
   add_foreign_key "tag_similarities", "tags"
   add_foreign_key "tag_similarities", "tags", column: "target_tag_id"
+  add_foreign_key "tag_versions", "tags"
   add_foreign_key "tag_versions", "users", column: "created_by_user_id"
   add_foreign_key "tags", "tag_names"
   add_foreign_key "theatre_comments", "theatres"

backend/lib/tasks/sync_nico.rake

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

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

backend/spec/db/external_tags_spec.rb

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

backend/spec/factories/external_tags.rb

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

backend/spec/factories/tags.rb

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

backend/spec/models/external_tag_spec.rb

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

backend/spec/models/nico_tag_relation_spec.rb

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

backend/spec/models/post_external_tag_spec.rb

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

backend/spec/models/post_spec.rb

@@ -6,25 +6,6 @@ RSpec.describe Post, type: :model do
     PostUrlSanitisationRule.unscoped.delete_all
   end
 
-  describe '#snapshot_tags_json' do
-    it 'keeps internal snapshots and external identifiers distinct, even with the same id' do
-      post = create(:post)
-      tag = create(:tag)
-      external = create(:external_tag, id: tag.id)
-      create(:post_tag, post:, tag:)
-      create(:post_tag_section, post:, tag:, begin_ms: 2000, end_ms: nil)
-      PostExternalTag.create!(post:, external_tag: external)
-
-      expect(post.snapshot_tags_json).to eq([
-          { 'tag_id' => tag.id,
-            'version_no' => tag.version_no,
-            'name' => tag.name,
-            'category' => tag.category,
-            'sections' => [{ 'begin_ms' => 2000, 'end_ms' => nil }] },
-          { 'external_tag_id' => external.id }])
-    end
-  end
-
   describe 'URL normalisation' do
     it 'normalises the HTTP URL before applying sanitisation rules' do
       PostUrlSanitisationRule.create!(

backend/spec/models/post_tag_spec.rb

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

backend/spec/models/tag_name_sanitisation_rule_spec.rb

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

backend/spec/models/tag_spec.rb

@@ -1,29 +1,6 @@
 require 'rails_helper'
 
 RSpec.describe Tag, type: :model do
-  describe 'external tag separation' do
-    ['nico:reserved', 'NiCo:reserved'].each do |name|
-      it "rejects the reserved prefix #{ name } for internal tags" do
-        tag = build(:tag, name:)
-
-        expect(tag).to be_invalid
-        expect(tag.errors[:name]).to be_present
-        expect {
-          described_class.normalise_tags!([name])
-        }.to raise_error(Tag::NicoTagNormalisationError)
-      end
-    end
-
-    it 'finds external links by the internal tag id even when ids differ' do
-      tag = create(:tag)
-      external = create(:external_tag, id: tag.id + 10_000)
-      # The migration retains existing links without running model validation.
-      NicoTagRelation.insert_all!([{ tag_id: tag.id, nico_tag_id: external.id }])
-
-      expect(tag.linked_nico_tags).to contain_exactly(external)
-    end
-  end
-
   describe '.normalise_tags!' do
     it 'rejects deprecated tags when deny_deprecated is enabled' do
       tag_name = TagName.create!(name: 'normalise deprecated tag')
@@ -182,44 +159,17 @@ 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
+  describe 'deprecated validation' do
+    it 'rejects deprecated nico tags' do
+      tag = build(
+        :tag,
+        name: 'nico:deprecated_validation',
+        category: :nico,
+        deprecated_at: Time.current
+      )
 
-      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)
+      expect(tag).not_to be_valid
+      expect(tag.errors[:deprecated_at]).to include('ニコタグは廃止できません．')
     end
   end
 
@@ -235,14 +185,18 @@ RSpec.describe Tag, type: :model do
     context 'when merging a simple source tag' do
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
 
-      it 'deletes the source tag, moves its post link, and keeps its name as an alias' do
+      it 'discards the source post_tag, creates an active target post_tag, discards the source tag, and aliases the source tag_name' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        target_link = PostTag.find_by(post: post_record, tag: target_tag)
+        source_pt = PostTag.with_discarded.find(source_post_tag.id)
+        active_target = PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)
 
-        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
-        expect(target_link).to be_present
-        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
+        expect(source_pt.discarded_at).to be_present
+        expect(source_pt.tag_id).to eq(source_tag.id)
+        expect(active_target).to be_present
+
+        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
+        expect(TagName.with_discarded.find(source_tag_name.id)).not_to be_discarded
         expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
         expect(target_tag.reload.post_count).to eq(1)
       end
@@ -252,101 +206,38 @@ RSpec.describe Tag, type: :model do
       let!(:target_post_tag) { PostTag.create!(post: post_record, tag: target_tag) }
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
 
-      it 'deletes the source link and preserves the existing target link' do
-        create(:post_tag_section, post: post_record, tag: source_tag,
-                                  begin_ms: 1000, end_ms: 2000)
-        target_section = create(:post_tag_section, post: post_record,
-                                                   tag: target_tag,
-                                                   begin_ms: 3000, end_ms: nil)
-
+      it 'discards the source post_tag, keeps one active target post_tag, discards the source tag, and aliases the source tag_name' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        target_links = PostTag.where(post: post_record, tag: target_tag)
+        source_pt = PostTag.with_discarded.find(source_post_tag.id)
+        active = PostTag.kept.where(post_id: post_record.id, tag_id: target_tag.id)
 
-        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
-        expect(target_links).to contain_exactly(target_post_tag)
-        expect(PostTagSection.where(post: post_record, tag: source_tag)).to be_empty
-        expect(target_post_tag.reload.sections).to contain_exactly(target_section)
+        expect(source_pt.discarded_at).to be_present
+        expect(source_pt.tag_id).to eq(source_tag.id)
+        expect(active.count).to eq(1)
+        expect(active.first.id).to eq(target_post_tag.id)
 
-        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
+        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
+        expect(TagName.with_discarded.find(source_tag_name.id)).not_to be_discarded
         expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
         expect(target_tag.reload.post_count).to eq(1)
       end
     end
 
-    it 'keeps source history and records the new target alias after deleting the source' do
-      user = create_member_user!
-      source_name = source_tag.name
-      source_alias = TagName.create!(name: 'merge_alias', canonical: source_tag_name)
-      TagVersioning.ensure_snapshot!(source_tag, created_by_user: user)
-      original_version = source_tag.tag_versions.first
-
-      described_class.merge_tags!(target_tag, [source_tag], created_by_user: user)
-
-      versions = TagVersion.where(tag_id: source_tag.id).order(:version_no)
-      expect(versions.pluck(:version_no, :event_type))
-          .to eq([[1, 'create'], [2, 'discard']])
-      expect(versions.first).to eq(original_version)
-      expect(versions.last).to have_attributes(
-          name: source_name, aliases: source_alias.name, created_by_user: user)
-      expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
-
-      target_versions = target_tag.tag_versions.order(:version_no)
-      expect(target_versions.pluck(:event_type)).to eq(['create', 'update'])
-      expect(target_versions.last.aliases.split).to eq([source_name])
-    end
-
-    it 'deletes source relationships while preserving unrelated relationships' do
-      parent = create(:tag)
-      child = create(:tag)
-      nico_tag = create(:external_tag)
-      TagImplication.create!(tag: source_tag, parent_tag: parent)
-      TagImplication.create!(tag: child, parent_tag: source_tag)
-      kept_implication = TagImplication.create!(tag: target_tag, parent_tag: parent)
-      NicoTagRelation.create!(tag: source_tag, nico_tag:)
-      kept_relation = NicoTagRelation.create!(tag: target_tag, nico_tag:)
-      TagSimilarity.create!(tag: source_tag, target_tag:, cos: 0.5)
-      TagSimilarity.create!(tag: target_tag, target_tag: source_tag, cos: 0.5)
-      kept_similarity = TagSimilarity.create!(tag: target_tag, target_tag: parent,
-                                               cos: 0.5)
-
-      described_class.merge_tags!(target_tag, [source_tag])
-
-      expect(TagImplication.all).to contain_exactly(kept_implication)
-      expect(NicoTagRelation.all).to contain_exactly(kept_relation)
-      expect(TagSimilarity.all).to contain_exactly(kept_similarity)
-      expect(TagVersion.where(tag_id: source_tag.id).order(:version_no).last.parent_tag_ids)
-          .to eq(parent.id.to_s)
-    end
-
-    it 'preserves material history referencing the deleted source tag' do
-      source_tag.update!(category: :material)
-      target_tag.update!(category: :material)
-      material = Material.create!(tag: source_tag, url: 'https://example.com/material')
-      version = MaterialVersionRecorder.record!(
-          material:, event_type: :create, created_by_user: nil)
-      material.update!(tag: target_tag)
-
-      described_class.merge_tags!(target_tag, [source_tag])
-
-      expect(version.reload).to have_attributes(
-          tag_id: source_tag.id, tag_name: source_tag_name.name, tag_category: 'material')
-      expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
-      expect(material.reload.tag).to eq(target_tag)
-    end
-
     context 'when source_tags includes the target itself' do
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
 
       it 'ignores the target in source_tags while still merging the source tag' do
         described_class.merge_tags!(target_tag, [source_tag, target_tag])
 
