#419 フロントのテスト追加

backend/app/controllers/nico_tags_controller.rb

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

backend/app/controllers/post_versions_controller.rb

@@ -2,6 +2,9 @@ class PostVersionsController < ApplicationController
   def index
     post_id = params[:post].presence
     tag_id = params[:tag].presence&.to_i
+    external_tag_id = params[:external_tag].presence&.to_i
+    return head :bad_request if tag_id && external_tag_id
+
     page = (params[:page].presence || 1).to_i
     limit = (params[:limit].presence || 20).to_i
 
@@ -26,9 +29,14 @@ class PostVersionsController < ApplicationController
                 'prev.original_created_from AS prev_original_created_from',
                 'prev.original_created_before AS prev_original_created_before')
     q = q.where('post_versions.post_id = ?', post_id) if post_id
-    if tag_id
-      q = q.where("JSON_CONTAINS(post_versions.tags_json, JSON_OBJECT('id', #{ tag_id })) " +
-                  "OR JSON_CONTAINS(prev.tags_json, JSON_OBJECT('id', #{ tag_id }))")
+    if external_tag_id || (tag_id && !(Tag.exists?(id: tag_id)))
+      q = q.where('JSON_CONTAINS(post_versions.tags_json,' +
+                  "JSON_OBJECT('external_tag_id', #{ external_tag_id || tag_id })) " +
+                  'OR JSON_CONTAINS(prev.tags_json,' +
+                  "JSON_OBJECT('external_tag_id', #{ external_tag_id || tag_id }))")
+    elsif tag_id
+      q = q.where("JSON_CONTAINS(post_versions.tags_json, JSON_OBJECT('tag_id', #{ tag_id })) " +
+                  "OR JSON_CONTAINS(prev.tags_json, JSON_OBJECT('tag_id', #{ tag_id }))")
     end
 
     count = q.except(:select, :order, :limit, :offset).count
@@ -43,50 +51,19 @@ class PostVersionsController < ApplicationController
   private
 
   def serialise_versions rows
+    rows = rows.to_a
     user_ids = rows.map(&:created_by_user_id).compact.uniq
     users_by_id = User.where(id: user_ids).pluck(:id, :name).to_h
+    snapshots = rows.flat_map { |row|
+      [normalise_json(row.tags_json),
+       normalise_json(row.attributes['prev_tags_json']) || []]
+    }
+    external_tag_names = external_tag_names_for(snapshots)
 
     rows.map do |row|
-      cur_tags =
-          normalise_json(row.tags_json)
-          .sort_by { [(case _1.fetch('category')
-                       when 'deerjikist'
-                         0
-                       when 'meme'
-                         1
-                       when 'character'
-                         2
-                       when 'general'
-                         3
-                       when 'material'
-                         4
-                       when 'meta'
-                         5
-                       else
-                         6
-                       end),
-                      _1.fetch('name').downcase] }
-          .map { Post.tag_snapshot_literal(_1) }
-      prev_tags =
-          (normalise_json(row.attributes['prev_tags_json']) || [])
-          .sort_by { [(case _1.fetch('category')
-                       when 'deerjikist'
-                         0
-                       when 'meme'
-                         1
-                       when 'character'
-                         2
-                       when 'general'
-                         3
-                       when 'material'
-                         4
-                       when 'meta'
-                         5
-                       else
-                         6
-                       end),
-                      _1.fetch('name').downcase] }
-          .map { Post.tag_snapshot_literal(_1) }
+      cur_tags = snapshot_tag_literals(normalise_json(row.tags_json), external_tag_names)
+      prev_tags = snapshot_tag_literals(
+          normalise_json(row.attributes['prev_tags_json']) || [], external_tag_names)
 
       { post_id: row.post_id,
         version_no: row.version_no,
@@ -113,21 +90,52 @@ class PostVersionsController < ApplicationController
     end
   end
 
-  def build_version_tags(cur_tags, prev_tags)
+  def external_tag_names_for snapshots
+    ids = snapshots.flatten.filter_map { _1['external_tag_id'] }.uniq
+    names = ExternalTag.where(id: ids).pluck(:id, :platform, :name).to_h { |id, platform, name|
+      [id, "#{ platform }:#{ name }"]
+    }
+    missing_ids = ids - names.keys
+
+    NicoTagVersion
+        .where(tag_id: missing_ids)
+        .order(:tag_id, version_no: :desc)
+        .pluck(:tag_id, :name)
+        .each { |id, name| names[id] ||= name }
+
+    names
+  end
+
+  def snapshot_tag_literals snapshots, external_tag_names
+    snapshots
+        .filter_map { |snapshot|
+          if snapshot.key?('tag_id')
+            [tag_category_order(snapshot['category']),
+             Post.tag_snapshot_literal(snapshot)]
+          elsif external_tag_names[snapshot['external_tag_id']]
+            [6, external_tag_names[snapshot['external_tag_id']]]
+          end
+        }
+        .sort_by { |order, name| [order, name.downcase] }
+        .map(&:second)
+  end
+
+  def tag_category_order category
+    ['deerjikist', 'meme', 'character', 'general', 'material', 'meta'].index(category) || 6
+  end
+
+  def build_version_tags cur_tags, prev_tags
     (cur_tags | prev_tags).map do |name|
       type =
-        if cur_tags.include?(name) && prev_tags.include?(name)
-          '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

@@ -49,7 +49,7 @@ class PostsController < ApplicationController
         filtered_posts
         .joins("LEFT JOIN (#{ pt_max_sql }) pt_max ON pt_max.post_id = posts.id")
         .reselect('posts.*', Arel.sql("#{ updated_at_all_sql } AS updated_at_all"))
-        .preload(:uploaded_user, :parents, :children,
+        .preload(:external_tags, :uploaded_user, :parents, :children,
                  post_tags: [:sections, { tag: [:deerjikists, :materials,
                                                 { tag_name: :wiki_page }] }])
         .with_attached_thumbnail
@@ -102,12 +102,14 @@ class PostsController < ApplicationController
   end
 
   def random
-    post = filtered_posts.preload(:uploaded_user, :parents, :children,
-                                  post_tags: [:sections, { tag: [:deerjikists, :materials,
-                                                                 { tag_name: :wiki_page }] }])
-                         .with_attached_thumbnail
-                         .order('RAND()')
-                         .first
+    post =
+        filtered_posts
+        .preload(:uploaded_user, :parents, :children,
+                 post_tags: [:sections, { tag: [:deerjikists, :materials,
+                                                { tag_name: :wiki_page }] }])
+        .with_attached_thumbnail
+        .order('RAND()')
+        .first
     return head :not_found unless post
 
     render json: PostRepr.base(post, current_user)
@@ -426,8 +428,20 @@ class PostsController < ApplicationController
     end
   end
 
-  def tagged_post_ids_for(name) =
-      Post.joins(tags: :tag_name).where(tag_names: { name: }).select(:id)
+  def tagged_post_ids_for(name)
+    posts_by_internal_tags =
+        Post
+        .joins(tags: :tag_name)
+        .where(tag_names: { name: })
+        .select(:id)
+
+    posts_by_external_tags =
+        Post
+        .joins(:external_tags)
+        .where("CONCAT(external_tags.platform, ':', external_tags.name) = ?", name)
+
+    (posts_by_internal_tags + posts_by_external_tags).uniq(&:id)
+  end
 
   def sync_post_tags! post, desired_tags, sections
     desired_tags.each do |t|
@@ -505,7 +519,10 @@ class PostsController < ApplicationController
       memo[tag_id] = TagRepr.inline(tag).merge(children:, sections:)
     end
 
