#354

backend/app/controllers/application_controller.rb

@@ -92,6 +92,4 @@ class ApplicationController < ActionController::API
 
     value
   end
-
-  def resolve_locale! = Locale.find_by(code: params[:locale]) || Locale.nipponese
 end

backend/app/controllers/materials_controller.rb

@@ -72,7 +72,6 @@ class MaterialsController < ApplicationController
     return head :unauthorized unless current_user
     return head :forbidden unless current_user.gte_member?
 
-    locale = resolve_locale!
     tag_name_raw = params[:tag].to_s.strip
     file = params[:file]
     file_sha256 = MaterialFileSha256.from_upload(file)
@@ -90,7 +89,7 @@ class MaterialsController < ApplicationController
 
     begin
       Material.transaction do
-        tag = resolve_material_tag!(locale, tag_name_raw)
+        tag = resolve_material_tag!(tag_name_raw)
         material = Material.new(tag:, url:,
                                 created_by_user: current_user,
                                 updated_by_user: current_user)
@@ -237,8 +236,10 @@ class MaterialsController < ApplicationController
     nil
   end
 
-  def resolve_material_tag! locale, tag_name_raw
-    Tag.find_or_create_by_tag_name!(locale, tag_name_raw, category: :material)
+  def resolve_material_tag! tag_name_raw
+    tag_name = TagName.find_undiscard_or_create_by!(name: tag_name_raw)
+    tag = tag_name.tag
+    tag || Tag.create!(tag_name:, category: :material)
   end
 
   def material_index_needs_tag_name? filters

backend/app/controllers/nico_tags_controller.rb

@@ -70,8 +70,6 @@ class NicoTagsController < ApplicationController
     return head :unauthorized unless current_user
     return head :forbidden unless current_user.gte_member?
 
-    locale = resolve_locale!
-
     id = params[:id].to_i
 
     tag = Tag.find(id)
@@ -81,8 +79,7 @@ class NicoTagsController < ApplicationController
     linked_tags = nil
 
     ApplicationRecord.transaction do
-      linked_tags = Tag.normalise_tags!(locale, linked_tag_names,
-                                        with_tagme: false,
+      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

backend/app/controllers/posts_controller.rb

@@ -36,8 +36,8 @@ class PostsController < ApplicationController
     offset = (page - 1) * limit
 
     pt_max_sql =
-        PostVersion
-        .select('post_id, MAX(created_at) AS max_updated_at')
+        PostTag
+        .select('post_id, MAX(updated_at) AS max_updated_at')
         .group('post_id')
         .to_sql
 
@@ -50,7 +50,8 @@ class PostsController < ApplicationController
         .joins("LEFT JOIN (#{ pt_max_sql }) pt_max ON pt_max.post_id = posts.id")
         .reselect('posts.*', Arel.sql("#{ updated_at_all_sql } AS updated_at_all"))
         .preload(:uploaded_user, :parents, :children,
-                 post_tags: [:sections, { tag: [:deerjikists, :materials,
+                 active_post_tags: [:sections,
+                                    { tag: [:deerjikists, :materials,
                                             { tag_name: :wiki_page }] }])
         .with_attached_thumbnail
 
@@ -103,7 +104,8 @@ class PostsController < ApplicationController
 
   def random
     post = filtered_posts.preload(:uploaded_user, :parents, :children,
-                                  post_tags: [:sections, { tag: [:deerjikists, :materials,
+                                  active_post_tags: [:sections,
+                                                     { tag: [:deerjikists, :materials,
                                                              { tag_name: :wiki_page }] }])
                          .with_attached_thumbnail
                          .order('RAND()')
@@ -188,7 +190,8 @@ class PostsController < ApplicationController
     post =
         Post
         .includes(:uploaded_user, :parents, :children,
-                  post_tags: [:sections, { tag: [:deerjikists, :materials,
+                  active_post_tags: [:sections,
+                                     { tag: [:deerjikists, :materials,
                                              { tag_name: :wiki_page }] }])
         .with_attached_thumbnail
         .find_by(id: params[:id])
@@ -308,7 +311,6 @@ class PostsController < ApplicationController
     base_version_no = parse_base_version_no
     return render_bad_request('base_version_no は必須です．') if !(force) && !(base_version_no)
 
-    locale = Locale.find_by(code: params[:locale].presence) || Locale.nipponese
     title = params[:title].presence
     tag_names = params[:tags].to_s.split
     original_created_from = params[:original_created_from]
@@ -330,8 +332,7 @@ class PostsController < ApplicationController
         base_snapshot = post_snapshot_from_version(base_version)
         current_snapshot = post_snapshot_from_record(post)
       end
-      incoming_snapshot = post_incoming_snapshot(locale:,
-                                                 title:,
+      incoming_snapshot = post_incoming_snapshot(title:,
                                                  original_created_from:,
                                                  original_created_before:,
                                                  tag_names:,
@@ -360,7 +361,7 @@ class PostsController < ApplicationController
             end
           end
 
-      apply_post_snapshot!(locale, post, snapshot_to_apply)
+      apply_post_snapshot!(post, snapshot_to_apply)
     end
 
     return render json: conflict_json, status: :conflict if conflict_json
@@ -383,6 +384,50 @@ class PostsController < ApplicationController
     render_post_form_record_invalid e.record
   end
 
+  def changes
+    id = params[:id].presence
+    tag_id = params[:tag].presence
+    page = (params[:page].presence || 1).to_i
+    limit = (params[:limit].presence || 20).to_i
+
+    page = 1 if page < 1
+    limit = 1 if limit < 1
+
+    offset = (page - 1) * limit
+
+    pts = PostTag.with_discarded
+    pts = pts.where(post_id: id) if id.present?
+    pts = pts.where(tag_id:) if tag_id.present?
+    pts = pts.includes(:post, :created_user, :deleted_user,
+                       tag: [:deerjikists, :materials, { tag_name: :wiki_page }])
+
+    events = []
+    pts.each do |pt|
+      tag = TagRepr.base(pt.tag)
+      post = pt.post
+
+      events << Event.new(
+          post:,
+          tag:,
+          user: pt.created_user && { id: pt.created_user.id, name: pt.created_user.name },
+          change_type: 'add',
+          timestamp: pt.created_at)
+
+      if pt.discarded_at
+        events << Event.new(
+            post:,
+            tag:,
+            user: pt.deleted_user && { id: pt.deleted_user.id, name: pt.deleted_user.name },
+            change_type: 'remove',
+            timestamp: pt.discarded_at)
+      end
+    end
+    events.sort_by!(&:timestamp)
+    events.reverse!
+
+    render json: { changes: (events.slice(offset, limit) || []).as_json, count: events.size }
+  end
+
   private
 
   def filtered_posts
@@ -457,13 +502,13 @@ class PostsController < ApplicationController
       end
     end
 