-        target_link = PostTag.find_by(post: post_record, tag: target_tag)
+        source_pt = PostTag.with_discarded.find(source_post_tag.id)
+        active_target = PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)
 
         expect(Tag.find(target_tag.id)).to be_present
-        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
-        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
-        expect(target_link).to be_present
+        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
+        expect(source_pt.discarded_at).to be_present
+        expect(source_pt.tag_id).to eq(source_tag.id)
+        expect(active_target).to be_present
         expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
         expect(target_tag.reload.post_count).to eq(1)
       end
@@ -369,16 +260,18 @@ RSpec.describe Tag, type: :model do
         )
       end
 
-      it 'still merges and keeps the source name as an alias without validating it' do
+      it 'still merges, but discards the source tag_name instead of aliasing it' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        target_link = PostTag.find_by(post: post_record, tag: target_tag)
+        source_pt = PostTag.with_discarded.find(source_post_tag.id)
+        active_target = PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)
+        discarded_source_tag_name = TagName.with_discarded.find(source_tag_name.id)
 
-        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
-        expect(target_link).to be_present
+        expect(source_pt.discarded_at).to be_present
+        expect(source_pt.tag_id).to eq(source_tag.id)
+        expect(active_target).to be_present
 
-        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
-        expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
+        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
         expect(target_tag.reload.post_count).to eq(1)
       end
     end
@@ -395,32 +288,38 @@ RSpec.describe Tag, type: :model do
             message: 'init')
       end
 
-      it 'rolls back earlier deletions, links, and history when a later source has a wiki' do
-        earlier_source = create(:tag)
-        earlier_name = earlier_source.tag_name
-        source_section = create(:post_tag_section, post: post_record,
-                                                   tag: source_tag,
-                                                   begin_ms: 1000, end_ms: 2000)
-
+      it 'rolls back the transaction' do
         expect {
-          described_class.merge_tags!(target_tag, [earlier_source, source_tag])
+          described_class.merge_tags!(target_tag, [source_tag])
         }.to raise_error(ActiveRecord::RecordInvalid)
 
-        expect(Tag.unscoped.exists?(earlier_source.id)).to be(true)
-        expect(earlier_name.reload.canonical_id).to be_nil
-        expect(TagVersion.where(tag_id: [earlier_source.id, source_tag.id, target_tag.id]))
-            .to be_empty
-        expect(Tag.unscoped.exists?(source_tag.id)).to be(true)
-        expect(TagName.unscoped.exists?(source_tag_name.id)).to be(true)
-        expect(source_post_tag.reload.tag_id).to eq(source_tag.id)
-        expect(source_post_tag.sections).to contain_exactly(source_section)
-        expect(PostTag.find_by(post: post_record, tag: target_tag)).to be_nil
-        expect(source_tag.reload.post_count).to eq(1)
+        expect(Tag.with_discarded.find(source_tag.id)).not_to be_discarded
+        expect(TagName.with_discarded.find(source_tag_name.id)).not_to be_discarded
+        expect(PostTag.kept.find(source_post_tag.id).tag_id).to eq(source_tag.id)
+        expect(PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)).to be_nil
         expect(source_tag_name.reload.canonical_id).to be_nil
         expect(target_tag.reload.post_count).to eq(0)
       end
     end
 
+    context 'when merging a nico source tag' do
+      let!(:target_tag) { create(:tag, category: :nico, name: 'nico:foo') }
+      let!(:source_tag) { create(:tag, category: :nico, name: 'nico:bar') }
+      let!(:source_tag_name_id) { source_tag.tag_name_id }
+
+      it 'discards the source tag_name instead of aliasing it' do
+        described_class.merge_tags!(target_tag, [source_tag])
+
+        discarded_source_tag = Tag.with_discarded.find(source_tag.id)
+        discarded_source_tag_name = TagName.with_discarded.find(source_tag_name_id)
+
+        expect(discarded_source_tag).to be_discarded
+        expect(discarded_source_tag_name).to be_discarded
+        expect(discarded_source_tag_name.canonical_id).to be_nil
+        expect(target_tag.reload.post_count).to eq(0)
+      end
+    end
+
     def snapshot_tags(post)
       post.snapshot_tag_names.join(' ')
     end
@@ -466,15 +365,12 @@ RSpec.describe Tag, type: :model do
         expect(latest.event_type).to eq('update')
         expect(latest.created_by_user).to be_nil
         expect(latest.tags).to eq(snapshot_tags(post_record.reload))
-        expect(latest.tags_json.map { |item| item.fetch('tag_id') }).to eq([target_tag.id])
-        expect(affected_versions.first.tags_json.map { |item| item.fetch('tag_id') })
-            .to eq([source_tag.id])
 
         expect(unaffected_post.reload.post_versions.count).to eq(1)
       end
     end
 
-    context 'when the source tag has no post_tags' do
+    context 'when the source tag has no active post_tags' do
       let!(:another_post) do
         Post.create!(url: 'https://example.com/posts/3', title: 'another post')
       end

backend/spec/models/version_record_spec.rb

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

backend/spec/requests/nico_tags_spec.rb

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

backend/spec/requests/posts_spec.rb

@@ -17,7 +17,7 @@ RSpec.describe 'Posts API', type: :request do
   end
 
   def create_nico_tag!(name)
-    ExternalTag.find_or_create_by!(platform: :nico, name: name.delete_prefix('nico:'))
+    Tag.find_or_create_by_tag_name!(name, category: :nico)
   end
 
   def dummy_upload
@@ -93,26 +93,6 @@ RSpec.describe 'Posts API', type: :request do
     count
   end
 
-  def expect_external_tag_json tag_json, external_tag
-    external_tag.reload
-
-    expect(tag_json).to include(
-        'id' => external_tag.id,
-        'name' => "#{ external_tag.platform }:#{ external_tag.name }",
-        'category' => 'nico',
-        'created_at' => external_tag.created_at.as_json,
-        'updated_at' => external_tag.created_at.as_json,
-        'deprecated_at' => nil,
-        'aliases' => [],
-        'parents' => [],
-        'post_count' => external_tag.post_count,
-        'has_wiki' => false,
-        'material_id' => nil,
-        'has_deerjikists' => false,
-        'children' => [],
-        'sections' => [])
-  end
-
   let!(:tag_name) { TagName.create!(name: 'spec_tag') }
   let!(:tag)      { Tag.create!(tag_name: tag_name, category: :general) }
 
@@ -168,15 +148,12 @@ RSpec.describe 'Posts API', type: :request do
 
     it 'keeps children and sections keys in non-detail tag responses' do
       PostTagSection.create!(post: hit_post, tag:, begin_ms: 1_000, end_ms: nil)
-      deprecated_tag = create(:tag, deprecated_at: Time.current)
-      create(:post_tag, post: hit_post, tag: deprecated_tag)
 
       get '/posts'
 
       expect(response).to have_http_status(:ok)
 
       hit_json = json.fetch('posts').find { |post| post['id'] == hit_post.id }
-      expect(hit_json.fetch('tags').map { |item| item.fetch('id') }).to eq([tag.id])
       tag_json = hit_json.fetch('tags').find { |item| item['name'] == 'spec_tag' }
 
       expect(tag_json.fetch('children')).to eq([])
@@ -185,26 +162,6 @@ RSpec.describe 'Posts API', type: :request do
       ])
     end
 
