From 31fc32b37721f298b93b2bac92031e55f5374a47 Mon Sep 17 00:00:00 2001
From: miteruzo <miteruzo@naver.com>
Date: Tue, 22 Sep 2026 19:22:16 +0900
Subject: [PATCH] #419

---
 .../app/controllers/nico_tags_controller.rb   | 31 +++-----
 .../controllers/post_versions_controller.rb   | 25 +++---
 backend/app/controllers/posts_controller.rb   | 17 ++--
 .../controllers/tag_children_controller.rb    |  2 -
 backend/app/controllers/tags_controller.rb    | 79 +++++++++++--------
 backend/app/models/external_tag.rb            | 15 ++++
 backend/app/models/nico_tag_version.rb        |  2 +-
 backend/app/models/post.rb                    | 37 ++++++---
 backend/app/models/post_external_tag.rb       |  4 +
 backend/app/models/tag.rb                     | 36 ++-------
 .../app/services/nico_tag_version_recorder.rb |  3 +-
 backend/app/services/post_create_plan.rb      | 44 +++++------
 backend/app/services/tag_versioning.rb        | 24 +-----
 backend/db/schema.rb                          | 23 +++++-
 backend/lib/tasks/sync_nico.rake              | 10 +--
 15 files changed, 181 insertions(+), 171 deletions(-)
 create mode 100644 backend/app/models/external_tag.rb
 create mode 100644 backend/app/models/post_external_tag.rb

diff --git a/backend/app/controllers/nico_tags_controller.rb b/backend/app/controllers/nico_tags_controller.rb
index ce63840..68c35f5 100644
--- a/backend/app/controllers/nico_tags_controller.rb
+++ b/backend/app/controllers/nico_tags_controller.rb
@@ -15,23 +15,22 @@ class NicoTagsController < ApplicationController
     limit = 1 if limit < 1
 
     post_tag_max_sql =
-        PostTag
-        .select('tag_id, MAX(created_at) AS max_created_at')
-        .group('tag_id')
+        PostExternalTag
+        .select('external_tag_id, MAX(created_at) AS max_created_at')
+        .group('external_tag_id')
         .to_sql
 
-    q = Tag.nico_tags
-           .joins(:tag_name)
-           .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')
+    q = q.where('external_tags.name LIKE ?', "%#{ name }%") if name
     if linked_tag
       linked_tag_ids =
-        Tag
-        .joins(:tag_name)
-        .where('tag_names.name LIKE ?', "%#{ linked_tag }%")
-        .pluck(:id)
+          Tag
+          .joins(:tag_name)
+          .where('tag_names.name LIKE ?', "%#{ linked_tag }%")
+          .pluck(:id)
       linked_nico_tag_ids = NicoTagRelation.where(tag_id: linked_tag_ids).pluck(:nico_tag_id)
       q = q.where(id: linked_nico_tag_ids)
     end
@@ -72,8 +71,7 @@ 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
@@ -81,9 +79,6 @@ class NicoTagsController < ApplicationController
     ApplicationRecord.transaction do
       linked_tags = Tag.normalise_tags!(linked_tag_names, with_tagme: false,
                                                           with_no_deerjikist: false)
-      if linked_tags.any? { |t| t.nico? }
-        raise Tag::NicoTagNormalisationError
-      end
 
       TagVersioning.record_tag_snapshots!(linked_tags, created_by_user: current_user)
 
diff --git a/backend/app/controllers/post_versions_controller.rb b/backend/app/controllers/post_versions_controller.rb
index e938a11..d337fe1 100644
--- a/backend/app/controllers/post_versions_controller.rb
+++ b/backend/app/controllers/post_versions_controller.rb
@@ -27,8 +27,8 @@ class PostVersionsController < ApplicationController
                 'prev.original_created_before AS prev_original_created_before')
     q = q.where('post_versions.post_id = ?', post_id) if post_id
     if tag_id
-      q = q.where("JSON_CONTAINS(post_versions.tags_json, JSON_OBJECT('id', #{ tag_id })) " +
-                  "OR JSON_CONTAINS(prev.tags_json, JSON_OBJECT('id', #{ 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
@@ -113,21 +113,18 @@ class PostVersionsController < ApplicationController
     end
   end
 