-    PostTag.where(post_id: post.id, tag_id: to_remove.to_a).find_each do |pt|
-      pt.destroy!
+    PostTag.where(post_id: post.id, tag_id: to_remove.to_a).kept.find_each do |pt|
+      pt.discard_by!(current_user)
     end
   end
 
   def build_tag_tree_for post
-    post_tags = post.post_tags.reject { |post_tag| post_tag.tag.deprecated? }
+    post_tags = post.active_post_tags.reject { |post_tag| post_tag.tag.deprecated? }
     tags = post_tags.map(&:tag)
     tag_ids = tags.map(&:id)
 
@@ -672,6 +717,7 @@ class PostsController < ApplicationController
   def editable_tag_names_from_post post
     post
         .post_tags
+        .kept
         .joins(tag: :tag_name)
         .merge(Tag.not_nico)
         .merge(Tag.where(deprecated_at: nil))
@@ -687,13 +733,12 @@ class PostsController < ApplicationController
     end
   end
 
-  def post_incoming_snapshot locale:, title:, original_created_from:, original_created_before:,
+  def post_incoming_snapshot title:, original_created_from:, original_created_before:,
                              tag_names:, video_ms_param:, duration_param:, parent_post_ids:
     validate_original_created_values!(original_created_from, original_created_before)
-    Tag.normalise_tags!(locale, tag_names,
-                        with_tagme: false,
-                        deny_deprecated: true,
-                        with_sections: true) => { tags:, sections: }
+    Tag.normalise_tags!(tag_names, with_tagme: false, deny_deprecated: true,
+                                        with_sections: true) =>
+        { tags:, sections: }
 
     tags = Tag.expand_parent_tags(tags).reject(&:deprecated?)
     video_ms = normalise_video_ms(tags, video_ms_param:, duration_param:)
@@ -828,7 +873,7 @@ class PostsController < ApplicationController
     (added_by_current & removed_by_me).present? || (removed_by_current & added_by_me).present?
   end
 
-  def apply_post_snapshot! locale, post, snapshot
+  def apply_post_snapshot! post, snapshot
     PostVersionRecorder.ensure_snapshot!(post, created_by_user: current_user)
 
     post.update!(title: snapshot[:title],
@@ -836,10 +881,10 @@ class PostsController < ApplicationController
                  original_created_from: snapshot[:original_created_from],
                  original_created_before: snapshot[:original_created_before])
 
-    Tag.normalise_tags!(locale, snapshot[:tag_names],
-                        with_tagme: false,
+    Tag.normalise_tags!(snapshot[:tag_names], with_tagme: false,
                                                deny_deprecated: true,
-                        with_sections: true) => { tags: editable_tags, sections: }
+                                               with_sections: true) =>
+        { tags: editable_tags, sections: }
     TagVersioning.record_tag_snapshots!(editable_tags, created_by_user: current_user)
 
     readonly_tags = post.tags.nico.to_a

backend/app/controllers/tags_controller.rb

@@ -200,45 +200,14 @@ class TagsController < ApplicationController
              .find_by(id: params[:id])
     return head :not_found unless tag
 
-    rows = normalise_deerjikist_rows(tag)
-    return if performed?
-
     ApplicationRecord.transaction do
-      tag.lock!
-
-      requested_keys = rows.map { |row| [row[:platform], row[:code]] }.uniq
-      row_indexes_by_key = rows_by_key(rows)
-      locked_deerjikists = lock_deerjikists_for_tag_update(tag.id, requested_keys)
-      current_deerjikists = locked_deerjikists.filter { |deerjikist|
-        deerjikist.tag_id == tag.id
-      }
-      requested_deerjikists = locked_deerjikists.filter { |deerjikist|
-        row_indexes_by_key.key?([deerjikist.platform, deerjikist.code])
-      }
-
-      render_deerjikist_conflicts(requested_deerjikists, row_indexes_by_key, tag)
-      raise ActiveRecord::Rollback if performed?
-
-      requested_keys_set = requested_keys.to_set
-      current_deerjikists.each do |deerjikist|
-        key = [deerjikist.platform, deerjikist.code]
-        deerjikist.destroy! unless requested_keys_set.include?(key)
-      end
-
-      existing_keys = requested_deerjikists.to_h { |deerjikist|
-        [[deerjikist.platform, deerjikist.code], true]
-      }
-      requested_keys.each do |platform, code|
-        next if existing_keys[[platform, code]]
-
-        deerjikist = Deerjikist.new(platform:, code:, tag:)
-        row_index = row_indexes_by_key[[platform, code]].first
-        begin
-          render_deerjikist_form_record_invalid(deerjikist, row_index) unless deerjikist.save
-        rescue ActiveRecord::RecordNotUnique
-          conflicts = lock_deerjikists_for_tag_update(tag.id, [[platform, code]])
-          render_deerjikist_conflicts(conflicts, row_indexes_by_key, tag)
-        end
+      tag.deerjikists = []
+      params[:_json].each.with_index do |item, i|
+        platform = item[:platform]
+        code = normalise_deerjikist_code(platform, item[:code])
+        deerjikist = Deerjikist.find_or_initialize_by(platform:, code:)
+        deerjikist.tag = tag
+        render_deerjikist_form_record_invalid(deerjikist, i) unless deerjikist.save
         raise ActiveRecord::Rollback if performed?
       end
     end
@@ -278,8 +247,6 @@ class TagsController < ApplicationController
 
     return unless validate_tag_rename(tag, name)
 
-    locale = resolve_locale!
-
     alias_names = params[:aliases].to_s.split.uniq
     parent_names = params[:parent_tags].to_s.split.uniq
     deprecated = bool?(:deprecated)
@@ -310,7 +277,7 @@ class TagsController < ApplicationController
       alias_names.delete(name)
 
       update_aliases!(tag, alias_names)
-      update_parent_tags!(locale, tag, parent_names)
+      update_parent_tags!(tag, parent_names)
 
       tag.reload
 
@@ -575,7 +542,7 @@ class TagsController < ApplicationController
       return false
     end
 
-    target_tag_name = TagName.find_by(name:)
+    target_tag_name = TagName.with_discarded.find_by(name:)
     return true if target_tag_name.nil?
     return true if target_tag_name.canonical_id?
 