-    it 'preloads tag details and sections as the number of posts grows' do
-      5.times do
-        link = create(:post_tag, post: create(:post, uploaded_user: user))
-        create(:post_tag_section, post: link.post, tag: link.tag,
-                                  begin_ms: 1000, end_ms: 2000)
-      end
-      get '/posts', params: { limit: 1 }
-
-      one_post_queries = count_sql_queries do
-        get '/posts', params: { limit: 1 }
-      end
-      many_post_queries = count_sql_queries do
-        get '/posts', params: { limit: 20 }
-      end
-
-      expect(response).to have_http_status(:ok)
-      expect(json.fetch('posts').size).to eq(8)
-      expect(many_post_queries).to be <= one_post_queries
-    end
-
     context 'when q is provided' do
       it 'filters posts by q (hit case)' do
         get '/posts', params: { tags: 'spec_tag' }
@@ -253,93 +210,6 @@ RSpec.describe 'Posts API', type: :request do
       end
     end
 
-    context 'with legacy external tag name searches' do
-      let!(:external_tag) { create(:external_tag, id: tag.id, name: 'search_external') }
-      let!(:both_post) do
-        create(:post).tap do |post|
-          PostTag.create!(post:, tag:)
-          PostExternalTag.create!(post:, external_tag:)
-        end
-      end
-
-      before do
-        PostExternalTag.create!(post: miss_post, external_tag:)
-      end
-
-      it 'keeps internal name searches independent of colliding external ids' do
-        get '/posts', params: { tags: tag.name }
-
-        expect(response).to have_http_status(:ok)
-        expect(json.fetch('count')).to eq(3)
-        expect(json.fetch('posts').map { _1.fetch('id') })
-            .to contain_exactly(post_record.id, hit_post.id, both_post.id)
-      end
-
-      it 'finds posts through PostExternalTag by the qualified legacy name' do
-        get '/posts', params: { tags: 'nico:search_external' }
-
-        expect(response).to have_http_status(:ok)
-        expect(json.fetch('count')).to eq(2)
-        expect(json.fetch('posts').map { _1.fetch('id') })
-            .to contain_exactly(miss_post.id, both_post.id)
-      end
-
-      [nil, 'all'].each do |match|
-        it "intersects internal and external matches with match=#{ match || 'omitted' }" do
-          params = { tags: "#{ tag.name } nico:search_external" }
-          params[:match] = match if match
-
-          get '/posts', params: params
-
-          expect(response).to have_http_status(:ok)
-          expect(json.fetch('count')).to eq(1)
-          expect(json.fetch('posts').map { _1.fetch('id') }).to eq([both_post.id])
-        end
-      end
-
-      it 'unions internal alias and external matches without duplicate posts' do
-        get '/posts', params: { tags: 'manko nico:search_external', match: 'any' }
-
-        expect(response).to have_http_status(:ok)
-        expect(json.fetch('count')).to eq(4)
-        expect(json.fetch('posts').map { _1.fetch('id') })
-            .to contain_exactly(post_record.id, hit_post.id, miss_post.id, both_post.id)
-      end
-
-      it 'excludes external matches from an internal tag search' do
-        get '/posts', params: { tags: "#{ tag.name } not:nico:search_external" }
-
-        expect(response).to have_http_status(:ok)
-        expect(json.fetch('count')).to eq(2)
-        expect(json.fetch('posts').map { _1.fetch('id') })
-            .to contain_exactly(post_record.id, hit_post.id)
-      end
-
-      it 'unions an external match with a negated internal match' do
-        get '/posts', params: { tags: "nico:search_external not:#{ tag.name }", match: 'any' }
-
-        expect(response).to have_http_status(:ok)
-        expect(json.fetch('count')).to eq(2)
-        expect(json.fetch('posts').map { _1.fetch('id') })
-            .to contain_exactly(miss_post.id, both_post.id)
-      end
-
-      it 'keeps a missing qualified external name empty' do
-        get '/posts', params: { tags: 'nico:missing_search_external' }
-
-        expect(response).to have_http_status(:ok)
-        expect(json.fetch('count')).to eq(0)
-        expect(json.fetch('posts')).to be_empty
-      end
-
-      it 'applies the same mixed name search to the existing random endpoint' do
-        get '/posts/random', params: { tags: "#{ tag.name } nico:search_external", match: 'all' }
-
-        expect(response).to have_http_status(:ok)
-        expect(json.fetch('id')).to eq(both_post.id)
-      end
-    end
-
     context 'when tags contain not:' do
       let!(:foo_tag_name) { TagName.create!(name: 'not_spec_foo') }
       let!(:foo_tag) { Tag.create!(tag_name: foo_tag_name, category: :general) }
@@ -590,71 +460,6 @@ RSpec.describe 'Posts API', type: :request do
       end
     end
 
-    context 'when update times include version history' do
-      let(:t0) { Time.zone.parse('2020-01-01 12:00:00') }
-      let(:t1) { t0 + 1.day }
-      let(:t2) { t0 + 2.days }
-      let(:t3) { t0 + 3.days }
-      let!(:history_post) do
-        create(:post, url: 'https://example.com/version-time/history',
-                      created_at: t0, updated_at: t0)
-      end
-      let!(:plain_post) do
-        create(:post, url: 'https://example.com/version-time/plain',
-                      created_at: t1, updated_at: t1)
-      end
-      let!(:newer_post) do
-        create(:post, url: 'https://example.com/version-time/newer',
-                      created_at: t0, updated_at: t3)
-      end
-
-      before do
-        link = create(:post_tag, post: history_post, tag:)
-        travel_to(t0) do
-          PostVersionRecorder.record!(post: history_post,
-                                      event_type: :create, created_by_user: nil)
-          PostVersionRecorder.record!(post: newer_post,
-                                      event_type: :create, created_by_user: nil)
-        end
-        travel_to(t2) do
-          link.destroy!
-          PostVersionRecorder.record!(post: history_post,
-                                      event_type: :update, created_by_user: nil)
-        end
-        create(:post_tag, post: plain_post, tag:, created_at: t3)
-      end
-
-      ['asc', 'desc'].each do |direction|
-        it "sorts by the later of post update and latest version time (#{ direction })" do
-          get '/posts', params: { url: '/version-time/', order: "updated_at:#{ direction }" }
-
-          expect(response).to have_http_status(:ok)
-          expected_ids = [plain_post.id, history_post.id, newer_post.id]
-          expected_ids.reverse! if direction == 'desc'
-          expect(json.fetch('posts').map { |item| item.fetch('id') }).to eq(expected_ids)
-          expect(json.fetch('count')).to eq(3)
-
-          times = json.fetch('posts').to_h do |item|
-            [item.fetch('id'), Time.zone.parse(item.fetch('updated_at'))]
-          end
-          expect(times).to eq({ plain_post.id => t1,
-                               history_post.id => t2,
-                               newer_post.id => t3 })
-          expect(history_post.reload.updated_at).to eq(t0)
-        end
-      end
-
-      it 'filters inclusively by the latest version time after a tag is deleted' do
-        get '/posts', params: { url: '/version-time/',
-                               updated_from: t2.iso8601,
-                               updated_to: t2.iso8601 }
-
-        expect(response).to have_http_status(:ok)
-        expect(json.fetch('posts').map { |item| item.fetch('id') }).to eq([history_post.id])
-        expect(json.fetch('count')).to eq(1)
-      end
-    end
-
     context 'when original_created_from/original_created_to are provided' do
       # 注意: controller の現状ロジックに合わせてる
       # original_created_from は `original_created_before > ?`
@@ -719,30 +524,6 @@ RSpec.describe 'Posts API', type: :request do
         expect(json.fetch('count')).to eq(2)
       end
     end
-
-    it 'returns internal and external tags with colliding ids in the legacy tags array' do
-      external_tag = create(:external_tag, id: tag.id, name: 'post_index_external')
-      PostExternalTag.create!(post: hit_post, external_tag:)
-
-      get '/posts'
-
-      expect(response).to have_http_status(:ok)
-
-      post_json =
-          json
-          .fetch('posts')
-          .find { _1.fetch('id') == hit_post.id }
-
-      external_json =
-          post_json
-          .fetch('tags')
-          .find { _1['name'] == 'nico:post_index_external' }
-
-      expect(post_json.fetch('tags')).to include(
-          a_hash_including('id' => tag.id, 'name' => tag.name, 'category' => 'general'))
-      expect(external_json).not_to be_nil
-      expect_external_tag_json(external_json, external_tag)
-    end
   end
 