-  def build_version_tags(cur_tags, prev_tags)
+  def build_version_tags cur_tags, prev_tags
     (cur_tags | prev_tags).map do |name|
       type =
-        if cur_tags.include?(name) && prev_tags.include?(name)
-          '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
diff --git a/backend/app/controllers/posts_controller.rb b/backend/app/controllers/posts_controller.rb
index edbc392..0239644 100644
--- a/backend/app/controllers/posts_controller.rb
+++ b/backend/app/controllers/posts_controller.rb
@@ -671,7 +671,6 @@ 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 +688,7 @@ class PostsController < ApplicationController
                              tag_names:, video_ms_param:, duration_param:, parent_post_ids:
     validate_original_created_values!(original_created_from, original_created_before)
     Tag.normalise_tags!(tag_names, with_tagme: false, deny_deprecated: true,
-                                        with_sections: true) =>
-        { 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 +831,12 @@ class PostsController < ApplicationController
                  original_created_from: snapshot[:original_created_from],
                  original_created_before: snapshot[:original_created_before])
 
-    Tag.normalise_tags!(snapshot[:tag_names], with_tagme: false,
-                                               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
diff --git a/backend/app/controllers/tag_children_controller.rb b/backend/app/controllers/tag_children_controller.rb
index cfdde30..ebffad7 100644
--- a/backend/app/controllers/tag_children_controller.rb
+++ b/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)
diff --git a/backend/app/controllers/tags_controller.rb b/backend/app/controllers/tags_controller.rb
index f152d19..85ef6d2 100644
--- a/backend/app/controllers/tags_controller.rb
+++ b/backend/app/controllers/tags_controller.rb
@@ -100,13 +100,14 @@ class TagsController < ApplicationController
 
   def autocomplete
     q = params[:q].to_s.strip.sub(/\Anot:/i, '')
+    prefix = "#{ ActiveRecord::Base.sanitize_sql_like(q) }%"
 
     with_nico = bool?(:nico, default: true)
     present_only = bool?(:present, default: true)
 
     alias_rows =
         TagName
-        .where('name LIKE ?', "#{ q }%")
+        .where('name LIKE ?', prefix)
         .where.not(canonical_id: nil)
         .pluck(:canonical_id, :name)
 
@@ -118,29 +119,57 @@ class TagsController < ApplicationController
       matched_alias_by_tag_name_id[canonical_id] ||= alias_name
     end
 
-    base = Tag.joins(:tag_name)
-              .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?
+    internal_tags =
+        if with_nico
           canonical_hit.or(base.where(tag_name_id: canonical_ids.uniq))
         else
           canonical_hit
         end
 
-    tags = tags.order(Arel.sql('post_count DESC, tag_names.name')).limit(20).to_a
+    internal_rows =
+        internal_tags
+        .order(Arel.sql('tags.post_count DESC, tag_names.name'))
+        .limit(20)
+        .map { |tag|
+          TagRepr.base(tag).merge(matched_alias: matched_alias_by_tag_name_id[tag.tag_name_id])
+        }
 
-    render json: tags.map { |tag|
-      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 { |tag|
+          { 'id' => tag.id,
+            'name' => "#{ tag.platform }:#{ tag.name }",
+            'category' => 'nico',
+            'deprecated_at' => nil,
+            'created_at' => tag.created_at,
+            'updated_at' => tag.created_at,
+            'post_count' => tag.post_count,
+            'matched_alias' => nil }
+        }
+
+    rows =
+        (internal_rows + external_rows)
+        .sort_by { |row| [-row[:post_count], row[:name]] }
+        .first(20)
+
+    render json: rows
   end
 
   def show
@@ -282,11 +311,7 @@ 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 +359,9 @@ class TagsController < ApplicationController
 
     tag = Tag.find(params[:id])
 
-    if tag.nico? && deprecated_given && deprecated
-      return render_unprocessable_entity 'ニコタグは廃止できません．', field: :deprecated
-    end
-
     return unless validate_tag_rename(tag, name)
 
-    if tag.nico? || (category.present? && category == 'nico')
+    if category.present? && category == 'nico'
       return render_unprocessable_entity 'ニコタグは変更できません．', field: :category
     end
 
@@ -547,11 +568,6 @@ class TagsController < ApplicationController
   end
 
   def record_tag_version! tag, event_type:, created_by_user:, name_changed: false, wiki_page: nil
-    if tag.nico?
-      NicoTagVersionRecorder.record!(tag:, event_type:, created_by_user:)
-      return
-    end
-
     TagVersionRecorder.record!(tag:, event_type:, created_by_user:)
 
     return unless name_changed
@@ -661,8 +677,7 @@ class TagsController < ApplicationController
 
   def update_parent_tags! tag, parent_names
     parent_tags = Tag.normalise_tags!(parent_names, with_tagme: false,
-                                                    with_no_deerjikist: false,
-                                                    deny_nico: true)
+                                                    with_no_deerjikist: false)
 