@@ -587,14 +554,17 @@ class TagsController < ApplicationController
     return if name == tag.name
 
     current_tag_name = tag.tag_name
-    target_tag_name = TagName.find_by(name:)
+    target_tag_name = TagName.with_discarded.find_by(name:)
 
     if target_tag_name.nil?
       current_tag_name.update!(name:)
       return
     end
 
-    promote_tag_alias!(tag, current_tag_name:, promoted_tag_name: target_tag_name)
+    promote_tag_alias!(
+        tag,
+        current_tag_name:,
+        promoted_tag_name: target_tag_name)
   end
 
   def promote_tag_alias! tag, current_tag_name:, promoted_tag_name:
@@ -604,9 +574,11 @@ class TagsController < ApplicationController
       TagVersioning.ensure_snapshot!(old_owner_tag, created_by_user: current_user)
     end
 
+    promoted_tag_name.undiscard! if promoted_tag_name.discarded?
     promoted_tag_name.update!(canonical: nil)
 
-    TagName.where(canonical_id: current_tag_name.id)
+    TagName.with_discarded
+           .where(canonical_id: current_tag_name.id)
            .where.not(id: promoted_tag_name.id)
            .find_each do |alias_tag_name|
       alias_tag_name.update!(canonical: promoted_tag_name)
@@ -637,7 +609,7 @@ class TagsController < ApplicationController
     end
 
     alias_names.each do |alias_name|
-      alias_tag_name = TagName.find_or_create_by!(name: alias_name)
+      alias_tag_name = TagName.find_undiscard_or_create_by!(name: alias_name)
       affected_tags << alias_tag_name.canonical&.tag
     end
 
@@ -652,7 +624,7 @@ class TagsController < ApplicationController
     end
 
     alias_names.each do |alias_name|
-      alias_tag_name = TagName.find_or_create_by!(name: alias_name)
+      alias_tag_name = TagName.find_undiscard_or_create_by!(name: alias_name)
       alias_tag_name.update!(canonical: tag.tag_name)
     end
 
@@ -661,9 +633,8 @@ class TagsController < ApplicationController
     end
   end
 
-  def update_parent_tags! locale, tag, parent_names
-    parent_tags = Tag.normalise_tags!(locale, parent_names,
-                                      with_tagme: false,
+  def update_parent_tags! tag, parent_names
+    parent_tags = Tag.normalise_tags!(parent_names, with_tagme: false,
                                                     with_no_deerjikist: false,
                                                     deny_nico: true)
 
@@ -682,7 +653,6 @@ class TagsController < ApplicationController
   end
 
   def normalise_deerjikist_code platform, code
-    code = code.to_s
     return code if platform != 'youtube' || code[0] != '@'
 
     url = "https://www.youtube.com/#{ code }"
@@ -699,82 +669,6 @@ class TagsController < ApplicationController
     nil
   end
 
-  def normalise_deerjikist_rows tag
-    rows = []
-
-    params[:_json].each.with_index do |item, index|
-      platform = item[:platform]
-      unless Deerjikist.platforms.key?(platform)
-        render_deerjikist_platform_invalid(index)
-        return rows
-      end
-
-      code = normalise_deerjikist_code(platform, item[:code])
-      deerjikist = Deerjikist.new(platform:, code:, tag:)
-      unless deerjikist.valid?
-        render_deerjikist_form_record_invalid(deerjikist, index)
-        return rows
-      end
-
-      rows << { index:, platform:, code: }
-    end
-
-    rows
-  end
-
-  def lock_deerjikists_for_tag_update tag_id, keys
-    clauses = ['tag_id = ?']
-    values = [tag_id]
-
-    keys.each do |platform, code|
-      clauses << '(platform = ? AND code = ?)'
-      values << platform << code
-    end
-
-    Deerjikist
-      .where(clauses.join(' OR '), *values)
-      .order(:platform, :code)
-      .lock
-      .to_a
-  end
-
-  def render_deerjikist_conflicts deerjikists, row_indexes_by_key, tag
-    conflicts = deerjikists.filter { |deerjikist| deerjikist.tag_id != tag.id }
-    return if conflicts.empty?
-
-    tag_names_by_id = Tag
-                      .joins(:tag_name)
-                      .where(id: conflicts.map(&:tag_id).uniq)
-                      .pluck('tags.id', 'tag_names.name')
-                      .to_h
-    fields = { }
-
-    conflicts.each do |deerjikist|
-      message = "この情報は既に「#{ tag_names_by_id[deerjikist.tag_id] }」に紐づいてゐます．"
-      row_indexes_by_key[[deerjikist.platform, deerjikist.code]].each do |index|
-        field = :"deerjikists.#{ index }.code"
-        fields[field] ||= []
-        fields[field] << message
-      end
-    end
-
-    render_validation_error fields:
-  end
-
-  def render_deerjikist_platform_invalid index
-    render_validation_error fields: {
-      :"deerjikists.#{ index }.platform" => ['値が不正です．'],
-    }
-  end
-
-  def rows_by_key rows
-    rows.each_with_object({ }) do |row, result|
-      key = [row[:platform], row[:code]]
-      result[key] ||= []
-      result[key] << row[:index]
-    end
-  end
-
   def render_deerjikist_form_record_invalid deerjikist, index
     fields = { }
 

backend/app/controllers/wiki_pages_controller.rb

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

backend/app/models/language.rb

@@ -1,3 +0,0 @@
-class Language < ApplicationRecord
-  has_many :languages, class_name: 'Locale', foreign_key: :language_code
-end