-    root_ids.filter_map { |id| build_node.call(id, []) }
+    internal_tags = root_ids.filter_map { |id| build_node.call(id, []) }
+    external_tags =
+        post.external_tags.map { ExternalTagRepr.inline(_1).merge(children: [], sections: []) }
+    internal_tags + external_tags
   end
 
   def sibling_posts_by_parent parent_post_ids
@@ -653,7 +670,7 @@ class PostsController < ApplicationController
 
   def editable_tag_names_from_version version
     version.tags_json
-        .reject { _1.fetch('category') == 'nico' }
+        .select { _1.key?('tag_id') }
         .map { Post.tag_snapshot_literal(_1) }
         .sort
   end
@@ -671,7 +688,6 @@ class PostsController < ApplicationController
     post
         .post_tags
         .joins(tag: :tag_name)
-        .merge(Tag.not_nico)
         .merge(Tag.where(deprecated_at: nil))
         .includes(:sections, tag: :tag_name)
         .order('tag_names.name')
@@ -689,8 +705,7 @@ class PostsController < ApplicationController
                              tag_names:, video_ms_param:, duration_param:, parent_post_ids:
     validate_original_created_values!(original_created_from, original_created_before)
     Tag.normalise_tags!(tag_names, with_tagme: false, deny_deprecated: true,
-                                        with_sections: true) =>
-        { 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:)
@@ -833,15 +848,12 @@ class PostsController < ApplicationController
                  original_created_from: snapshot[:original_created_from],
                  original_created_before: snapshot[:original_created_before])
 
-    Tag.normalise_tags!(snapshot[:tag_names], with_tagme: false,
-                                               deny_deprecated: true,
-                                               with_sections: true) =>
-        { tags: editable_tags, sections: }
-    TagVersioning.record_tag_snapshots!(editable_tags, created_by_user: current_user)
+    Tag.normalise_tags!(snapshot[:tag_names],
+                        with_tagme: false,
+                        deny_deprecated: true,
+                        with_sections: true) => { tags:, sections: }
+    TagVersioning.record_tag_snapshots!(tags, created_by_user: current_user)
 
-    readonly_tags = post.tags.nico.to_a
-
-    tags = readonly_tags + editable_tags
     tags = Tag.expand_parent_tags(tags).reject(&:deprecated?)
 
     post.video_ms = tags.any? { _1.id == Tag.video.id } ? snapshot[:video_ms] : nil

backend/app/controllers/tag_children_controller.rb

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

backend/app/controllers/tags_controller.rb

@@ -34,49 +34,155 @@ class TagsController < ApplicationController
 
     offset = (page - 1) * limit
 
-    q =
+    tags =
         if post_id.present?
-          Tag.joins(:posts, :tag_name)
+          Tag.joins(:posts, :tag_name).where(posts: { id: post_id })
         else
           Tag.joins(:tag_name)
         end
-        .includes(:tag_name, :materials, tag_name: :wiki_page)
-    q = q.where(posts: { id: post_id }) if post_id.present?
 
-    q = q.where('tag_names.name LIKE ?', "%#{ name }%") if name
-    q = q.where(category:) if category
-    q = q.where('tags.post_count >= ?', post_count_between[0]) if post_count_between[0]
-    q = q.where('tags.post_count <= ?', post_count_between[1]) if post_count_between[1]
-    q = q.where('tags.created_at >= ?', created_between[0]) if created_between[0]
-    q = q.where('tags.created_at <= ?', created_between[1]) if created_between[1]
-    q = q.where('tags.updated_at >= ?', updated_between[0]) if updated_between[0]
-    q = q.where('tags.updated_at <= ?', updated_between[1]) if updated_between[1]
-    if deprecated_given
-      q = deprecated ? q.where.not(deprecated_at: nil) : q.where(deprecated_at: nil)
+    external_tags =
+        if post_id.present?
+          ExternalTag.joins(:posts).where(posts: { id: post_id })
+        else
+          ExternalTag.all
+        end
+
+    if name
+      tags = tags.where('tag_names.name LIKE ?', "%#{ name }%")
+      external_tags =
+          external_tags.where("CONCAT(external_tags.platform, ':', external_tags.name) LIKE ?",
+                              "%#{ name }%")
     end
 
+    if category == 'nico'
+      tags = tags.none
+    elsif category
+      tags = tags.where(category:)
+      external_tags = external_tags.none
+    end
+
+    if post_count_between[0]
+      tags = tags.where('tags.post_count >= ?', post_count_between[0])
+      external_tags = external_tags.where('external_tags.post_count >= ?', post_count_between[0])
+    end
+
+    if post_count_between[1]
+      tags = tags.where('tags.post_count <= ?', post_count_between[1])
+      external_tags = external_tags.where('external_tags.post_count <= ?', post_count_between[1])
+    end
+
+    if created_between[0]
+      tags = tags.where('tags.created_at >= ?', created_between[0])
+      external_tags = external_tags.where('external_tags.created_at >= ?', created_between[0])
+    end
+
+    if created_between[1]
+      tags = tags.where('tags.created_at <= ?', created_between[1])
+      external_tags = external_tags.where('external_tags.created_at <= ?', created_between[1])
+    end
+
+    if updated_between[0]
+      tags = tags.where('tags.updated_at >= ?', updated_between[0])
+      external_tags = external_tags.where('external_tags.created_at >= ?', updated_between[0])
+    end
+
+    if updated_between[1]
+      tags = tags.where('tags.updated_at <= ?', updated_between[1])
+      external_tags = external_tags.where('external_tags.created_at <= ?', updated_between[1])
+    end
+
+    if deprecated_given
+      if deprecated
+        tags = tags.where.not(deprecated_at: nil)
+        external_tags = external_tags.none
+      else
+        tags = tags.where(deprecated_at: nil)
+      end
+    end
+
+    tag_sql = tags.select("'tag' AS source",
+                          'tags.id',
+                          'tag_names.name',
+                          'tags.category',
+                          'tags.post_count',
+                          'tags.created_at',
+                          'tags.updated_at',
+                          'tags.deprecated_at').to_sql
+
+    external_tag_sql = external_tags.select(
+        "'external_tag' AS source",
+        'external_tags.id',
+        "CONCAT(external_tags.platform, ':', external_tags.name) AS name",
+        "'nico' AS category",
+        'external_tags.post_count',
+        'external_tags.created_at',
+        'external_tags.created_at AS updated_at',
+        'NULL AS deprecated_at').to_sql
+
+    union_sql = "#{ tag_sql } UNION ALL #{ external_tag_sql }"
+
     sort_sql =
-        case order[0]
-        when 'name'
-          'tag_names.name'
-        when 'category'
-          'CASE tags.category ' +
+        if order[0] == 'category'
+          'CASE category ' +
           "WHEN 'deerjikist' THEN 0 " +
           "WHEN 'meme' THEN 1 " +
           "WHEN 'character' THEN 2 " +
           "WHEN 'general' THEN 3 " +
           "WHEN 'material' THEN 4 " +
           "WHEN 'meta' THEN 5 " +
-          "WHEN 'nico' THEN 6 END"
+          "WHEN 'nico' THEN 6 " +
+          'END'
         else
-          "tags.#{ order[0] }"
+          order[0]
         end
-    tags = q.order(Arel.sql("#{ sort_sql } #{ order[1] }, tags.id #{ order[1] }"))
-            .limit(limit)
-            .offset(offset)
-            .to_a
 