     old_parent_tags = tag.parents.to_a
 
diff --git a/backend/app/models/external_tag.rb b/backend/app/models/external_tag.rb
new file mode 100644
index 0000000..305c641
--- /dev/null
+++ b/backend/app/models/external_tag.rb
@@ -0,0 +1,15 @@
+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: :tag_id, dependent: :destroy
+  has_many :linked_tags, through: :nico_tag_relations, source: :nico_tag
+
+  def snapshot_linked_tag_names
+    linked_tags.joins(:tag_name).order('tag_names.name').pluck('tag_names.name')
+  end
+end
diff --git a/backend/app/models/nico_tag_version.rb b/backend/app/models/nico_tag_version.rb
index b57252a..e83ecff 100644
--- a/backend/app/models/nico_tag_version.rb
+++ b/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
diff --git a/backend/app/models/post.rb b/backend/app/models/post.rb
index e4fd44c..f10eb07 100644
--- a/backend/app/models/post.rb
+++ b/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: :delete_all
+  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
diff --git a/backend/app/models/post_external_tag.rb b/backend/app/models/post_external_tag.rb
new file mode 100644
index 0000000..38cb528
--- /dev/null
+++ b/backend/app/models/post_external_tag.rb
@@ -0,0 +1,4 @@
+class PostExternalTag < ApplicationRecord
+  belongs_to :post
+  belongs_to :external_tag
+end
diff --git a/backend/app/models/tag.rb b/backend/app/models/tag.rb
index 8a09ef4..1e37685 100644
--- a/backend/app/models/tag.rb
+++ b/backend/app/models/tag.rb
@@ -31,10 +31,6 @@ class Tag < ApplicationRecord
   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
-  has_many :linked_nico_tags, through: :reversed_nico_tag_relations, source: :nico_tag
-
   has_many :tag_implications, foreign_key: :parent_tag_id, dependent: :destroy
   has_many :children, through: :tag_implications, source: :tag
 
@@ -50,7 +46,6 @@ 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 +60,15 @@ class Tag < ApplicationRecord
                   character:   'character',
                   general:     'general',
                   material:    'material',
-                  nico:        'nico',
                   meta:        'meta'
 
   validates :category, presence: true, inclusion: { in: Tag.categories.keys }
 
-  validate :nico_tag_name_must_start_with_nico
+  validate :tag_name_mustnt_start_with_nico
   validate :tag_name_must_be_canonical
   validate :category_must_be_deerjikist_with_deerjikists
-  validate :nico_tags_cannot_be_deprecated
 
-  scope :nico_tags, -> { nico }
+  def self.nico_tags = ExternalTag.where(platform: :nico)
 
   CATEGORY_PREFIXES = {
       'general:'    => :general,
@@ -110,10 +103,9 @@ 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 +265,8 @@ 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 +293,9 @@ 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 +336,4 @@ 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
diff --git a/backend/app/services/nico_tag_version_recorder.rb b/backend/app/services/nico_tag_version_recorder.rb
index 8f1be7f..34dfbde 100644
--- a/backend/app/services/nico_tag_version_recorder.rb
+++ b/backend/app/services/nico_tag_version_recorder.rb
@@ -14,6 +14,7 @@ class NicoTagVersionRecorder < VersionRecorder
   def record_key = :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
 end
diff --git a/backend/app/services/post_create_plan.rb b/backend/app/services/post_create_plan.rb
index a6f38b7..fedd574 100644
--- a/backend/app/services/post_create_plan.rb
+++ b/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
diff --git a/backend/app/services/tag_versioning.rb b/backend/app/services/tag_versioning.rb
index ae5b3dd..d874a3e 100644
--- a/backend/app/services/tag_versioning.rb
+++ b/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
 
diff --git a/backend/db/schema.rb b/backend/db/schema.rb
index 1cb6423..c4ce56a 100644
--- a/backend/db/schema.rb
+++ b/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"
diff --git a/backend/lib/tasks/sync_nico.rake b/backend/lib/tasks/sync_nico.rake
index f95fba1..2331f5b 100644
--- a/backend/lib/tasks/sync_nico.rake
+++ b/backend/lib/tasks/sync_nico.rake
@@ -115,7 +115,7 @@ namespace :nico do
       # 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_non_nico_tag_ids = tags.pluck(:id).to_set
 
       # 記載すべき外部タグ Id. および連携される内部タグ Id. のリスト
       desired_nico_tag_based_ids = []
@@ -123,11 +123,11 @@ namespace :nico do
       desired_non_nico_tag_ids = []
 
       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!(tag:, event_type: :create, created_by_user: nil)
+        end
 
         desired_nico_tag_based_ids << tag.id