backend/app/models/locale.rb

@@ -1,24 +0,0 @@
-class Locale < ApplicationRecord
-  after_create_commit :generate_tag_names!
-
-  belongs_to :language, foreign_key: :language_code, primary_key: :code
-  belongs_to :script, foreign_key: :script_code, primary_key: :code
-
-  def self.nipponese = Locale.find('ja')
-
-  private
-
-  def generate_tag_names!
-    tag_ids = TagName.where(language_code:, primary_flg: true).pluck(:tag_id)
-    Tag.where.not(id: tag_ids).find_each do
-      TagName.create!(tag_id: _1.id,
-                      language_code:,
-                      name: TagName.generate_name(self, _1, _1.name),
-                      script_code:,
-                      primary_flg: true,
-                      # TODO: 公証実装したら書く．
-                      # auto_generated: true,
-                      canonical_id: nil)
-    end
-  end
-end

backend/app/models/post.rb

@@ -55,27 +55,25 @@ class Post < ApplicationRecord
   belongs_to :uploaded_user, class_name: 'User', optional: true
 
   has_many :post_tags, dependent: :destroy, inverse_of: :post
-  has_many :tags, through: :post_tags
+  has_many :active_post_tags, -> { kept }, class_name: 'PostTag', inverse_of: :post
+  has_many :post_tags_with_discarded, -> { with_discarded }, class_name: 'PostTag'
+  has_many :tags, through: :active_post_tags
   has_many :active_tags, -> { where(tags: { deprecated_at: nil }) },
-           through: :post_tags,
-           source: :tag
+           through: :active_post_tags, source: :tag
 
   has_many :user_post_views, dependent: :delete_all
   has_many :post_similarities, dependent: :delete_all
   has_many :post_versions
-
   has_many :gekanator_guessed_games,
            class_name: 'GekanatorGame',
            foreign_key: :guessed_post_id,
            dependent: :delete_all,
            inverse_of: :guessed_post
-
   has_many :gekanator_correct_games,
            class_name: 'GekanatorGame',
            foreign_key: :correct_post_id,
            dependent: :delete_all,
            inverse_of: :correct_post
-
   has_many :gekanator_question_examples, dependent: :delete_all
 
   has_many :parent_post_implications,
@@ -125,6 +123,7 @@ class Post < ApplicationRecord
 
   def snapshot_tag_names
     post_tags
+        .kept
         .joins(tag: :tag_name)
         .includes(:sections, tag: :tag_name)
         .order('tag_names.name')
@@ -151,6 +150,7 @@ class Post < ApplicationRecord
 
   def snapshot_tags_json
     post_tags
+        .kept
         .joins(tag: :tag_name)
         .includes(:sections, tag: :tag_name)
         .order('tags.id')

backend/app/models/post_tag.rb

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

backend/app/models/post_tag_section.rb

@@ -4,7 +4,7 @@ class PostTagSection < ApplicationRecord
   belongs_to :post
   belongs_to :tag
 
-  belongs_to :post_tag, foreign_key: [:post_id, :tag_id],
+  belongs_to :post_tag, -> { kept }, foreign_key: [:post_id, :tag_id],
                                      primary_key: [:post_id, :tag_id],
                                      inverse_of: :sections,
                                      optional: true

backend/app/models/script.rb

@@ -1,3 +0,0 @@
-class Script < ApplicationRecord
-  ;
-end

backend/app/models/tag.rb

@@ -2,6 +2,8 @@ require 'set'
 
 
 class Tag < ApplicationRecord
+  include MyDiscard
+
   class NicoTagNormalisationError < ArgumentError
     ;
   end
@@ -26,7 +28,9 @@ class Tag < ApplicationRecord
   end
 
   has_many :post_tags, inverse_of: :tag
-  has_many :posts, through: :post_tags
+  has_many :active_post_tags, -> { kept }, class_name: 'PostTag', inverse_of: :tag
+  has_many :post_tags_with_discarded, -> { with_discarded }, class_name: 'PostTag'
+  has_many :posts, through: :active_post_tags
 
   has_many :nico_tag_relations, foreign_key: :nico_tag_id, dependent: :destroy
   has_many :linked_tags, through: :nico_tag_relations, source: :tag
@@ -52,7 +56,6 @@ class Tag < ApplicationRecord
   has_many :tag_versions
   has_many :nico_tag_versions
 
-  has_many :tag_names
   belongs_to :tag_name
   delegate :wiki_page, to: :tag_name
 
@@ -102,16 +105,14 @@ class Tag < ApplicationRecord
 
   def has_deerjikists = deerjikists.loaded? ? deerjikists.any? : deerjikists.exists?
 
-  def self.tagme = find_or_create_by_tag_name!(Locale.nipponese, 'タグ希望', category: :meta)
-  def self.bot = find_or_create_by_tag_name!(Locale.nipponese, 'bot操作', category: :meta)
-  def self.no_deerjikist =
-      find_or_create_by_tag_name!(Locale.nipponese, 'ニジラー情報不詳', category: :meta)
-  def self.video = find_or_create_by_tag_name!(Locale.nipponese, '動画', category: :meta)
-  def self.niconico = find_or_create_by_tag_name!(Locale.nipponese, 'ニコニコ', category: :meta)
-  def self.youtube = find_or_create_by_tag_name!(Locale.nipponese, 'YouTube', category: :meta)
+  def self.tagme = find_or_create_by_tag_name!('タグ希望', category: :meta)
+  def self.bot = find_or_create_by_tag_name!('bot操作', category: :meta)
+  def self.no_deerjikist = find_or_create_by_tag_name!('ニジラー情報不詳', category: :meta)
+  def self.video = find_or_create_by_tag_name!('動画', category: :meta)
+  def self.niconico = find_or_create_by_tag_name!('ニコニコ', category: :meta)
+  def self.youtube = find_or_create_by_tag_name!('YouTube', category: :meta)
 
-  def self.normalise_tags! locale, tag_names,
-                           with_tagme: true,
+  def self.normalise_tags! tag_names, with_tagme: true,
                                       with_no_deerjikist: true,
                                       deny_nico: true,
                                       deny_deprecated: false,
@@ -144,7 +145,7 @@ class Tag < ApplicationRecord
 
       name = TagName.canonicalise(name).first
 
-      find_or_create_by_tag_name!(locale, name, category: (cat || :general)).tap do |tag|
+      find_or_create_by_tag_name!(name, category: (cat || :general)).tap do |tag|
         if deny_deprecated && tag.deprecated?
           raise DeprecatedTagNormalisationError, [tag.name]
         end
@@ -232,25 +233,13 @@ class Tag < ApplicationRecord
     [left_end_ms, right_end_ms].max
   end
 
-  def self.find_or_create_by_tag_name! locale, name, category:
-    language_code = locale.language_code
-    name = name.to_s.strip
+  def self.find_or_create_by_tag_name! name, category:
+    tn = TagName.find_undiscard_or_create_by!(name: name.to_s.strip)
+    tn = tn.canonical if tn.canonical_id?
 