-    render json: { tags: TagRepr.many(tags), count: q.size }
+    connection = ApplicationRecord.connection
+
+    count = connection.select_value(<<~SQL)
+      SELECT
+        COUNT(0)
+      FROM
+        (#{ union_sql }) legacy_tags
+    SQL
+
+    rows = connection.select_all(<<~SQL).to_a
+      SELECT
+        source
+      , id
+      FROM
+        (#{ union_sql }) legacy_tags
+      ORDER BY
+        #{ sort_sql } #{ order[1] }
+      , id #{ order[1] }
+      , source #{ order[1] }
+      LIMIT
+        #{ limit }
+      OFFSET
+        #{ offset }
+    SQL
+
+    tag_ids = rows.filter { _1['source'] == 'tag' }.map { _1['id'] }
+    external_tag_ids = rows.filter { _1['source'] == 'external_tag' }.map { _1['id'] }
+
+    tags_by_id =
+        Tag
+        .joins(:tag_name)
+        .includes(:tag_name, :materials, tag_name: :wiki_page)
+        .where(id: tag_ids)
+        .index_by(&:id)
+    external_tags_by_id =
+        ExternalTag
+        .where(id: external_tag_ids)
+        .index_by(&:id)
+
+    render json: { tags: rows.map { |row|
+      if row['source'] == 'tag'
+        TagRepr.base(tags_by_id.fetch(row['id']))
+      else
+        external_tag_json(external_tags_by_id.fetch(row['id']))
+      end
+    }, count: }
   end
 
   def with_depth
@@ -100,13 +206,14 @@ class TagsController < ApplicationController
 
   def autocomplete
     q = params[:q].to_s.strip.sub(/\Anot:/i, '')
+    prefix = "#{ ActiveRecord::Base.sanitize_sql_like(q) }%"
 
     with_nico = bool?(:nico, default: true)
     present_only = bool?(:present, default: true)
 
     alias_rows =
         TagName
-        .where('name LIKE ?', "#{ q }%")
+        .where('name LIKE ?', prefix)
         .where.not(canonical_id: nil)
         .pluck(:canonical_id, :name)
 
@@ -118,29 +225,43 @@ class TagsController < ApplicationController
       matched_alias_by_tag_name_id[canonical_id] ||= alias_name
     end
 
-    base = Tag.joins(:tag_name)
-              .includes(:tag_name, :materials, tag_name: :wiki_page)
-              .where(deprecated_at: nil)
+    base =
+        Tag
+        .joins(:tag_name)
+        .includes(:tag_name, :materials, tag_name: :wiki_page)
+        .where(deprecated_at: nil)
     base = base.where('tags.post_count > 0') if present_only
 
-    canonical_hit =
-        base
-        .where(((with_nico ? '(tags.category = ? AND tag_names.name LIKE ?) OR ' : '') +
-                'tag_names.name LIKE ?'),
-               *(with_nico ? ['nico', "nico:#{ q }%"] : []), "#{ q }%")
+    canonical_hit = base.where('tag_names.name LIKE ?', prefix)
 
-    tags =
-        if canonical_ids.present?
-          canonical_hit.or(base.where(tag_name_id: canonical_ids.uniq))
-        else
-          canonical_hit
-        end
+    internal_tags = canonical_hit.or(base.where(tag_name_id: canonical_ids.uniq))
 
-    tags = tags.order(Arel.sql('post_count DESC, tag_names.name')).limit(20).to_a
+    internal_rows =
+        internal_tags
+        .order(Arel.sql('tags.post_count DESC, tag_names.name'))
+        .limit(20)
+        .map { |tag|
+          TagRepr.base(tag).merge(matched_alias: matched_alias_by_tag_name_id[tag.tag_name_id])
+        }
 
-    render json: tags.map { |tag|
-      TagRepr.base(tag).merge(matched_alias: matched_alias_by_tag_name_id[tag.tag_name_id])
-    }
+    return render json: internal_rows unless with_nico
+
+    external_base = ExternalTag.all
+    external_base = external_base.where('post_count > 0') if present_only
+
+    external_rows =
+        external_base
+        .where("CONCAT(platform, ':', name) LIKE ? OR name LIKE ?", prefix, prefix)
+        .order(post_count: :desc, name: :asc)
+        .limit(20)
+        .map { external_tag_json(_1) }
+
+    rows =
+        (internal_rows + external_rows)
+        .sort_by { |row| [-row['post_count'], row['name']] }
+        .first(20)
+
+    render json: rows
   end
 
   def show
@@ -158,14 +279,20 @@ class TagsController < ApplicationController
     name = params[:name].to_s.strip
     return render_bad_request('name は必須です．') if name.blank?
 
-    tag = Tag.joins(:tag_name)
-             .includes(:tag_name, :materials, tag_name: :wiki_page)
-             .find_by(tag_names: { name: })
-    if tag
-      render json: TagRepr.base(tag)
-    else
-      head :not_found
-    end
+    tag =
+        Tag
+        .joins(:tag_name)
+        .includes(:tag_name, :materials, tag_name: :wiki_page)
+        .find_by(tag_names: { name: })
+    return render json: TagRepr.base(tag) if tag
+
+    platform, external_name = name.split(':', 2)
+    return head :not_found unless external_name
+
+    external_tag = ExternalTag.find_by(platform:, name: external_name)
+    return head :not_found unless external_tag
+
+    render json: ExternalTagRepr.base(external_tag)
   end
 
   def deerjikists
@@ -282,11 +409,7 @@ class TagsController < ApplicationController
     parent_names = params[:parent_tags].to_s.split.uniq
     deprecated = bool?(:deprecated)
 
-    if tag.nico? && deprecated
-      return render_unprocessable_entity 'ニコタグは廃止できません．', field: :deprecated
-    end
-
-    if tag.nico? || category == 'nico'
+    if category == 'nico'
       return render_unprocessable_entity 'ニコタグは変更できません．', field: :category
     end
 
@@ -334,13 +457,9 @@ class TagsController < ApplicationController
 
     tag = Tag.find(params[:id])
 
-    if tag.nico? && deprecated_given && deprecated
-      return render_unprocessable_entity 'ニコタグは廃止できません．', field: :deprecated
-    end
-
     return unless validate_tag_rename(tag, name)
 
-    if tag.nico? || (category.present? && category == 'nico')
+    if category.present? && category == 'nico'
       return render_unprocessable_entity 'ニコタグは変更できません．', field: :category
     end
 
@@ -547,11 +666,6 @@ class TagsController < ApplicationController
   end
 
   def record_tag_version! tag, event_type:, created_by_user:, name_changed: false, wiki_page: nil
-    if tag.nico?
-      NicoTagVersionRecorder.record!(tag:, event_type:, created_by_user:)
-      return
-    end
-
     TagVersionRecorder.record!(tag:, event_type:, created_by_user:)
 
     return unless name_changed
@@ -661,8 +775,7 @@ class TagsController < ApplicationController
 
   def update_parent_tags! tag, parent_names
     parent_tags = Tag.normalise_tags!(parent_names, with_tagme: false,
-                                                    with_no_deerjikist: false,
-                                                    deny_nico: true)
+                                                    with_no_deerjikist: false)
 
     old_parent_tags = tag.parents.to_a
 
@@ -790,4 +903,20 @@ class TagsController < ApplicationController
 
     render_validation_error fields:
   end
+
+  def external_tag_json tag
+    { 'id' => tag.id,
+      'name' => "#{ tag.platform }:#{ tag.name }",
+      'category' => 'nico',
+      'deprecated_at' => nil,
+      'created_at' => tag.created_at,
+      'updated_at' => tag.created_at,
+      'post_count' => tag.post_count,
+      'matched_alias' => nil,
+      'aliases' => [],
+      'parents' => [],
+      'has_wiki' => false,
+      'material_id' => nil,
+      'has_deerjikists' => false }
+  end
 end

backend/app/models/external_tag.rb

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

backend/app/models/nico_tag_relation.rb

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

backend/app/models/nico_tag_version.rb

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

backend/app/models/post.rb

@@ -92,6 +92,9 @@ class Post < ApplicationRecord
            inverse_of: :parent_post
   has_many :children, through: :child_post_implications, source: :post
 