   describe 'GET /posts/:id' do
@@ -900,25 +681,6 @@ RSpec.describe 'Posts API', type: :request do
         expect(response).to have_http_status(:ok)
         expect(query_count).to be <= 45
       end
-
-      it 'returns external tags as root nodes in the legacy tag tree' do
-        external_tag = create(:external_tag, id: tag.id, name: 'post_detail_external')
-        PostExternalTag.create!(post: post_record, external_tag:)
-
-        request
-
-        expect(response).to have_http_status(:ok)
-
-        expect(json.fetch('tags')).to include(
-            a_hash_including('id' => tag.id, 'name' => tag.name, 'category' => 'general'))
-        external_json =
-            json
-            .fetch('tags')
-            .find { _1['name'] == 'nico:post_detail_external' }
-
-        expect(external_json).not_to be_nil
-        expect_external_tag_json(external_json, external_tag)
-      end
     end
 
     context 'when post does not exist' do
@@ -1400,9 +1162,11 @@ RSpec.describe 'Posts API', type: :request do
       )
     end
 
-    context 'when the external nico tag already exists' do
+    context 'when nico tag already exists in tags' do
       before do
-        create(:external_tag, name: 'nico_tag')
+        Tag.find_undiscard_or_create_by!(
+            tag_name: TagName.find_undiscard_or_create_by!(name: 'nico:nico_tag'),
+            category: :nico)
       end
 
       it 'returns 422 with tag field errors' do
@@ -1655,11 +1419,9 @@ RSpec.describe 'Posts API', type: :request do
 
     it '200 and updates title + resync tags when member' do
       sign_in_as(member)
-      create(:post_tag_section, post: post_record, tag:,
-                                begin_ms: 1000, end_ms: 2000)
 
       tn2 = TagName.create!(name: 'spec_tag_2')
-      replacement_tag = Tag.create!(tag_name: tn2, category: :general)
+      Tag.create!(tag_name: tn2, category: :general)
 
       put "/posts/#{post_record.id}", params: post_update_params(
           post_record,
@@ -1672,38 +1434,6 @@ RSpec.describe 'Posts API', type: :request do
 
       names = json['tags'].map { |n| n['name'] }
       expect(names).to include('spec_tag_2')
-      expect(names).not_to include('spec_tag')
-      expect(PostTag.exists?(post: post_record, tag:)).to be(false)
-      expect(PostTagSection.exists?(post: post_record, tag:)).to be(false)
-      expect(tag.reload.post_count).to eq(0)
-      expect(replacement_tag.reload.post_count).to eq(1)
-
-      versions = post_record.post_versions.order(:version_no)
-      expect(versions.first.tags_json).to include(
-          a_hash_including('tag_id' => tag.id,
-                           'sections' => [{ 'begin_ms' => 1000, 'end_ms' => 2000 }]))
-      expect(versions.last.tags_json.map { |item| item.fetch('tag_id') })
-          .not_to include(tag.id)
-    end
-
-    it 'can add a removed tag again and records both changes' do
-      sign_in_as(member)
-
-      put "/posts/#{ post_record.id }", params: post_update_params(post_record, tags: '')
-      expect(response).to have_http_status(:ok)
-      expect(PostTag.exists?(post: post_record, tag:)).to be(false)
-
-      put "/posts/#{ post_record.id }", params: post_update_params(
-          post_record, tags: 'spec_tag')
-
-      expect(response).to have_http_status(:ok)
-      expect(PostTag.where(post: post_record, tag:).count).to eq(1)
-      expect(PostTag.find_by!(post: post_record, tag:).created_user).to eq(member)
-      expect(tag.reload.post_count).to eq(1)
-      snapshots = post_record.post_versions.order(:version_no).map do |version|
-        version.tags_json.map { |item| item.fetch('tag_id') }
-      end
-      expect(snapshots.map { |ids| ids.include?(tag.id) }).to eq([true, false, true])
     end
 
     it 'rejects a deprecated tag specified directly' do
@@ -1726,9 +1456,11 @@ RSpec.describe 'Posts API', type: :request do
       )
     end
 
-    context 'when the external nico tag already exists' do
+    context 'when nico tag already exists in tags' do
       before do
-        create(:external_tag, name: 'nico_tag')
+        Tag.find_undiscard_or_create_by!(
+            tag_name: TagName.find_undiscard_or_create_by!(name: 'nico:nico_tag'),
+            category: :nico)
       end
 
       it 'returns 422 with tag field errors' do
@@ -2045,7 +1777,7 @@ RSpec.describe 'Posts API', type: :request do
         base_version = create_post_version_for!(post_record.reload)
 
         nico_tag = create_nico_tag!('nico:optimistic_lock_nico')
-        PostExternalTag.create!(post: post_record, external_tag: nico_tag)
+        PostTag.create!(post: post_record, tag: nico_tag, created_user: member)
 
         PostVersionRecorder.record!(
             post: post_record.reload,
@@ -2065,14 +1797,14 @@ RSpec.describe 'Posts API', type: :request do
 
         expect(names).to include('spec_tag')
         expect(names).to include(Tag.no_deerjikist.name)
-        expect(post_record.external_tags).to contain_exactly(nico_tag)
+        expect(names).to include(nico_tag.name)
       end
 
       it 'keeps nico tags even when they are not included in PUT tags' do
         sign_in_as(member)
 
         nico_tag = create_nico_tag!('nico:readonly_update_nico')
-        PostExternalTag.create!(post: post_record, external_tag: nico_tag)
+        PostTag.create!(post: post_record, tag: nico_tag, created_user: member)
 
         base_version = create_post_version_for!(post_record.reload)
 
@@ -2087,15 +1819,7 @@ RSpec.describe 'Posts API', type: :request do
 
         expect(names).to include('spec_tag')
         expect(names).to include(Tag.no_deerjikist.name)
-        expect(post_record.external_tags).to contain_exactly(nico_tag)
-
-        external_json =
-            json
-            .fetch('tags')
-            .find { _1['name'] == "nico:#{ nico_tag.name }" }
-
-        expect(external_json).not_to be_nil
-        expect_external_tag_json(external_json, nico_tag)
+        expect(names).to include(nico_tag.name)
       end
 
       it 'allows non-nico tags linked from nico tags to be removed by normal post update' do
@@ -2105,7 +1829,7 @@ RSpec.describe 'Posts API', type: :request do
         linked_tag = Tag.find_or_create_by_tag_name!('relation_linked_tag', category: :general)
 
         NicoTagRelation.create!(nico_tag:, tag: linked_tag)
-        PostExternalTag.create!(post: post_record, external_tag: nico_tag)
+        PostTag.create!(post: post_record, tag: nico_tag, created_user: member)
         PostTag.create!(post: post_record, tag: linked_tag, created_user: member)
 
         base_version = create_post_version_for!(post_record.reload)
@@ -2119,7 +1843,7 @@ RSpec.describe 'Posts API', type: :request do
 
         names = post_record.reload.tags.map(&:name)
 
-        expect(post_record.external_tags).to contain_exactly(nico_tag)
+        expect(names).to include(nico_tag.name)
         expect(names).to include('spec_tag')
         expect(names).to include(Tag.no_deerjikist.name)
         expect(names).not_to include(linked_tag.name)
@@ -2155,29 +1879,123 @@ RSpec.describe 'Posts API', type: :request do
       expect(response).to have_http_status(:not_found)
     end
 
-    it 'returns viewed state and current tags with their sections' do
-      create(:post_tag_section, post: post_record, tag:,
-                                begin_ms: 1000, end_ms: nil)
-      deprecated_tag = create(:tag, deprecated_at: Time.current)
-      create(:post_tag, post: post_record, tag: deprecated_tag)
-
+    it '200 and returns viewed boolean' do
       get '/posts/random'
-
       expect(response).to have_http_status(:ok)
       expect(json).to have_key('viewed')
       expect([true, false]).to include(json['viewed'])
-      expect(json.fetch('tags')).to contain_exactly(
-          a_hash_including('id' => tag.id,
-                           'children' => [],
-                           'sections' => [{ 'begin_ms' => 1000, 'end_ms' => nil }]))
     end
   end
 
   describe 'GET /posts/changes' do
-    it 'returns 404 for the retired history endpoint' do
-      get '/posts/changes'
+    let(:member) { create(:user, :member) }
 