-    tn = TagName.find_or_create_by!(language_code:, name:) do
-      _1.script_code = locale.script_code
-      _1.primary_flg = true
+    Tag.find_undiscard_or_create_by!(tag_name_id: tn.id) do |t|
+      t.category = category
     end
-
-    tag = tn.tag
-    if tag
-      tag.update!(tag_name_id: tn.id) if tag.tag_name_id != tn.id
-      return tag
-    end
-
-    tag = Tag.create!(tag_name: tn, category:)
-    tn.update!(tag:)
-
-    tag
   rescue ActiveRecord::RecordNotUnique
     retry
   end
@@ -270,11 +259,11 @@ class Tag < ApplicationRecord
 
         TagVersioning.ensure_snapshot!(source_tag, created_by_user:)
 
-        source_tag.post_tags.find_each do |source_pt|
+        source_tag.post_tags.kept.find_each do |source_pt|
           post_id = source_pt.post_id
           affected_post_ids << post_id
-          source_pt.destroy!
-          unless PostTag.exists?(post_id:, tag: target_tag)
+          source_pt.discard_by!(created_by_user)
+          unless PostTag.kept.exists?(post_id:, tag: target_tag)
             PostTag.create!(post_id:, tag: target_tag)
           end
         end
@@ -286,10 +275,10 @@ class Tag < ApplicationRecord
         end
 
         TagVersioning.record!(source_tag, event_type: :discard, created_by_user:)
-        source_tag.destroy!
+        source_tag.discard!
 
         if source_tag.nico?
-          source_tag_name.destroy!
+          source_tag_name.discard!
         else
           source_tag_name.update_columns(canonical_id: target_tag.tag_name_id,
                                          updated_at: Time.current)
@@ -304,13 +293,13 @@ class Tag < ApplicationRecord
       end
 
       # 投稿件数を再集計
-      target_tag.update_columns(post_count: PostTag.where(tag: target_tag).count)
+      target_tag.update_columns(post_count: PostTag.kept.where(tag: target_tag).count)
     end
 
     target_tag.reload
   end
 
-  def snapshot_aliases = tag_name.aliases.order(:name).pluck(:name)
+  def snapshot_aliases = tag_name.aliases.kept.order(:name).pluck(:name)
 
   def snapshot_parent_tag_ids = parents.order(:id).pluck(:id)
 

backend/app/models/tag_name.rb

@@ -1,5 +1,7 @@
 class TagName < ApplicationRecord
-  belongs_to :tag, optional: true
+  include MyDiscard
+
+  has_one :tag
   has_one :wiki_page
 
   belongs_to :canonical, class_name: 'TagName', optional: true
@@ -21,12 +23,6 @@ class TagName < ApplicationRecord
     names.map { |name| tns[name]&.canonical&.name || name }.uniq
   end
 
-  def self.generate_name locale, tag, name
-    # TODO: 言語ごとの自動命名ロジック完成したら書く．
-
-    "Tag_##{ tag.id }"
-  end
-
   private
 
   def canonical_must_be_canonical

backend/app/models/tag_name_sanitisation_rule.rb

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

backend/app/representations/post_repr.rb

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

backend/app/services/post_creator.rb

@@ -9,7 +9,7 @@ class PostCreator
     @field_warnings = { }
   end
 
-  def create!(locale)
+  def create!
     thumbnail_attachment = prepare_thumbnail_attachment
     post = Post.new(title: @attributes[:title].presence,
                     url: @attributes[:url],
@@ -25,7 +25,7 @@ class PostCreator
       post_tags = planned_post_tags
       sections = planned_sections
       TagVersioning.record_tag_snapshots!(snapshot_tags, created_by_user: @actor)
-      post.video_ms = planned_video_ms(locale)
+      post.video_ms = planned_video_ms
       post.save!
       sync_post_tags!(post, post_tags, sections)
       sync_parent_posts!(post, planned_parent_post_ids)
@@ -53,15 +53,15 @@ class PostCreator
 
   def planned_parent_post_ids = planned_create_attributes[:normalised_parent_post_ids]
 
-  def planned_video_ms locale
-    planned_create_attributes(locale)[:video_ms]
+  def planned_video_ms
+    planned_create_attributes[:video_ms]
   end
 
-  def planned_create_attributes locale
+  def planned_create_attributes
     @planned_create_attributes ||= begin
       if @attributes.key?(:snapshot_tag_specs)
-        snapshot_tags = materialise_tags(locale, @attributes[:snapshot_tag_specs] || [])
-        post_tags = materialise_tags(locale, @attributes[:post_tag_specs] || [])
+        snapshot_tags = materialise_tags(@attributes[:snapshot_tag_specs] || [])
+        post_tags = materialise_tags(@attributes[:post_tag_specs] || [])
         {
             snapshot_tags: snapshot_tags,
             post_tags: post_tags,
@@ -92,13 +92,13 @@ class PostCreator
         video_ms: plan[:video_ms] }
   end
 
-  def materialise_tags locale, specs
+  def materialise_tags specs
     Array(specs).each_with_object({ }) do |spec, tags|
       name = spec[:name] || spec['name']
       category = spec[:category] || spec['category']
       next if name.blank? || category.blank?
 