+  has_many :post_external_tags, dependent: :destroy
+  has_many :external_tags, through: :post_external_tags
+
   has_one_attached :thumbnail
 
   attribute :version_no, :integer, default: 1
@@ -150,19 +153,27 @@ class Post < ApplicationRecord
   end
 
   def snapshot_tags_json
-    post_tags
-        .joins(tag: :tag_name)
-        .includes(:sections, tag: :tag_name)
-        .order('tags.id')
-        .map do |pt|
-      { 'id' => pt.tag.id,
-        'version_no' => pt.tag.version_no,
-        'name' => pt.tag.name,
-        'category' => pt.tag.category,
-        'sections' => pt.sections.sort_by(&:begin_ms).map {
-          { 'begin_ms' => _1.begin_ms, 'end_ms' => _1.end_ms }
-        } }
-    end
+    tag_snapshots =
+        post_tags
+            .joins(tag: :tag_name)
+            .includes(:sections, tag: :tag_name)
+            .order('tags.id')
+            .map { |pt|
+          { 'tag_id' => pt.tag.id,
+            'version_no' => pt.tag.version_no,
+            'name' => pt.tag.name,
+            'category' => pt.tag.category,
+            'sections' => pt.sections.sort_by(&:begin_ms).map {
+                            { 'begin_ms' => _1.begin_ms, 'end_ms' => _1.end_ms }
+                          } }
+        }
+
+    external_tag_snapshots =
+        post_external_tags.order(:external_tag_id).map {
+          { 'external_tag_id' => _1.external_tag_id }
+        }
+
+    tag_snapshots + external_tag_snapshots
   end
 
   def self.section_literal section

backend/app/models/post_external_tag.rb

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

backend/app/models/tag.rb

@@ -28,11 +28,10 @@ class Tag < ApplicationRecord
   has_many :post_tags, inverse_of: :tag
   has_many :posts, through: :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
@@ -50,7 +49,6 @@ class Tag < ApplicationRecord
   has_many :materials
 
   has_many :tag_versions
-  has_many :nico_tag_versions
 
   belongs_to :tag_name
   delegate :wiki_page, to: :tag_name
@@ -65,17 +63,13 @@ class Tag < ApplicationRecord
                   character:   'character',
                   general:     'general',
                   material:    'material',
-                  nico:        'nico',
                   meta:        'meta'
 
   validates :category, presence: true, inclusion: { in: Tag.categories.keys }
 
-  validate :nico_tag_name_must_start_with_nico
+  validate :tag_name_mustnt_start_with_nico
   validate :tag_name_must_be_canonical
   validate :category_must_be_deerjikist_with_deerjikists
-  validate :nico_tags_cannot_be_deprecated
-
-  scope :nico_tags, -> { nico }
 
   CATEGORY_PREFIXES = {
       'general:'    => :general,
@@ -110,10 +104,9 @@ class Tag < ApplicationRecord
 
   def self.normalise_tags! tag_names, with_tagme: true,
                                       with_no_deerjikist: true,
-                                      deny_nico: true,
                                       deny_deprecated: false,
                                       with_sections: false
-    if deny_nico && tag_names.any? { |n| n.downcase.start_with?('nico:') }
+    if tag_names.any? { |n| n.downcase.start_with?('nico:') }
       raise NicoTagNormalisationError
     end
 
@@ -273,12 +266,8 @@ class Tag < ApplicationRecord
         TagVersioning.record!(source_tag, event_type: :discard, created_by_user:)
         source_tag.destroy!
 
-        if source_tag.nico?
-          source_tag_name.destroy!
-        else
-          source_tag_name.update_columns(canonical_id: target_tag.tag_name_id,
-                                         updated_at: Time.current)
-        end
+        source_tag_name.update_columns(canonical_id: target_tag.tag_name_id,
+                                       updated_at: Time.current)
 
         TagVersioning.record!(target_tag, event_type: :update, created_by_user:)
       end
@@ -305,11 +294,9 @@ class Tag < ApplicationRecord
 
   private
 
-  def nico_tag_name_must_start_with_nico
-    n = name.to_s
-    if ((nico? && !(n.downcase.start_with?('nico:'))) || 
-        (!(nico?) && n.downcase.start_with?('nico:')))
-      errors.add :name, 'ニコニコ・タグの命名規則に反してゐます．'
+  def tag_name_mustnt_start_with_nico
+    if name.to_s.downcase.start_with?('nico:')
+      errors.add :name, 'タグの命名規則に反してゐます．'
     end
   end
 
@@ -350,10 +337,4 @@ class Tag < ApplicationRecord
 
     total_s * 1_000 + match[:ms].to_s.ljust(3, '0')[0, 3].to_i
   end
-
-  def nico_tags_cannot_be_deprecated
-    if nico? && deprecated_at.present?
-      errors.add :deprecated_at, 'ニコタグは廃止できません．'
-    end
-  end
 end

backend/app/representations/external_tag_repr.rb

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

backend/app/representations/post_repr.rb

@@ -87,15 +87,24 @@ module PostRepr
   end
 
   def tag_json post
-    post
+    internal_tags =
+        post
         .post_tags
         .reject { _1.tag.deprecated? }
         .sort_by { _1.tag.name }
-        .map do |post_tag|
-      TagRepr.inline(post_tag.tag).merge(
-          'children' => [],
-          'sections' => post_tag.sections.as_json(only: [:begin_ms, :end_ms]))
-    end
+        .map { |post_tag|
+          TagRepr.inline(post_tag.tag).merge(
+              'children' => [],
+              'sections' => post_tag.sections.as_json(only: [:begin_ms, :end_ms]))
+        }
+
+    external_tags =
+        post
+        .external_tags
+        .sort_by { _1.name }
+        .map { ExternalTagRepr.base(_1).merge('children' => [], 'sections' => []) }
+
+    internal_tags + external_tags
   end
 
   def thumbnail_url post, host: nil

backend/app/services/nico_tag_version_recorder.rb

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

backend/app/services/post_create_plan.rb

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

backend/app/services/tag_versioning.rb

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

backend/app/services/version_recorder.rb

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

backend/db/migrate/20260921040000_create_external_tags.rb

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

backend/db/migrate/20260921050000_create_post_external_tags.rb

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

backend/db/migrate/20260921060000_change_foreign_key_on_nico_tag_relations.rb

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

backend/db/migrate/20260921230000_migrate_external_tags.rb

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

backend/db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
+ActiveRecord::Schema[8.0].define(version: 2026_09_21_230000) do
   create_table "active_storage_attachments", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "name", null: false
     t.string "record_type", null: false
@@ -48,6 +48,14 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
     t.index ["tag_id"], name: "index_deerjikists_on_tag_id"
   end
 
+  create_table "external_tags", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.string "platform", limit: 16, null: false
+    t.string "name", null: false
+    t.integer "post_count", default: 0, null: false
+    t.datetime "created_at", null: false
+    t.index ["platform", "name"], name: "index_external_tags_on_platform_and_name", unique: true
+  end
+
   create_table "gekanator_ai_runs", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "model", null: false
     t.integer "input_tokens", default: 0, null: false
@@ -281,6 +289,13 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) 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
@@ -352,7 +367,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
     t.check_constraint "(`video_ms` is null) or (`video_ms` > 0)", name: "chk_post_versions_video_ms_positive"
     t.check_constraint "`event_type` in (_utf8mb4'create',_utf8mb4'update',_utf8mb4'discard',_utf8mb4'restore')", name: "post_versions_event_type_valid"
     t.check_constraint "`version_no` > 0", name: "post_versions_version_no_positive"
-    t.check_constraint "json_schema_valid(_utf8mb4'{\"type\":\"array\",\"items\":{\"type\":\"object\",\"properties\":{\"id\":{\"type\":\"integer\",\"minimum\":1},\"version_no\":{\"type\":\"integer\",\"minimum\":1},\"name\":{\"type\":\"string\",\"minLength\":1},\"category\":{\"type\":\"string\",\"enum\":[\"deerjikist\",\"meme\",\"character\",\"general\",\"material\",\"meta\",\"nico\"]},\"sections\":{\"type\":\"array\",\"items\":{\"type\":\"object\",\"properties\":{\"begin_ms\":{\"type\":\"integer\",\"minimum\":0},\"end_ms\":{\"type\":[\"integer\",\"null\"],\"minimum\":0}},\"required\":[\"begin_ms\",\"end_ms\"],\"additionalProperties\":false}}},\"required\":[\"id\",\"version_no\",\"name\",\"category\",\"sections\"],\"additionalProperties\":false}}',`tags_json`)", name: "chk_post_versions_tags_json_schema"
+    t.check_constraint "json_schema_valid(_utf8mb4'{\"type\":\"array\",\"items\":{\"oneOf\":[{\"type\":\"object\",\"properties\":{\"tag_id\":{\"type\":\"integer\",\"minimum\":1},\"version_no\":{\"type\":\"integer\",\"minimum\":1},\"name\":{\"type\":\"string\",\"minLength\":1},\"category\":{\"type\":\"string\",\"enum\":[\"deerjikist\",\"meme\",\"character\",\"general\",\"material\",\"meta\"]},\"sections\":{\"type\":\"array\",\"items\":{\"type\":\"object\",\"properties\":{\"begin_ms\":{\"type\":\"integer\",\"minimum\":0},\"end_ms\":{\"type\":[\"integer\",\"null\"],\"minimum\":0}},\"required\":[\"begin_ms\",\"end_ms\"],\"additionalProperties\":false}}},\"required\":[\"tag_id\",\"version_no\",\"sections\"],\"additionalProperties\":false},{\"type\":\"object\",\"properties\":{\"external_tag_id\":{\"type\":\"integer\",\"minimum\":1}},\"required\":[\"external_tag_id\"],\"additionalProperties\":false}]}}',`tags_json`)", name: "chk_post_versions_tags_json_schema"
   end
 