-      expect(response).to have_http_status(:not_found)
+    it 'returns add/remove events (history) for a post' do
+      # add
+      tn2 = TagName.create!(name: 'spec_tag2')
+      tag2 = Tag.create!(tag_name: tn2, category: :general)
+      pt = PostTag.create!(post: post_record, tag: tag2, created_user: member)
+
+      # remove (discard)
+      pt.discard_by!(member)
+
+      get '/posts/changes', params: { id: post_record.id }
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include('changes', 'count')
+      expect(json['changes']).to be_an(Array)
+      expect(json['count']).to be >= 2
+
+      types = json['changes'].map { |e| e['change_type'] }.uniq
+      expect(types).to include('add')
+      expect(types).to include('remove')
+    end
+
+    it 'filters history by tag' do
+      tn2 = TagName.create!(name: 'history_tag_hit')
+      tag2 = Tag.create!(tag_name: tn2, category: :general)
+
+      tn3 = TagName.create!(name: 'history_tag_miss')
+      tag3 = Tag.create!(tag_name: tn3, category: :general)
+
+      other_post = Post.create!(
+        title: 'other post',
+        url: 'https://example.com/history-other'
+      )
+
+      # hit: add
+      PostTag.create!(post: post_record, tag: tag2, created_user: member)
+
+      # hit: add + remove
+      pt2 = PostTag.create!(post: other_post, tag: tag2, created_user: member)
+      pt2.discard_by!(member)
+
+      # miss: add + remove
+      pt3 = PostTag.create!(post: post_record, tag: tag3, created_user: member)
+      pt3.discard_by!(member)
+
+      get '/posts/changes', params: { tag: tag2.id }
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include('changes', 'count')
+      expect(json['count']).to eq(3)
+
+      changes = json.fetch('changes')
+
+      expect(changes.map { |e| e.dig('tag', 'id') }.uniq).to eq([tag2.id])
+      expect(changes.map { |e| e['change_type'] }).to match_array(%w[add add remove])
+      expect(changes.map { |e| e.dig('post', 'id') }).to match_array([
+        post_record.id,
+        other_post.id,
+        other_post.id
+      ])
+    end
+
+    it 'filters history by post and tag together' do
+      tn2 = TagName.create!(name: 'history_tag_combo_hit')
+      tag2 = Tag.create!(tag_name: tn2, category: :general)
+
+      tn3 = TagName.create!(name: 'history_tag_combo_miss')
+      tag3 = Tag.create!(tag_name: tn3, category: :general)
+
+      other_post = Post.create!(
+        title: 'other combo post',
+        url: 'https://example.com/history-combo-other'
+      )
+
+      # hit
+      PostTag.create!(post: post_record, tag: tag2, created_user: member)
+
+      # miss by post
+      pt2 = PostTag.create!(post: other_post, tag: tag2, created_user: member)
+      pt2.discard_by!(member)
+
+      # miss by tag
+      pt3 = PostTag.create!(post: post_record, tag: tag3, created_user: member)
+      pt3.discard_by!(member)
+
+      get '/posts/changes', params: { id: post_record.id, tag: tag2.id }
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include('changes', 'count')
+      expect(json['count']).to eq(1)
+
+      changes = json.fetch('changes')
+      expect(changes.size).to eq(1)
+      expect(changes[0]['change_type']).to eq('add')
+      expect(changes[0].dig('post', 'id')).to eq(post_record.id)
+      expect(changes[0].dig('tag', 'id')).to eq(tag2.id)
+    end
+
+    it 'returns empty history when tag does not match' do
+      tn2 = TagName.create!(name: 'history_tag_no_hit')
+      tag2 = Tag.create!(tag_name: tn2, category: :general)
+
+      get '/posts/changes', params: { tag: tag2.id }
+
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('changes')).to eq([])
+      expect(json.fetch('count')).to eq(0)
     end
   end
 
@@ -2229,7 +2047,7 @@ RSpec.describe 'Posts API', type: :request do
     end
 
     let!(:v2) do
-      post_record.post_tags.find_by!(tag: tag).destroy!
+      post_record.post_tags.kept.find_by!(tag: tag).discard_by!(member)
       PostTag.create!(post: post_record, tag: tag2, created_user: member)
       post_record.update!(
         title: 'updated spec post',
@@ -2338,88 +2156,6 @@ RSpec.describe 'Posts API', type: :request do
       expect(first.fetch('created_at')).to eq(t_v1.iso8601)
     end
 
-    context 'with external tag history' do
-      let(:external_id) { tag.id }
-      let(:external) { create(:external_tag, id: external_id) }
-      let(:external_post) { create(:post) }
-
-      before do
-        PostExternalTag.create!(post: external_post, external_tag: external)
-        PostVersionRecorder.record!(
-            post: external_post, event_type: :create, created_by_user: member)
-
-        unrelated_post = create(:post)
-        PostExternalTag.create!(post: unrelated_post, external_tag: create(:external_tag))
-        PostVersionRecorder.record!(
-            post: unrelated_post, event_type: :create, created_by_user: member)
-      end
-
-      it 'prefers Tag over ExternalTag for the legacy tag parameter' do
-        get '/posts/versions', params: { tag: tag.id }
-
-        expect(response).to have_http_status(:ok)
-        expect(json.fetch('count')).to eq(3)
-        expect(json.fetch('versions').map { [_1.fetch('post_id'), _1.fetch('version_no')] })
-            .to contain_exactly(
-                [post_record.id, 1], [post_record.id, 2], [other_post_version.post_id, 1])
-      end
-
-      it 'explicitly filters ExternalTag even when its id collides with Tag' do
-        get '/posts/versions', params: { external_tag: external.id }
-
-        expect(response).to have_http_status(:ok)
-        expect(json.fetch('count')).to eq(1)
-        expect(json.fetch('versions')).to contain_exactly(
-            a_hash_including('post_id' => external_post.id, 'version_no' => 1))
-      end
-
-      # Temporary compatibility shim until the frontend sends external_tag explicitly.
-      context 'with legacy tag fallback and no internal Tag with the external id' do
-        let(:external_id) { Tag.maximum(:id).to_i + 10_000 }
-
-        it 'falls back to ExternalTag for the legacy tag parameter' do
-          expect(Tag.exists?(external.id)).to be(false)
-
-          get '/posts/versions', params: { tag: external.id }
-
-          expect(response).to have_http_status(:ok)
-          expect(json.fetch('count')).to eq(1)
-          expect(json.fetch('versions')).to contain_exactly(
-              a_hash_including('post_id' => external_post.id, 'version_no' => 1))
-        end
-
-        [:tag, :external_tag].each do |parameter|
-          it "includes external removal history through the legacy API's #{ parameter }" do
-            external_post.post_external_tags.destroy_all
-            PostVersionRecorder.record!(
-                post: external_post.reload, event_type: :update, created_by_user: member)
-
-            get '/posts/versions', params: { parameter => external.id }
-
-            expect(response).to have_http_status(:ok)
-            expect(json.fetch('count')).to eq(2)
-            expect(json.fetch('versions')).to contain_exactly(
-                a_hash_including('post_id' => external_post.id, 'version_no' => 1),
-                a_hash_including('post_id' => external_post.id, 'version_no' => 2))
-          end
-        end
-      end
-    end
-
-    it 'can render history containing external identifiers' do
-      PostExternalTag.create!(post: post_record, external_tag: create(:external_tag))
-      post_record.update_columns(version_no: 2)
-      PostVersionRecorder.record!(post: post_record,
-                                  event_type: :update, created_by_user: member)
-
-      get '/posts/versions', params: { post: post_record.id }
-
-      expect(response).to have_http_status(:ok)
-      expect(json.fetch('count')).to eq(3)
-      expect(json.fetch('versions').first.fetch('tags')).to include(
-          'name' => tag2.name, 'type' => 'context')
-    end
-
     it 'filters versions by tag when the current snapshot includes the tag' do
       get '/posts/versions', params: { post: post_record.id, tag: tag2.id }
 