-      tag = Tag.find_or_create_by_tag_name!(locale, name, category:)
+      tag = Tag.find_or_create_by_tag_name!(name, category:)
       tag.update!(category:) if tag.category.to_sym != category.to_sym
       tags[name] ||= tag
     end.values
@@ -121,11 +121,9 @@ class PostCreator
   def sync_post_tags! post, desired_tags, sections
     desired_ids = desired_tags.map(&:id).to_set
     current_ids = post.tags.pluck(:id).to_set
-
     Tag.where(id: desired_ids - current_ids).find_each do |tag|
       PostTag.create_or_find_by!(post:, tag:, created_user: @actor)
     end
-
     PostTagSection.where(post_id: post.id).destroy_all
     sections.each do |tag_id, ranges|
       ranges.each do |begin_ms, end_ms|
@@ -135,9 +133,10 @@ class PostCreator
                                end_ms:)
       end
     end
-
     PostTag.where(post_id: post.id,
-                  tag_id: (current_ids - desired_ids).to_a).destroy_all
+                  tag_id: (current_ids - desired_ids).to_a).kept.find_each do |post_tag|
+      post_tag.discard_by!(@actor)
+    end
   end
 
   def sync_parent_posts! post, ids

backend/app/services/youtube/sync.rb

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

backend/config/routes.rb

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

backend/db/migrate/20260921000000_delete_inactive_records_from_post_tags.rb

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

backend/db/migrate/20260921010000_add_foreign_key_on_post_id_and_tag_id_in_post_tag_sections.rb

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

backend/db/migrate/20260921020000_delete_discarded_records_from_tags.rb

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

backend/db/migrate/20260921030000_delete_discarded_records_from_tag_names.rb

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

backend/db/migrate/20260922000000_create_languages.rb

@@ -1,20 +0,0 @@
-class CreateLanguages < ActiveRecord::Migration[8.0]
-  def up
-    create_table :languages, id: { type: :string, limit: 16 }, primary_key: :code do |t|
-      t.string :name, null: false
-      t.datetime :deprecated_at, index: true
-      t.datetime :created_at, null: false
-    end
-
-    execute <<~SQL
-      INSERT INTO
-        languages(code, name, created_at)
-      VALUES
-        ('ja', '日本語', #{ connection.quote Time.current })
-    SQL
-  end
-
-  def down
-    drop_table :languages
-  end
-end

backend/db/migrate/20260922010000_create_scripts.rb

@@ -1,20 +0,0 @@
-class CreateScripts < ActiveRecord::Migration[8.0]
-  def up
-    create_table :scripts, id: { type: 'CHAR(4)' }, primary_key: :code do |t|
-      t.string :name, null: false
-      t.datetime :deprecated_at, index: true
-      t.datetime :created_at, null: false
-    end
-
-    execute <<~SQL
-      INSERT INTO
-        scripts(code, name, created_at)
-      VALUES
-        ('Jpan', '漢字および仮名文字', #{ connection.quote Time.current })
-    SQL
-  end
-
-  def down
-    drop_table :scripts
-  end
-end

backend/db/migrate/20260922020000_create_locales.rb

@@ -1,25 +0,0 @@
-class CreateLocales < ActiveRecord::Migration[8.0]
-  def up
-    create_table :locales, id: { type: :string, limit: 32 }, primary_key: :code do |t|
-      t.string :language_code, limit: 16, null: false, index: true
-      t.column :script_code, 'CHAR(4)', null: false
-      t.string :name, null: false
-      t.datetime :deprecated_at, index: true
-      t.datetime :created_at, null: false
-
-      t.foreign_key :languages, column: :language_code, primary_key: :code
-      t.foreign_key :scripts, column: :script_code, primary_key: :code
-    end
-
-    execute <<~SQL
-      INSERT INTO
-        locales(code, language_code, script_code, name, created_at)
-      VALUES
-        ('ja', 'ja', 'Jpan', '日本語', #{ connection.quote Time.current })
-    SQL
-  end
-
-  def down
-    drop_table :locales
-  end
-end

backend/db/migrate/20260922030000_add_columns_to_tag_names.rb

@@ -1,59 +0,0 @@
-class AddColumnsToTagNames < ActiveRecord::Migration[8.0]
-  def up
-    add_reference :tag_names, :tag, after: :id, foreign_key: true
-    add_column :tag_names, :language_code, :string,
-               limit: 16, null: false, after: :tag_id, default: 'ja'
-    add_column :tag_names, :script_code, 'CHAR(4)',
-               null: false, after: :name, default: 'Jpan'
-    add_column :tag_names, :primary_flg, :boolean,
-               null: false, after: :script_code, default: true
-
-    add_foreign_key :tag_names, :languages, column: :language_code, primary_key: :code
-    add_foreign_key :tag_names, :scripts, column: :script_code, primary_key: :code
-
-    remove_index :tag_names, :name
-    add_index :tag_names, [:language_code, :name], unique: true
-
-    change_column_default :tag_names, :language_code, from: 'ja', to: nil
-    change_column_default :tag_names, :script_code, from: 'Jpan', to: nil
-
-    execute <<~SQL
-      UPDATE
-        tag_names tn
-        LEFT JOIN
-          tags AS t
-          ON
-            t.tag_name_id = COALESCE(tn.canonical_id, tn.id)
-      SET
-        tn.tag_id = t.id
-      , tn.primary_flg = CASE
-                         WHEN tn.canonical_id IS NULL THEN
-                           1
-                         ELSE
-                           0
-                         END
-    SQL
-
-    change_column_default :tag_names, :primary_flg, from: true, to: nil
-
-    add_column :tag_names, :primary_tag_id, :bigint,
-               as: 'CASE WHEN primary_flg THEN tag_id ELSE NULL END'
-    add_index :tag_names, [:primary_tag_id, :language_code], unique: true
-  end
-
-  def down
-    remove_index :tag_names, [:primary_tag_id, :language_code]
-    remove_column :tag_names, :primary_tag_id
-
-    remove_foreign_key :tag_names, column: :language_code
-    remove_foreign_key :tag_names, column: :script_code
-
-    remove_index :tag_names, [:language_code, :name]
-    add_index :tag_names, :name, unique: true
-
-    remove_column :tag_names, :primary_flg
-    remove_column :tag_names, :script_code
-    remove_column :tag_names, :language_code
-    remove_reference :tag_names, :tag, foreign_key: true
-  end
-end

backend/db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
+ActiveRecord::Schema[8.0].define(version: 2026_07_27_123600) do
   create_table "active_storage_attachments", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "name", null: false
     t.string "record_type", null: false
@@ -311,12 +311,23 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
     t.check_constraint "`begin_ms` >= 0", name: "chk_post_tag_sections_begin_ms_natural"
   end
 