   create_table "posts", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
@@ -700,9 +715,11 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
   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", "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"

backend/lib/tasks/sync_nico.rake

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

backend/spec/db/external_tags_spec.rb

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

backend/spec/factories/external_tags.rb

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

backend/spec/factories/tags.rb

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

backend/spec/models/external_tag_spec.rb

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

backend/spec/models/nico_tag_relation_spec.rb

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

backend/spec/models/post_external_tag_spec.rb

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

backend/spec/models/post_spec.rb

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

backend/spec/models/tag_spec.rb

@@ -1,6 +1,29 @@
 require 'rails_helper'
 
 RSpec.describe Tag, type: :model do
+  describe 'external tag separation' do
+    ['nico:reserved', 'NiCo:reserved'].each do |name|
+      it "rejects the reserved prefix #{ name } for internal tags" do
+        tag = build(:tag, name:)
+
+        expect(tag).to be_invalid
+        expect(tag.errors[:name]).to be_present
+        expect {
+          described_class.normalise_tags!([name])
+        }.to raise_error(Tag::NicoTagNormalisationError)
+      end
+    end
+
+    it 'finds external links by the internal tag id even when ids differ' do
+      tag = create(:tag)
+      external = create(:external_tag, id: tag.id + 10_000)
+      # The migration retains existing links without running model validation.
+      NicoTagRelation.insert_all!([{ tag_id: tag.id, nico_tag_id: external.id }])
+
+      expect(tag.linked_nico_tags).to contain_exactly(external)
+    end
+  end
+
   describe '.normalise_tags!' do
     it 'rejects deprecated tags when deny_deprecated is enabled' do
       tag_name = TagName.create!(name: 'normalise deprecated tag')
@@ -159,20 +182,6 @@ RSpec.describe Tag, type: :model do
     end
   end
 
-  describe 'deprecated validation' do
-    it 'rejects deprecated nico tags' do
-      tag = build(
-        :tag,
-        name: 'nico:deprecated_validation',
-        category: :nico,
-        deprecated_at: Time.current
-      )
-
-      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
-
   describe '.find_or_create_by_tag_name!' do
     it 'creates a tag and name with the requested category after stripping whitespace' do
       tag = nil
@@ -290,7 +299,7 @@ RSpec.describe Tag, type: :model do
     it 'deletes source relationships while preserving unrelated relationships' do
       parent = create(:tag)
       child = create(:tag)
-      nico_tag = create(:tag, :nico)
+      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)
@@ -412,48 +421,6 @@ RSpec.describe Tag, type: :model do
       end
     end
 
-    context 'when merging a nico source tag' do
-      let!(:target_tag) do
-        create(:tag, category: :nico, tag_name: create(:tag_name, name: 'nico:foo'))
-      end
-      let!(:source_tag) do
-        create(:tag, category: :nico, tag_name: create(:tag_name, name: 'nico:bar'))
-      end
-      let!(:source_tag_name_id) { source_tag.tag_name_id }
-
-      it 'deletes the source tag and name instead of keeping an alias' do
-        described_class.merge_tags!(target_tag, [source_tag])
-
-        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
-        expect(TagName.unscoped.exists?(source_tag_name_id)).to be(false)
-        expect(target_tag.reload.post_count).to eq(0)
-      end
-
-      it 'keeps nico history while deleting source links and allows recreating the name' do
-        linked_tag = create(:tag)
-        NicoTagRelation.create!(nico_tag: source_tag, tag: linked_tag)
-        kept_relation = NicoTagRelation.create!(nico_tag: target_tag, tag: linked_tag)
-        user = create_member_user!
-        source_name = source_tag.name
-
-        described_class.merge_tags!(target_tag, [source_tag], created_by_user: user)
-
-        expect(NicoTagRelation.all).to contain_exactly(kept_relation)
-        versions = NicoTagVersion.where(tag_id: source_tag.id).order(:version_no)
-        expect(versions.pluck(:version_no, :event_type))
-            .to eq([[1, 'create'], [2, 'discard']])
-        expect(versions.last).to have_attributes(
-            name: source_name, linked_tags: linked_tag.name, created_by_user: user)
-
-        recreated = described_class.find_or_create_by_tag_name!(source_name, category: :nico)
-
-        expect(recreated.id).not_to eq(source_tag.id)
-        expect(recreated.tag_name_id).not_to eq(source_tag_name_id)
-        expect(recreated.nico_tag_versions).to be_empty
-        expect(versions.reload.size).to eq(2)
-      end
-    end
-
     def snapshot_tags(post)
       post.snapshot_tag_names.join(' ')
     end
@@ -499,8 +466,8 @@ RSpec.describe Tag, type: :model do
         expect(latest.event_type).to eq('update')
         expect(latest.created_by_user).to be_nil
         expect(latest.tags).to eq(snapshot_tags(post_record.reload))
-        expect(latest.tags_json.map { |item| item.fetch('id') }).to eq([target_tag.id])
-        expect(affected_versions.first.tags_json.map { |item| item.fetch('id') })
+        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)

backend/spec/models/version_record_spec.rb

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

backend/spec/requests/nico_tags_spec.rb

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

backend/spec/requests/posts_spec.rb

@@ -17,7 +17,7 @@ RSpec.describe 'Posts API', type: :request do
   end
 
   def create_nico_tag!(name)
-    Tag.find_or_create_by_tag_name!(name, category: :nico)
+    ExternalTag.find_or_create_by!(platform: :nico, name: name.delete_prefix('nico:'))
   end
 
   def dummy_upload
@@ -93,6 +93,26 @@ RSpec.describe 'Posts API', type: :request do
     count
   end
 
+  def expect_external_tag_json tag_json, external_tag
+    external_tag.reload
+
+    expect(tag_json).to include(
+        'id' => external_tag.id,
+        'name' => "#{ external_tag.platform }:#{ external_tag.name }",
+        'category' => 'nico',
+        'created_at' => external_tag.created_at.as_json,
+        'updated_at' => external_tag.created_at.as_json,
+        'deprecated_at' => nil,
+        'aliases' => [],
+        'parents' => [],
+        'post_count' => external_tag.post_count,
+        'has_wiki' => false,
+        'material_id' => nil,
+        'has_deerjikists' => false,
+        'children' => [],
+        'sections' => [])
+  end
+
   let!(:tag_name) { TagName.create!(name: 'spec_tag') }
   let!(:tag)      { Tag.create!(tag_name: tag_name, category: :general) }
 