backend/spec/requests/tag_children_spec.rb

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

backend/spec/requests/tags_deerjikists_spec.rb

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

backend/spec/requests/tags_spec.rb

@@ -30,30 +30,6 @@ RSpec.describe 'Tags API', type: :request do
   end
 
   describe 'GET /tags' do
-    it 'includes legacy external JSON alongside an internal tag with the same id' do
-      external = create(:external_tag, id: tag.id, name: 'spec_external', post_count: 3)
-
-      get '/tags', params: { name: 'spec_' }
-
-      expect(response).to have_http_status(:ok)
-      expect(json.fetch('count')).to eq(2)
-      expect(response_tags).to contain_exactly(
-          a_hash_including('id' => tag.id, 'name' => tag.name, 'category' => 'general'),
-          a_hash_including(
-              'id' => external.id,
-              'name' => 'nico:spec_external',
-              'category' => 'nico',
-              'post_count' => 3,
-              'created_at' => external.created_at.as_json,
-              'updated_at' => external.created_at.as_json,
-              'deprecated_at' => nil,
-              'aliases' => [],
-              'parents' => [],
-              'has_wiki' => false,
-              'material_id' => nil,
-              'has_deerjikists' => false))
-    end
-
     it 'returns tags with count and metadata' do
       get '/tags'
 
@@ -188,64 +164,20 @@ RSpec.describe 'Tags API', type: :request do
       Tag.create!(tag_name: TagName.create!(name: 'cat_general'), category: :general)
       Tag.create!(tag_name: TagName.create!(name: 'cat_material'), category: :material)
       Tag.create!(tag_name: TagName.create!(name: 'cat_meta'), category: :meta)
-      create(:external_tag, name: 'cat_nico')
+      Tag.create!(tag_name: TagName.create!(name: 'nico:cat_nico'), category: :nico)
 
       get '/tags', params: { name: 'cat_', order: 'category:asc', limit: 20 }
 
       expect(response).to have_http_status(:ok)
-      expect(response_names).to eq([
-          'cat_deerjikist', 'cat_meme', 'cat_character',
-          'cat_general', 'cat_material', 'cat_meta', 'nico:cat_nico'])
-      expect(json.fetch('count')).to eq(7)
-    end
-
-    context 'with mixed legacy pagination' do
-      let!(:first_tag) { create(:tag, name: 'a_mixed_page', category: :meme) }
-      let!(:middle_tag) { create(:tag, name: 'm_mixed_page', category: :meta) }
-      let!(:last_tag) { create(:tag, name: 'z_mixed_page', category: :general) }
-      let!(:first_external) do
-        create(:external_tag, id: first_tag.id, name: 'a_mixed_page')
-      end
-      let!(:last_external) do
-        create(:external_tag, id: last_tag.id, name: 'z_mixed_page')
-      end
-
-      let(:name_order) do
-        [
-            [first_tag.id, 'a_mixed_page'], [middle_tag.id, 'm_mixed_page'],
-            [first_external.id, 'nico:a_mixed_page'], [last_external.id, 'nico:z_mixed_page'],
-            [last_tag.id, 'z_mixed_page']]
-      end
-      let(:category_order) do
-        [
-            [first_tag.id, 'a_mixed_page'], [last_tag.id, 'z_mixed_page'],
-            [middle_tag.id, 'm_mixed_page'], [first_external.id, 'nico:a_mixed_page'],
-            [last_external.id, 'nico:z_mixed_page']]
-      end
-
-      ['name', 'category'].each do |order|
-        ['asc', 'desc'].each do |direction|
-          it "orders the combined records by #{ order }:#{ direction } before paging" do
-            ascending = order == 'name' ? name_order : category_order
-            expected = direction == 'asc' ? ascending : ascending.reverse
-
-            [2, 3].each do |limit|
-              pages = expected.each_slice(limit).to_a + [[]]
-              pages.each.with_index(1) do |expected_page, page|
-                get '/tags', params: {
-                    name: 'mixed_page', order: "#{ order }:#{ direction }", page:, limit: }
-
-                expect(response).to have_http_status(:ok)
-                expect(json.fetch('count')).to eq(5)
-                expect(response_tags.size).to eq(expected_page.size)
-                expect(response_tags.size).to be <= limit
-                expect(response_tags.map { [_1.fetch('id'), _1.fetch('name')] })
-                    .to eq(expected_page)
-              end
-            end
-          end
-        end
-      end
+      expect(response_names).to eq(%w[
+          cat_deerjikist
+          cat_meme
+          cat_character
+          cat_general
+          cat_material
+          cat_meta
+          nico:cat_nico
+      ])
     end
 
     it 'paginates and keeps total count' do
@@ -367,89 +299,6 @@ RSpec.describe 'Tags API', type: :request do
   end
 
   describe 'GET /tags/autocomplete' do
-    it 'combines internal and external matches without conflating equal ids' do
-      internal = Tag.create!(category: :general, name: 'mixed_internal', post_count: 2)
-      external = create(:external_tag, id: internal.id,
-                                       name: 'mixed_external', post_count: 3)
-
-      get '/tags/autocomplete', params: { q: 'not:mixed' }
-
-      expect(response).to have_http_status(:ok)
-      expect(json.map { |row| row.fetch('name') })
-          .to eq(['nico:mixed_external', 'mixed_internal'])
-      expect(json.first).to include(
-          'id' => external.id, 'category' => 'nico', 'post_count' => 3,
-          'created_at' => external.created_at.as_json,
-          'updated_at' => external.created_at.as_json,
-          'deprecated_at' => nil, 'matched_alias' => nil,
-          'aliases' => [], 'parents' => [], 'has_wiki' => false,
-          'material_id' => nil, 'has_deerjikists' => false)
-    end
-
-    it 'matches an external tag by its platform prefix' do
-      create(:external_tag, name: 'prefix_match', post_count: 1)
-
-      get '/tags/autocomplete', params: { q: 'nico:prefix' }
-
-      expect(json.map { |row| row.fetch('name') }).to eq(['nico:prefix_match'])
-    end
-
-    it 'excludes unused external tags unless present is false' do
-      create(:external_tag, name: 'unused_external')
-
-      get '/tags/autocomplete', params: { q: 'unused' }
-      expect(json).to be_empty
-
-      get '/tags/autocomplete', params: { q: 'unused', present: '0' }
-      expect(json.map { |row| row.fetch('name') }).to eq(['nico:unused_external'])
-    end
-
-    it 'limits the combined results to 20 and sorts ties by displayed name' do
-      11.times do |i|
-        name = "combined_#{ i.to_s.rjust(2, '0') }"
-        Tag.create!(category: :general, name:, post_count: 1)
-        create(:external_tag, name:, post_count: 1)
-      end
-
-      get '/tags/autocomplete', params: { q: 'combined' }
-
-      expected = 11.times.map { |i| "combined_#{ i.to_s.rjust(2, '0') }" }
-      expected += expected.map { |name| "nico:#{ name }" }
-      expect(json.map { |row| row.fetch('name') }).to eq(expected.first(20))
-    end
-
-    it 'excludes external tags but preserves internal alias matches when nico is false' do
-      internal = Tag.create!(category: :general, name: 'switch_internal', post_count: 1)
-      alias_target = Tag.create!(category: :general, name: 'alias_target', post_count: 1)
-      TagName.create!(name: 'switch_alias', canonical: alias_target.tag_name)
-      create(:external_tag, name: 'switch_external', post_count: 1)
-
-      get '/tags/autocomplete', params: { q: 'switch', nico: '0' }
-
-      expect(response).to have_http_status(:ok)
-      expect(json).to contain_exactly(
-          a_hash_including('id' => internal.id, 'name' => internal.name),
-          a_hash_including('id' => alias_target.id, 'name' => alias_target.name,
-                           'matched_alias' => 'switch_alias'))
-    end
-
-    ['%', '_'].each do |wildcard|
-      it "treats #{ wildcard } literally for canonical, alias, and external names" do
-        literal = "literal#{ wildcard }match"
-        Tag.create!(category: :general, name: literal, post_count: 1)
-        alias_target = Tag.create!(category: :general, name: 'literal_alias_target', post_count: 1)
-        TagName.create!(name: "#{ literal }_alias", canonical: alias_target.tag_name)
-        create(:external_tag, name: literal, post_count: 1)
-        Tag.create!(category: :general, name: 'literalXmatch', post_count: 2)
-        create(:external_tag, name: 'literalXmatch', post_count: 2)
-
-        get '/tags/autocomplete', params: { q: "literal#{ wildcard }" }
-
-        expect(json.map { |row| row.fetch('name') })
-            .to contain_exactly(literal, alias_target.name, "nico:#{ literal }")
-      end
-    end
-
     it 'returns matching tags by q' do
       get '/tags/autocomplete', params: { q: 'spec' }
 