-  create_table "post_tags", primary_key: ["post_id", "tag_id"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+  create_table "post_tags", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "post_id", null: false
     t.bigint "tag_id", null: false
     t.bigint "created_user_id"
+    t.bigint "deleted_user_id"
     t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.datetime "discarded_at"
+    t.virtual "is_active", type: :boolean, as: "(`discarded_at` is null)", stored: true
+    t.virtual "active_unique_key", type: :string, as: "(case when (`discarded_at` is null) then concat(`post_id`,_utf8mb4':',`tag_id`) else NULL end)", stored: true
+    t.index ["active_unique_key"], name: "idx_post_tags_active_unique", unique: true
     t.index ["created_user_id"], name: "index_post_tags_on_created_user_id"
+    t.index ["deleted_user_id"], name: "index_post_tags_on_deleted_user_id"
+    t.index ["discarded_at"], name: "index_post_tags_on_discarded_at"
+    t.index ["post_id", "discarded_at"], name: "index_post_tags_on_post_id_and_discarded_at"
+    t.index ["post_id"], name: "index_post_tags_on_post_id"
+    t.index ["tag_id", "discarded_at"], name: "index_post_tags_on_tag_id_and_discarded_at"
     t.index ["tag_id"], name: "index_post_tags_on_tag_id"
   end
 
@@ -384,6 +395,19 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
     t.index ["user_id"], name: "index_settings_on_user_id", unique: true
   end
 
+  create_table "wiki_assets", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "wiki_page_id", null: false
+    t.integer "no", null: false
+    t.string "alt_text"
+    t.binary "sha256", limit: 32, null: false
+    t.bigint "created_by_user_id", null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["created_by_user_id"], name: "index_wiki_assets_on_created_by_user_id"
+    t.index ["wiki_page_id", "no"], name: "index_wiki_assets_on_wiki_page_id_and_no", unique: true
+    t.index ["wiki_page_id", "sha256"], name: "index_wiki_assets_on_wiki_page_id_and_sha256", unique: true
+  end
+
   create_table "tag_implications", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "tag_id", null: false
     t.bigint "parent_tag_id", null: false
@@ -409,7 +433,9 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
     t.bigint "canonical_id"
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
+    t.datetime "discarded_at"
     t.index ["canonical_id"], name: "index_tag_names_on_canonical_id"
+    t.index ["discarded_at"], name: "index_tag_names_on_discarded_at"
     t.index ["name"], name: "index_tag_names_on_name", unique: true
   end
 
@@ -446,8 +472,10 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
     t.integer "post_count", default: 0, null: false
+    t.datetime "discarded_at"
     t.integer "version_no", null: false
     t.index ["deprecated_at"], name: "index_tags_on_deprecated_at"
+    t.index ["discarded_at"], name: "index_tags_on_discarded_at"
     t.index ["tag_name_id"], name: "index_tags_on_tag_name_id", unique: true
     t.check_constraint "(`deprecated_at` is null) or (`category` <> _utf8mb4'nico')", name: "chk_tags_deprecated_at_not_nico"
     t.check_constraint "`version_no` > 0", name: "chk_tags_version_no_positive"
@@ -588,19 +616,6 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
     t.index ["banned_at"], name: "index_users_on_banned_at"
   end
 
-  create_table "wiki_assets", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
-    t.bigint "wiki_page_id", null: false
-    t.integer "no", null: false
-    t.string "alt_text"
-    t.binary "sha256", limit: 32, null: false
-    t.bigint "created_by_user_id", null: false
-    t.datetime "created_at", null: false
-    t.datetime "updated_at", null: false
-    t.index ["created_by_user_id"], name: "index_wiki_assets_on_created_by_user_id"
-    t.index ["wiki_page_id", "no"], name: "index_wiki_assets_on_wiki_page_id_and_no", unique: true
-    t.index ["wiki_page_id", "sha256"], name: "index_wiki_assets_on_wiki_page_id_and_sha256", unique: true
-  end
-
   create_table "wiki_lines", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "sha256", limit: 64, null: false
     t.text "body", null: false
@@ -694,6 +709,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
   add_foreign_key "material_sync_suppressions", "users", column: "created_by_user_id"
   add_foreign_key "material_versions", "materials"
   add_foreign_key "material_versions", "materials", column: "parent_id"
+  add_foreign_key "material_versions", "tags"
   add_foreign_key "material_versions", "users", column: "created_by_user_id"
   add_foreign_key "material_versions", "users", column: "updated_by_user_id"
   add_foreign_key "materials", "materials", column: "parent_id"
@@ -702,15 +718,18 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
   add_foreign_key "materials", "users", column: "updated_by_user_id"
   add_foreign_key "nico_tag_relations", "tags"
   add_foreign_key "nico_tag_relations", "tags", column: "nico_tag_id"
+  add_foreign_key "nico_tag_versions", "tags"
   add_foreign_key "nico_tag_versions", "users", column: "created_by_user_id"
   add_foreign_key "post_implications", "posts"
   add_foreign_key "post_implications", "posts", column: "parent_post_id"
   add_foreign_key "post_similarities", "posts"
   add_foreign_key "post_similarities", "posts", column: "target_post_id"
-  add_foreign_key "post_tag_sections", "post_tags", column: ["post_id", "tag_id"], primary_key: ["post_id", "tag_id"], on_delete: :cascade
+  add_foreign_key "post_tag_sections", "posts"
+  add_foreign_key "post_tag_sections", "tags"
   add_foreign_key "post_tags", "posts"
   add_foreign_key "post_tags", "tags"
   add_foreign_key "post_tags", "users", column: "created_user_id"
+  add_foreign_key "post_tags", "users", column: "deleted_user_id"
   add_foreign_key "post_versions", "posts"
   add_foreign_key "post_versions", "users", column: "created_by_user_id"
   add_foreign_key "posts", "users", column: "uploaded_user_id"
@@ -720,6 +739,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_09_21_030000) do
   add_foreign_key "tag_names", "tag_names", column: "canonical_id"
   add_foreign_key "tag_similarities", "tags"
   add_foreign_key "tag_similarities", "tags", column: "target_tag_id"
+  add_foreign_key "tag_versions", "tags"
   add_foreign_key "tag_versions", "users", column: "created_by_user_id"
   add_foreign_key "tags", "tag_names"
   add_foreign_key "theatre_comments", "theatres"

backend/lib/tasks/sync_nico.rake

@@ -16,7 +16,7 @@ namespace :nico do
     end
 
     def sync_post_tags! post, desired_tag_ids, current_tag_ids: nil
-      current_tag_ids ||= PostTag.where(post_id: post.id).pluck(:tag_id).to_set
+      current_tag_ids ||= PostTag.kept.where(post_id: post.id).pluck(:tag_id).to_set
       desired_tag_ids = desired_tag_ids.compact.to_set
 
       to_add = desired_tag_ids - current_tag_ids
@@ -30,8 +30,8 @@ namespace :nico do
         end
       end
 