@@ -233,6 +253,93 @@ 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) }
@@ -612,6 +719,30 @@ RSpec.describe 'Posts API', type: :request do
         expect(json.fetch('count')).to eq(2)
       end
     end
+
+    it 'returns internal and external tags with colliding ids in the legacy tags array' do
+      external_tag = create(:external_tag, id: tag.id, name: 'post_index_external')
+      PostExternalTag.create!(post: hit_post, external_tag:)
+
+      get '/posts'
+
+      expect(response).to have_http_status(:ok)
+
+      post_json =
+          json
+          .fetch('posts')
+          .find { _1.fetch('id') == hit_post.id }
+
+      external_json =
+          post_json
+          .fetch('tags')
+          .find { _1['name'] == 'nico:post_index_external' }
+
+      expect(post_json.fetch('tags')).to include(
+          a_hash_including('id' => tag.id, 'name' => tag.name, 'category' => 'general'))
+      expect(external_json).not_to be_nil
+      expect_external_tag_json(external_json, external_tag)
+    end
   end
 
   describe 'GET /posts/:id' do
@@ -769,6 +900,25 @@ RSpec.describe 'Posts API', type: :request do
         expect(response).to have_http_status(:ok)
         expect(query_count).to be <= 45
       end
+
+      it 'returns external tags as root nodes in the legacy tag tree' do
+        external_tag = create(:external_tag, id: tag.id, name: 'post_detail_external')
+        PostExternalTag.create!(post: post_record, external_tag:)
+
+        request
+
+        expect(response).to have_http_status(:ok)
+
+        expect(json.fetch('tags')).to include(
+            a_hash_including('id' => tag.id, 'name' => tag.name, 'category' => 'general'))
+        external_json =
+            json
+            .fetch('tags')
+            .find { _1['name'] == 'nico:post_detail_external' }
+
+        expect(external_json).not_to be_nil
+        expect_external_tag_json(external_json, external_tag)
+      end
     end
 
     context 'when post does not exist' do
@@ -1250,9 +1400,9 @@ RSpec.describe 'Posts API', type: :request do
       )
     end
 
-    context 'when nico tag already exists in tags' do
+    context 'when the external nico tag already exists' do
       before do
-        Tag.find_or_create_by_tag_name!('nico:nico_tag', category: :nico)
+        create(:external_tag, name: 'nico_tag')
       end
 
       it 'returns 422 with tag field errors' do
@@ -1530,9 +1680,9 @@ RSpec.describe 'Posts API', type: :request do
 
       versions = post_record.post_versions.order(:version_no)
       expect(versions.first.tags_json).to include(
-          a_hash_including('id' => tag.id,
+          a_hash_including('tag_id' => tag.id,
                            'sections' => [{ 'begin_ms' => 1000, 'end_ms' => 2000 }]))
-      expect(versions.last.tags_json.map { |item| item.fetch('id') })
+      expect(versions.last.tags_json.map { |item| item.fetch('tag_id') })
           .not_to include(tag.id)
     end
 
@@ -1551,7 +1701,7 @@ RSpec.describe 'Posts API', type: :request do
       expect(PostTag.find_by!(post: post_record, tag:).created_user).to eq(member)
       expect(tag.reload.post_count).to eq(1)
       snapshots = post_record.post_versions.order(:version_no).map do |version|
-        version.tags_json.map { |item| item.fetch('id') }
+        version.tags_json.map { |item| item.fetch('tag_id') }
       end
       expect(snapshots.map { |ids| ids.include?(tag.id) }).to eq([true, false, true])
     end
@@ -1576,9 +1726,9 @@ RSpec.describe 'Posts API', type: :request do
       )
     end
 
-    context 'when nico tag already exists in tags' do
+    context 'when the external nico tag already exists' do
       before do
-        Tag.find_or_create_by_tag_name!('nico:nico_tag', category: :nico)
+        create(:external_tag, name: 'nico_tag')
       end
 
       it 'returns 422 with tag field errors' do
@@ -1895,7 +2045,7 @@ RSpec.describe 'Posts API', type: :request do
         base_version = create_post_version_for!(post_record.reload)
 
         nico_tag = create_nico_tag!('nico:optimistic_lock_nico')
-        PostTag.create!(post: post_record, tag: nico_tag, created_user: member)
+        PostExternalTag.create!(post: post_record, external_tag: nico_tag)
 
         PostVersionRecorder.record!(
             post: post_record.reload,
@@ -1915,14 +2065,14 @@ RSpec.describe 'Posts API', type: :request do
 
         expect(names).to include('spec_tag')
         expect(names).to include(Tag.no_deerjikist.name)
-        expect(names).to include(nico_tag.name)
+        expect(post_record.external_tags).to contain_exactly(nico_tag)
       end
 
       it 'keeps nico tags even when they are not included in PUT tags' do
         sign_in_as(member)
 
         nico_tag = create_nico_tag!('nico:readonly_update_nico')
-        PostTag.create!(post: post_record, tag: nico_tag, created_user: member)
+        PostExternalTag.create!(post: post_record, external_tag: nico_tag)
 
         base_version = create_post_version_for!(post_record.reload)
 
@@ -1937,7 +2087,15 @@ RSpec.describe 'Posts API', type: :request do
 
         expect(names).to include('spec_tag')
         expect(names).to include(Tag.no_deerjikist.name)
-        expect(names).to include(nico_tag.name)
+        expect(post_record.external_tags).to contain_exactly(nico_tag)
+
+        external_json =
+            json
+            .fetch('tags')
+            .find { _1['name'] == "nico:#{ nico_tag.name }" }
+
+        expect(external_json).not_to be_nil
+        expect_external_tag_json(external_json, nico_tag)
       end
 
       it 'allows non-nico tags linked from nico tags to be removed by normal post update' do
@@ -1947,7 +2105,7 @@ RSpec.describe 'Posts API', type: :request do
         linked_tag = Tag.find_or_create_by_tag_name!('relation_linked_tag', category: :general)
 
         NicoTagRelation.create!(nico_tag:, tag: linked_tag)
-        PostTag.create!(post: post_record, tag: nico_tag, created_user: member)
+        PostExternalTag.create!(post: post_record, external_tag: nico_tag)
         PostTag.create!(post: post_record, tag: linked_tag, created_user: member)
 
         base_version = create_post_version_for!(post_record.reload)
@@ -1961,7 +2119,7 @@ RSpec.describe 'Posts API', type: :request do
 
         names = post_record.reload.tags.map(&:name)
 
-        expect(names).to include(nico_tag.name)
+        expect(post_record.external_tags).to contain_exactly(nico_tag)
         expect(names).to include('spec_tag')
         expect(names).to include(Tag.no_deerjikist.name)
         expect(names).not_to include(linked_tag.name)
@@ -2180,6 +2338,88 @@ RSpec.describe 'Posts API', type: :request do
       expect(first.fetch('created_at')).to eq(t_v1.iso8601)
     end
 