@@ -491,27 +340,6 @@ RSpec.describe 'Tags API', type: :request do
   end
 
   describe 'GET /tags/name/:name' do
-    it 'preserves qualified external name lookup used by wiki pages' do
-      external = create(:external_tag, name: 'detail_external', post_count: 3)
-
-      get "/tags/name/#{ CGI.escape('nico:detail_external') }"
-
-      expect(response).to have_http_status(:ok)
-      expect(json).to include(
-          'id' => external.id,
-          'name' => 'nico:detail_external',
-          'category' => 'nico',
-          'post_count' => 3,
-          'created_at' => external.created_at.as_json,
-          'updated_at' => external.created_at.as_json,
-          'deprecated_at' => nil,
-          'aliases' => [],
-          'parents' => [],
-          'has_wiki' => false,
-          'material_id' => nil,
-          'has_deerjikists' => false)
-    end
-
     it 'returns tag by name' do
       get "/tags/name/#{ CGI.escape('spec_tag') }"
 
@@ -659,6 +487,18 @@ RSpec.describe 'Tags API', type: :request do
         expect(json.fetch('deprecated_at')).to be_present
       end
 
+      it 'rejects deprecating a nico tag' do
+        nico_tag = Tag.create!(name: 'nico:deprecated_update', category: :nico)
+
+        patch "/tags/#{ nico_tag.id }", params: { deprecated: '1' }
+
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(nico_tag.reload.deprecated_at).to be_nil
+        expect(json.fetch('errors')).to include(
+          'deprecated' => ['ニコタグは廃止できません．']
+        )
+      end
+
       it 'returns 422 when changing normal tag category to nico' do
         expect {
           patch "/tags/#{tag.id}", params: { category: 'nico' }
@@ -668,6 +508,32 @@ RSpec.describe 'Tags API', type: :request do
         expect(tag.reload.category).to eq('general')
       end
 
+      it 'returns 422 when updating nico tag name' do
+        nico_tag_name = TagName.create!(name: 'nico:tags_spec_source')
+        nico_tag = Tag.create!(tag_name: nico_tag_name, category: :nico)
+
+        expect {
+          patch "/tags/#{ nico_tag.id }", params: { name: 'nico:tags_spec_renamed' }
+        }.not_to change(NicoTagVersion, :count)
+
+        expect(response).to have_http_status(:unprocessable_entity)
+
+        expect(nico_tag.reload.name).to eq('nico:tags_spec_source')
+        expect(nico_tag.category).to eq('nico')
+      end
+
+      it 'returns 422 when changing nico tag category to normal category' do
+        nico_tag_name = TagName.create!(name: 'nico:category_change_ng')
+        nico_tag = Tag.create!(tag_name: nico_tag_name, category: :nico)
+
+        expect {
+          patch "/tags/#{nico_tag.id}", params: { category: 'general' }
+        }.not_to change(NicoTagVersion, :count)
+
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(nico_tag.reload.category).to eq('nico')
+      end
+
       it 'PATCH で tag の name を変更すると対応する wiki version を作成する' do
         wiki_page =
             Wiki::Commit.create_content!(
@@ -1303,6 +1169,28 @@ RSpec.describe 'Tags API', type: :request do
         expect(tag.category).to eq('general')
       end
 
+      it 'nico tag は更新できない' do
+        nico_tag = Tag.create!(
+          tag_name: TagName.create!(name: 'nico:put_update_all_ng'),
+          category: :nico
+        )
+
+        expect {
+          put "/tags/#{ nico_tag.id }", params: {
+            name: 'nico:put_update_all_renamed',
+            category: 'nico',
+            aliases: '',
+            parent_tags: '',
+            deprecated: '0',
+          }
+        }.not_to change(NicoTagVersion, :count)
+
+        expect(response).to have_http_status(:unprocessable_entity)
+
+        expect(nico_tag.reload.name).to eq('nico:put_update_all_ng')
+        expect(nico_tag.category).to eq('nico')
+      end
+
       it 'system tag の name は変更できない' do
         system_tag = Tag.tagme
         old_name = system_tag.name

backend/spec/services/nico_tag_version_recorder_spec.rb

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

backend/spec/services/post_create_plan_spec.rb

@@ -50,19 +50,6 @@ RSpec.describe PostCreatePlan do
     expect(plan[:video_ms]).to eq(60_000)
   end
 
-  it 'rejects direct external tag input without creating internal records' do
-    create(:external_tag, name: 'reserved')
-    counts = [Tag.count, TagName.count, ExternalTag.count]
-
-    expect {
-      described_class.new(attributes: { tags: 'NiCo:reserved' }).build!
-    }.to raise_error(ActiveRecord::RecordInvalid) { |error|
-      expect(error.record.errors[:tags]).to be_present
-    }
-
-    expect([Tag.count, TagName.count, ExternalTag.count]).to eq(counts)
-  end
-
   it 'validates a new tag name without persisting it' do
     long_name = 'a' * 256
     counts = [TagName.count, Tag.count]

backend/spec/services/youtube/sync_spec.rb

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

backend/spec/tasks/nico_sync_spec.rb

@@ -8,7 +8,8 @@ RSpec.describe 'nico:sync' do
   end
 
   def create_tag!(name, category:)
-    Tag.find_or_create_by_tag_name!(name, category:)
+    tn = TagName.find_undiscard_or_create_by!(name: name.to_s.strip)
+    Tag.find_undiscard_or_create_by!(tag_name_id: tn.id) { |t| t.category = category }
   end
 
   def link_nico_to_tag!(nico_tag, tag)
@@ -29,7 +30,7 @@ RSpec.describe 'nico:sync' do
 
     # 追加される linked tag を準備（nico tag に紐付く一般タグ）
     linked = create_tag!('spec_linked', category: 'general')
-    nico = create_external_tag!('AAA')
+    nico = create_tag!('nico:AAA', category: 'nico')
     link_nico_to_tag!(nico, linked)
 
     # bot / tagme は task 内で使うので作っておく（Tag.bot/tagme がある前提）
@@ -53,7 +54,7 @@ RSpec.describe 'nico:sync' do
     active_tag_names = post.tags.joins(:tag_name).pluck('tag_names.name')
 
     expect(active_tag_names).to include('spec_kept')
-    expect(post.external_tags).to contain_exactly(nico)
+    expect(active_tag_names).to include('nico:AAA')
     expect(active_tag_names).to include('spec_linked')
 
     expect(post.original_created_from).to eq(Time.iso8601('2026-01-01T03:34:00Z'))
@@ -107,7 +108,7 @@ RSpec.describe 'nico:sync' do
     expect(calls).to eq(2)
   end
 
-  it '古い nico tag の関連を物理削除し、変更前後の履歴を version に残す' do
+  it '既存 post にあった古い nico tag は active から外され、履歴として discard される' do
     post = Post.create!(
       title: 'old',
       url: 'https://www.nicovideo.jp/watch/sm9',
@@ -115,12 +116,12 @@ RSpec.describe 'nico:sync' do
     )
 
     # 旧nicoタグ（今回の同期結果に含まれない）
-    old_nico = create_external_tag!('OLD')
-    PostExternalTag.create!(post:, external_tag: old_nico)
-    create_post_version_for!(post)
+    old_nico = create_tag!('nico:OLD', category: 'nico')
+    old_pt = PostTag.create!(post: post, tag: old_nico)
+    expect(old_pt.discarded_at).to be_nil
 
     # 今回は NEW のみ欲しい
-    new_nico = create_external_tag!('NEW')
+    new_nico = create_tag!('nico:NEW', category: 'nico')
 
     # bot/tagme 念のため
     Tag.bot
@@ -131,21 +132,15 @@ RSpec.describe 'nico:sync' do
 
     run_rake_task('nico:sync')
 
-    expect(PostExternalTag.exists?(post:, external_tag: old_nico)).to be(false)
-    expect(old_nico.reload.post_count).to eq(0)
-    expect(new_nico.reload.post_count).to eq(1)
-
-    versions = post.post_versions.order(:version_no)
-    expect(versions.first.tags_json.filter_map { |item| item['external_tag_id'] })
-        .to include(old_nico.id)
-    expect(versions.last.tags_json.filter_map { |item| item['external_tag_id'] })
-        .to include(new_nico.id)
-    expect(versions.last.tags_json.filter_map { |item| item['external_tag_id'] })
-        .not_to include(old_nico.id)
+    # OLD は active から外れる（discarded_at が入る）
+    old_pts = PostTag.where(post_id: post.id, tag_id: old_nico.id).order(:id).to_a
+    expect(old_pts.last.discarded_at).to be_present
 
     # NEW は active にいる
     post.reload
-    expect(post.external_tags).to contain_exactly(new_nico)
+    active_names = post.tags.joins(:tag_name).pluck('tag_names.name')
+    expect(active_names).to include('nico:NEW')
+    expect(active_names).not_to include('nico:OLD')
   end
 
   def snapshot_tags(post)
@@ -211,7 +206,7 @@ RSpec.describe 'nico:sync' do
     create_post_version_for!(post)
 
     linked = create_tag!('spec_linked', category: 'general')
-    nico = create_external_tag!('AAA')
+    nico = create_tag!('nico:AAA', category: 'nico')
     link_nico_to_tag!(nico, linked)
 
     Tag.bot
@@ -239,7 +234,7 @@ RSpec.describe 'nico:sync' do
   end
 
   it '既存 post に差分が無いときは新しい version を作らない' do
-    nico = create_external_tag!('AAA')
+    nico = create_tag!('nico:AAA', category: 'nico')
     no_deerjikist = create_tag!('ニジラー情報不詳', category: 'meta')
 
     post = Post.create!(
@@ -250,7 +245,7 @@ RSpec.describe 'nico:sync' do
       original_created_before: Time.iso8601('2026-01-01T03:35:00Z')
     )
 
-    PostExternalTag.create!(post:, external_tag: nico)
+    PostTag.create!(post: post, tag: nico)
     PostTag.create!(post: post, tag: no_deerjikist)
     create_post_version_for!(post)
 
@@ -293,7 +288,7 @@ RSpec.describe 'nico:sync' do
       run_rake_task('nico:sync')
     }.to change(NicoTagVersion, :count).by(1)
 
-    nico_tag = ExternalTag.find_by!(platform: :nico, name: 'AAA')
+    nico_tag = Tag.joins(:tag_name).find_by!(tag_names: { name: 'nico:AAA' })
     version = nico_tag.nico_tag_versions.order(:version_no).last
 
     expect(version.version_no).to eq(1)
@@ -376,148 +371,4 @@ RSpec.describe 'nico:sync' do
     expect(versions.second.title).to eq('changed title')
     expect(versions.second.tags).to eq(snapshot_tags(post.reload))
   end
-
-  def create_external_tag!(name)
-    ExternalTag.create!(platform: :nico, name:)
-  end
-
-  def create_nico_sync_post!
-    post = Post.create!(
-      title: 't',
-      url: 'https://www.nicovideo.jp/watch/sm9',
-      uploaded_user: nil
-    )
-
-    PostTag.create!(post:, tag: Tag.no_deerjikist)
-
-    post
-  end
-
-  def run_nico_sync_with_tags! tags
-    stub_python([{
-      'code' => 'sm9',
-      'title' => 't',
-      'tags' => tags,
-      'user' => nil
-    }])
-
-    allow(URI).to receive(:open).and_return(StringIO.new('<html></html>'))
-
-    run_rake_task('nico:sync')
-  end
-
-  it '外部タグだけの変更では bot を付けず，投稿履歴を記録する' do
-    post = create_nico_sync_post!
-    PostVersionRecorder.record!(post:, event_type: :create, created_by_user: nil)
-
-    expect {
-      run_nico_sync_with_tags!(['raw tag[]', 'raw tag[]'])
-    }.to change(PostVersion, :count).by(1)
-        .and change(ExternalTag, :count).by(1)
-        .and change(PostExternalTag, :count).by(1)
-        .and change(NicoTagVersion, :count).by(1)
-        .and change(TagName, :count).by(0)
-
-    external = post.external_tags.sole
-    expect(external.name).to eq('raw tag[]')
-    expect(post.tags.map(&:name)).not_to include('bot操作')
-    expect(post.post_versions.order(:version_no).last.tags_json)
-        .to include('external_tag_id' => external.id)
-
-    expect {
-      run_nico_sync_with_tags!(['raw tag[]'])
-    }.to change(PostVersion, :count).by(0).and change(NicoTagVersion, :count).by(0)
-
-    expect {
-      run_nico_sync_with_tags!([])
-    }.to change(PostVersion, :count).by(1)
-    expect(post.reload.external_tags).to be_empty
-    expect(post.tags.map(&:name)).not_to include('bot操作')
-  end
-
-  it '外部タグが新規記載されたとき，その時点の連携タグを記載する' do
-    post = create_nico_sync_post!
-
-    external_tag = create_external_tag!('AAA')
-    linked_tag = create_tag!('spec_linked', category: :general)
-    link_nico_to_tag!(external_tag, linked_tag)
-
-    run_nico_sync_with_tags!(['AAA'])
-
-    expect(post.reload.external_tags).to include(external_tag)
-    expect(post.tags).to include(linked_tag)
-  end
-
-  it '外部タグに差分がない場合，内外マッピングが変はっても連携タグを再評価しない' do
-    post = create_nico_sync_post!
-
-    external_tag = create_external_tag!('AAA')
-    old_linked_tag = create_tag!('spec_old_linked', category: :general)
-    new_linked_tag = create_tag!('spec_new_linked', category: :general)
-
-    relation = link_nico_to_tag!(external_tag, old_linked_tag)
-
-    run_nico_sync_with_tags!(['AAA'])
-
-    expect(post.reload.tags).to include(old_linked_tag)
-
-    # 人手で連携タグを消除する．
-    PostTag.find_by!(post:, tag: old_linked_tag).destroy!
-
-    # 内外マッピングを変更する．
-    relation.destroy!
-    link_nico_to_tag!(external_tag, new_linked_tag)
-
-    # 外部タグ自体には差分が無い．
-    run_nico_sync_with_tags!(['AAA'])
-
-    post.reload
-
-    expect(post.external_tags).to include(external_tag)
-    expect(post.tags).not_to include(old_linked_tag)
-    expect(post.tags).not_to include(new_linked_tag)
-  end
-
-  it '外部タグの消除では連携タグを消除せず，再記載時にその時点の内外マッピングを適用する' do
-    post = create_nico_sync_post!
-
-    external_tag = create_external_tag!('AAA')
-    old_linked_tag = create_tag!('spec_old_linked', category: :general)
-    new_linked_tag = create_tag!('spec_new_linked', category: :general)
-
-    relation = link_nico_to_tag!(external_tag, old_linked_tag)
-
-    # 外部タグを新規記載する．
-    run_nico_sync_with_tags!(['AAA'])
-
-    post.reload
-    expect(post.external_tags).to include(external_tag)
-    expect(post.tags).to include(old_linked_tag)
-
-    # 外部タグを消除する．
-    run_nico_sync_with_tags!([])
-
-    post.reload
-    expect(post.external_tags).not_to include(external_tag)
-
-    # 外部タグの消除によって連携タグまでは消除されない．
-    expect(post.tags).to include(old_linked_tag)
-
-    # 外部タグが記載されてゐない間に内外マッピングを変更する．
-    relation.destroy!
-    link_nico_to_tag!(external_tag, new_linked_tag)
-
-    # 同じ外部タグを再記載する．
-    run_nico_sync_with_tags!(['AAA'])
-
-    post.reload
-
-    expect(post.external_tags).to include(external_tag)
-
-    # 旧連携タグは自動的には消除されない．
-    expect(post.tags).to include(old_linked_tag)
-
-    # 再記載時点の内外マッピングが新たに適用される．
-    expect(post.tags).to include(new_linked_tag)
-  end
 end