+    context 'with external tag history' do
+      let(:external_id) { tag.id }
+      let(:external) { create(:external_tag, id: external_id) }
+      let(:external_post) { create(:post) }
+
+      before do
+        PostExternalTag.create!(post: external_post, external_tag: external)
+        PostVersionRecorder.record!(
+            post: external_post, event_type: :create, created_by_user: member)
+
+        unrelated_post = create(:post)
+        PostExternalTag.create!(post: unrelated_post, external_tag: create(:external_tag))
+        PostVersionRecorder.record!(
+            post: unrelated_post, event_type: :create, created_by_user: member)
+      end
+
+      it 'prefers Tag over ExternalTag for the legacy tag parameter' do
+        get '/posts/versions', params: { tag: tag.id }
+
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch('count')).to eq(3)
+        expect(json.fetch('versions').map { [_1.fetch('post_id'), _1.fetch('version_no')] })
+            .to contain_exactly(
+                [post_record.id, 1], [post_record.id, 2], [other_post_version.post_id, 1])
+      end
+
+      it 'explicitly filters ExternalTag even when its id collides with Tag' do
+        get '/posts/versions', params: { external_tag: external.id }
+
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch('count')).to eq(1)
+        expect(json.fetch('versions')).to contain_exactly(
+            a_hash_including('post_id' => external_post.id, 'version_no' => 1))
+      end
+
+      # Temporary compatibility shim until the frontend sends external_tag explicitly.
+      context 'with legacy tag fallback and no internal Tag with the external id' do
+        let(:external_id) { Tag.maximum(:id).to_i + 10_000 }
+
+        it 'falls back to ExternalTag for the legacy tag parameter' do
+          expect(Tag.exists?(external.id)).to be(false)
+
+          get '/posts/versions', params: { tag: external.id }
+
+          expect(response).to have_http_status(:ok)
+          expect(json.fetch('count')).to eq(1)
+          expect(json.fetch('versions')).to contain_exactly(
+              a_hash_including('post_id' => external_post.id, 'version_no' => 1))
+        end
+
+        [:tag, :external_tag].each do |parameter|
+          it "includes external removal history through the legacy API's #{ parameter }" do
+            external_post.post_external_tags.destroy_all
+            PostVersionRecorder.record!(
+                post: external_post.reload, event_type: :update, created_by_user: member)
+
+            get '/posts/versions', params: { parameter => external.id }
+
+            expect(response).to have_http_status(:ok)
+            expect(json.fetch('count')).to eq(2)
+            expect(json.fetch('versions')).to contain_exactly(
+                a_hash_including('post_id' => external_post.id, 'version_no' => 1),
+                a_hash_including('post_id' => external_post.id, 'version_no' => 2))
+          end
+        end
+      end
+    end
+
+    it 'can render history containing external identifiers' do
+      PostExternalTag.create!(post: post_record, external_tag: create(:external_tag))
+      post_record.update_columns(version_no: 2)
+      PostVersionRecorder.record!(post: post_record,
+                                  event_type: :update, created_by_user: member)
+
+      get '/posts/versions', params: { post: post_record.id }
+
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('count')).to eq(3)
+      expect(json.fetch('versions').first.fetch('tags')).to include(
+          'name' => tag2.name, 'type' => 'context')
+    end
+
     it 'filters versions by tag when the current snapshot includes the tag' do
       get '/posts/versions', params: { post: post_record.id, tag: tag2.id }
 

backend/spec/requests/tag_children_spec.rb

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

backend/spec/requests/tags_spec.rb

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

backend/spec/services/nico_tag_version_recorder_spec.rb

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

backend/spec/services/post_create_plan_spec.rb

@@ -50,6 +50,19 @@ RSpec.describe PostCreatePlan do
     expect(plan[:video_ms]).to eq(60_000)
   end
 
+  it 'rejects direct external tag input without creating internal records' do
+    create(:external_tag, name: 'reserved')
+    counts = [Tag.count, TagName.count, ExternalTag.count]
+
+    expect {
+      described_class.new(attributes: { tags: 'NiCo:reserved' }).build!
+    }.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/tasks/nico_sync_spec.rb

@@ -29,7 +29,7 @@ RSpec.describe 'nico:sync' do
 
     # 追加される linked tag を準備（nico tag に紐付く一般タグ）
     linked = create_tag!('spec_linked', category: 'general')
-    nico = create_tag!('nico:AAA', category: 'nico')
+    nico = create_external_tag!('AAA')
     link_nico_to_tag!(nico, linked)
 
     # bot / tagme は task 内で使うので作っておく（Tag.bot/tagme がある前提）
@@ -53,7 +53,7 @@ RSpec.describe 'nico:sync' do
     active_tag_names = post.tags.joins(:tag_name).pluck('tag_names.name')
 
     expect(active_tag_names).to include('spec_kept')
-    expect(active_tag_names).to include('nico:AAA')
+    expect(post.external_tags).to contain_exactly(nico)
     expect(active_tag_names).to include('spec_linked')
 
     expect(post.original_created_from).to eq(Time.iso8601('2026-01-01T03:34:00Z'))
@@ -115,12 +115,12 @@ RSpec.describe 'nico:sync' do
     )
 
     # 旧nicoタグ（今回の同期結果に含まれない）
-    old_nico = create_tag!('nico:OLD', category: 'nico')
-    PostTag.create!(post:, tag: old_nico)
+    old_nico = create_external_tag!('OLD')
+    PostExternalTag.create!(post:, external_tag: old_nico)
     create_post_version_for!(post)
 
     # 今回は NEW のみ欲しい
-    new_nico = create_tag!('nico:NEW', category: 'nico')
+    new_nico = create_external_tag!('NEW')
 
     # bot/tagme 念のため
     Tag.bot
@@ -131,23 +131,21 @@ RSpec.describe 'nico:sync' do
 
     run_rake_task('nico:sync')
 
-    expect(PostTag.exists?(post:, tag: old_nico)).to be(false)
+    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.map { |item| item.fetch('id') })
+    expect(versions.first.tags_json.filter_map { |item| item['external_tag_id'] })
         .to include(old_nico.id)
-    expect(versions.last.tags_json.map { |item| item.fetch('id') })
+    expect(versions.last.tags_json.filter_map { |item| item['external_tag_id'] })
         .to include(new_nico.id)
-    expect(versions.last.tags_json.map { |item| item.fetch('id') })
+    expect(versions.last.tags_json.filter_map { |item| item['external_tag_id'] })
         .not_to include(old_nico.id)
 
     # NEW は active にいる
     post.reload
-    active_names = post.tags.joins(:tag_name).pluck('tag_names.name')
-    expect(active_names).to include('nico:NEW')
-    expect(active_names).not_to include('nico:OLD')
+    expect(post.external_tags).to contain_exactly(new_nico)
   end
 
   def snapshot_tags(post)
@@ -213,7 +211,7 @@ RSpec.describe 'nico:sync' do
     create_post_version_for!(post)
 
     linked = create_tag!('spec_linked', category: 'general')
-    nico = create_tag!('nico:AAA', category: 'nico')
+    nico = create_external_tag!('AAA')
     link_nico_to_tag!(nico, linked)
 
     Tag.bot
@@ -241,7 +239,7 @@ RSpec.describe 'nico:sync' do
   end
 
   it '既存 post に差分が無いときは新しい version を作らない' do
-    nico = create_tag!('nico:AAA', category: 'nico')
+    nico = create_external_tag!('AAA')
     no_deerjikist = create_tag!('ニジラー情報不詳', category: 'meta')
 
     post = Post.create!(
@@ -252,7 +250,7 @@ RSpec.describe 'nico:sync' do
       original_created_before: Time.iso8601('2026-01-01T03:35:00Z')
     )
 
-    PostTag.create!(post: post, tag: nico)
+    PostExternalTag.create!(post:, external_tag: nico)
     PostTag.create!(post: post, tag: no_deerjikist)
     create_post_version_for!(post)
 
@@ -295,7 +293,7 @@ RSpec.describe 'nico:sync' do
       run_rake_task('nico:sync')
     }.to change(NicoTagVersion, :count).by(1)
 
-    nico_tag = Tag.joins(:tag_name).find_by!(tag_names: { name: 'nico:AAA' })
+    nico_tag = ExternalTag.find_by!(platform: :nico, name: 'AAA')
     version = nico_tag.nico_tag_versions.order(:version_no).last
 
     expect(version.version_no).to eq(1)
@@ -378,4 +376,148 @@ RSpec.describe 'nico:sync' do
     expect(versions.second.title).to eq('changed title')
     expect(versions.second.tags).to eq(snapshot_tags(post.reload))
   end
+
+  def create_external_tag!(name)
+    ExternalTag.create!(platform: :nico, name:)
+  end
+
+  def create_nico_sync_post!
+    post = Post.create!(
+      title: 't',
+      url: 'https://www.nicovideo.jp/watch/sm9',
+      uploaded_user: nil
+    )
+
+    PostTag.create!(post:, tag: Tag.no_deerjikist)
+
+    post
+  end
+
+  def run_nico_sync_with_tags! tags
+    stub_python([{
+      'code' => 'sm9',
+      'title' => 't',
+      'tags' => tags,
+      'user' => nil
+    }])
+
+    allow(URI).to receive(:open).and_return(StringIO.new('<html></html>'))
+
+    run_rake_task('nico:sync')
+  end
+
+  it '外部タグだけの変更では bot を付けず，投稿履歴を記録する' do
+    post = create_nico_sync_post!
+    PostVersionRecorder.record!(post:, event_type: :create, created_by_user: nil)
+
+    expect {
+      run_nico_sync_with_tags!(['raw tag[]', 'raw tag[]'])
+    }.to change(PostVersion, :count).by(1)
+        .and change(ExternalTag, :count).by(1)
+        .and change(PostExternalTag, :count).by(1)
+        .and change(NicoTagVersion, :count).by(1)
+        .and change(TagName, :count).by(0)
+
+    external = post.external_tags.sole
+    expect(external.name).to eq('raw tag[]')
+    expect(post.tags.map(&:name)).not_to include('bot操作')
+    expect(post.post_versions.order(:version_no).last.tags_json)
+        .to include('external_tag_id' => external.id)
+
+    expect {
+      run_nico_sync_with_tags!(['raw tag[]'])
+    }.to change(PostVersion, :count).by(0).and change(NicoTagVersion, :count).by(0)
+
+    expect {
+      run_nico_sync_with_tags!([])
+    }.to change(PostVersion, :count).by(1)
+    expect(post.reload.external_tags).to be_empty
+    expect(post.tags.map(&:name)).not_to include('bot操作')
+  end
+
+  it '外部タグが新規記載されたとき，その時点の連携タグを記載する' do
+    post = create_nico_sync_post!
+
+    external_tag = create_external_tag!('AAA')
+    linked_tag = create_tag!('spec_linked', category: :general)
+    link_nico_to_tag!(external_tag, linked_tag)
+
+    run_nico_sync_with_tags!(['AAA'])
+
+    expect(post.reload.external_tags).to include(external_tag)
+    expect(post.tags).to include(linked_tag)
+  end
+
+  it '外部タグに差分がない場合，内外マッピングが変はっても連携タグを再評価しない' do
+    post = create_nico_sync_post!
+
+    external_tag = create_external_tag!('AAA')
+    old_linked_tag = create_tag!('spec_old_linked', category: :general)
+    new_linked_tag = create_tag!('spec_new_linked', category: :general)
+
+    relation = link_nico_to_tag!(external_tag, old_linked_tag)
+
+    run_nico_sync_with_tags!(['AAA'])
+
+    expect(post.reload.tags).to include(old_linked_tag)
+
+    # 人手で連携タグを消除する．
+    PostTag.find_by!(post:, tag: old_linked_tag).destroy!
+
+    # 内外マッピングを変更する．
+    relation.destroy!
+    link_nico_to_tag!(external_tag, new_linked_tag)
+
+    # 外部タグ自体には差分が無い．
+    run_nico_sync_with_tags!(['AAA'])
+
+    post.reload
+
+    expect(post.external_tags).to include(external_tag)
+    expect(post.tags).not_to include(old_linked_tag)
+    expect(post.tags).not_to include(new_linked_tag)
+  end
+
+  it '外部タグの消除では連携タグを消除せず，再記載時にその時点の内外マッピングを適用する' do
+    post = create_nico_sync_post!
+
+    external_tag = create_external_tag!('AAA')
+    old_linked_tag = create_tag!('spec_old_linked', category: :general)
+    new_linked_tag = create_tag!('spec_new_linked', category: :general)
+
+    relation = link_nico_to_tag!(external_tag, old_linked_tag)
+
+    # 外部タグを新規記載する．
+    run_nico_sync_with_tags!(['AAA'])
+
+    post.reload
+    expect(post.external_tags).to include(external_tag)
+    expect(post.tags).to include(old_linked_tag)
+
+    # 外部タグを消除する．
+    run_nico_sync_with_tags!([])
+
+    post.reload
+    expect(post.external_tags).not_to include(external_tag)
+
+    # 外部タグの消除によって連携タグまでは消除されない．
+    expect(post.tags).to include(old_linked_tag)
+
+    # 外部タグが記載されてゐない間に内外マッピングを変更する．
+    relation.destroy!
+    link_nico_to_tag!(external_tag, new_linked_tag)
+
+    # 同じ外部タグを再記載する．
+    run_nico_sync_with_tags!(['AAA'])
+
+    post.reload
+
+    expect(post.external_tags).to include(external_tag)
+
+    # 旧連携タグは自動的には消除されない．
+    expect(post.tags).to include(old_linked_tag)
+
+    # 再記載時点の内外マッピングが新たに適用される．
+    expect(post.tags).to include(new_linked_tag)
+  end
 end

frontend/src/components/DraggableDroppableTagRow.test.tsx

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

frontend/src/components/TagDetailSidebar.test.tsx

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

frontend/src/components/TagLink.test.tsx

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

frontend/src/lib/posts.test.ts

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

frontend/src/lib/prefetchers.test.ts

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

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

@@ -0,0 +1,44 @@
+import { waitFor } from '@testing-library/react'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+
+import PostHistoryPage from '@/pages/posts/PostHistoryPage'
+import { renderWithProviders } from '@/test/render'
+
+const postsApi = vi.hoisted (() => ({
+  fetchPostChanges: vi.fn (),
+  updatePost:       vi.fn (),
+}))
+
+const tagsApi = vi.hoisted (() => ({
+  fetchTag: vi.fn (),
+}))
+
+vi.mock ('@/lib/posts', () => postsApi)
+vi.mock ('@/lib/tags', () => tagsApi)
+
+describe ('PostHistoryPage', () => {
+  beforeEach (() => {
+    vi.clearAllMocks ()
+    postsApi.fetchPostChanges.mockResolvedValue ({
+      versions: [],
+      count:    0,
+    })
+  })
+
+  it ('filters by external_tag without resolving the id as an internal tag', async () => {
+    renderWithProviders (
+        <PostHistoryPage/>,
+        { route: '/posts/changes?external_tag=7' },
+    )
+
+    await waitFor (() => {
+      expect (postsApi.fetchPostChanges).toHaveBeenCalledWith ({
+        externalTag: '7',
+        page:        1,
+        limit:       20,
+      })
+    })
+
+    expect (tagsApi.fetchTag).not.toHaveBeenCalled ()
+  })
+})

frontend/src/pages/tags/TagListPage.test.tsx

@@ -1,4 +1,4 @@
-import { fireEvent, screen, waitFor } from '@testing-library/react'
+import { fireEvent, screen, waitFor, within } from '@testing-library/react'
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 
 import TagListPage from '@/pages/tags/TagListPage'
@@ -127,4 +127,49 @@ describe ('TagListPage', () => {
       )
     })
   })
+
+  it ('keeps colliding internal and external tags on their own routes', async () => {
+    const internal = buildTag ({
+      id:       7,
+      name:     'internal_collision',
+      category: 'general',
+    })
+    const external = buildTag ({
+      id:       7,
+      name:     'nico:external_collision',
+      category: 'nico',
+    })
+
+    tagsApi.fetchTags.mockResolvedValueOnce ({
+      tags:  [internal, external],
+      count: 2,
+    })
+
+    renderWithProviders (<TagListPage/>, { route: '/tags' })
+
+    const internalLink =
+	await screen.findByRole ('link', { name: 'internal_collision' })
+    const externalLink =
+	screen.getByRole ('link', { name: 'nico:external_collision' })
+
+    expect (internalLink).toHaveAttribute ('href', '/tags/7')
+    expect (externalLink).toHaveAttribute (
+	'href',
+	'/tags/nico?name=nico%3Aexternal_collision',
+    )
+
+    const internalRow = internalLink.closest ('tr')
+    const externalRow = externalLink.closest ('tr')
+
+    expect (internalRow).not.toBeNull ()
+    expect (externalRow).not.toBeNull ()
+
+    expect (
+	within (internalRow!).getByRole ('link', { name: '耕作履歴' }),
+    ).toHaveAttribute ('href', '/posts/changes?tag=7')
+
+    expect (
+	within (externalRow!).getByRole ('link', { name: '耕作履歴' }),
+    ).toHaveAttribute ('href', '/posts/changes?external_tag=7')
+  })
 })