-      PostTag.where(post_id: post.id, tag_id: to_remove.to_a).find_each do |pt|
-        pt.destroy!
+      PostTag.where(post_id: post.id, tag_id: to_remove.to_a).kept.find_each do |pt|
+        pt.discard_by!(nil)
       end
     end
 
@@ -124,7 +124,7 @@ namespace :nico do
 
       datum['tags'].each do |raw|
         name = TagNameSanitisationRule.sanitise("nico:#{ raw }")
-        tag = Tag.find_or_create_by_tag_name!(Locale.nipponese, name, category: :nico)
+        tag = Tag.find_or_create_by_tag_name!(name, category: :nico)
 
         event_type = tag.nico_tag_versions.exists? ? :update : :create
         NicoTagVersionRecorder.record!(tag:, event_type:, created_by_user: nil)

backend/spec/db/delete_discarded_tags_spec.rb

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

backend/spec/models/post_tag_spec.rb

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

backend/spec/models/tag_name_sanitisation_rule_spec.rb

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

backend/spec/models/tag_spec.rb

@@ -173,47 +173,6 @@ RSpec.describe Tag, type: :model do
     end
   end
 
-  describe '.find_or_create_by_tag_name!' do
-    it 'creates a tag and name with the requested category after stripping whitespace' do
-      tag = nil
-
-      expect {
-        tag = described_class.find_or_create_by_tag_name!(
-            '  lookup_new  ', category: :character)
-      }.to change(Tag, :count).by(1).and change(TagName, :count).by(1)
-
-      expect(tag.name).to eq('lookup_new')
-      expect(tag.category).to eq('character')
-    end
-
-    it 'reuses the canonical tag for an alias without changing its category' do
-      tag = create(:tag, category: :character)
-      alias_name = TagName.create!(name: 'lookup_alias', canonical: tag.tag_name)
-      found = nil
-
-      expect {
-        found = described_class.find_or_create_by_tag_name!(
-            alias_name.name, category: :general)
-      }.to change(Tag, :count).by(0).and change(TagName, :count).by(0)
-
-      expect(found).to eq(tag)
-      expect(found.category).to eq('character')
-    end
-
-    it 'creates a tag for an existing canonical name reached through an alias' do
-      canonical = create(:tag_name)
-      alias_name = TagName.create!(name: 'lookup_alias', canonical:)
-      tag = nil
-
-      expect {
-        tag = described_class.find_or_create_by_tag_name!(
-            alias_name.name, category: :general)
-      }.to change(Tag, :count).by(1).and change(TagName, :count).by(0)
-
-      expect(tag.tag_name).to eq(canonical)
-    end
-  end
-
   describe '.merge_tags!' do
     let!(:target_tag) { create(:tag, category: :general) }
     let!(:source_tag) { create(:tag, category: :general) }
@@ -226,14 +185,18 @@ RSpec.describe Tag, type: :model do
     context 'when merging a simple source tag' do
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
 
-      it 'deletes the source tag, moves its post link, and keeps its name as an alias' do
+      it 'discards the source post_tag, creates an active target post_tag, discards the source tag, and aliases the source tag_name' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        target_link = PostTag.find_by(post: post_record, tag: target_tag)
+        source_pt = PostTag.with_discarded.find(source_post_tag.id)
+        active_target = PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)
 
-        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
-        expect(target_link).to be_present
-        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
+        expect(source_pt.discarded_at).to be_present
+        expect(source_pt.tag_id).to eq(source_tag.id)
+        expect(active_target).to be_present
+
+        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
+        expect(TagName.with_discarded.find(source_tag_name.id)).not_to be_discarded
         expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
         expect(target_tag.reload.post_count).to eq(1)
       end
@@ -243,101 +206,38 @@ RSpec.describe Tag, type: :model do
       let!(:target_post_tag) { PostTag.create!(post: post_record, tag: target_tag) }
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
 
-      it 'deletes the source link and preserves the existing target link' do
-        create(:post_tag_section, post: post_record, tag: source_tag,
-                                  begin_ms: 1000, end_ms: 2000)
-        target_section = create(:post_tag_section, post: post_record,
-                                                   tag: target_tag,
-                                                   begin_ms: 3000, end_ms: nil)
-
+      it 'discards the source post_tag, keeps one active target post_tag, discards the source tag, and aliases the source tag_name' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        target_links = PostTag.where(post: post_record, tag: target_tag)
+        source_pt = PostTag.with_discarded.find(source_post_tag.id)
+        active = PostTag.kept.where(post_id: post_record.id, tag_id: target_tag.id)
 
-        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
-        expect(target_links).to contain_exactly(target_post_tag)
-        expect(PostTagSection.where(post: post_record, tag: source_tag)).to be_empty
-        expect(target_post_tag.reload.sections).to contain_exactly(target_section)
+        expect(source_pt.discarded_at).to be_present
+        expect(source_pt.tag_id).to eq(source_tag.id)
+        expect(active.count).to eq(1)
+        expect(active.first.id).to eq(target_post_tag.id)
 
-        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
+        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
+        expect(TagName.with_discarded.find(source_tag_name.id)).not_to be_discarded
         expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
         expect(target_tag.reload.post_count).to eq(1)
       end
     end
 
-    it 'keeps source history and records the new target alias after deleting the source' do
-      user = create_member_user!
-      source_name = source_tag.name
-      source_alias = TagName.create!(name: 'merge_alias', canonical: source_tag_name)
-      TagVersioning.ensure_snapshot!(source_tag, created_by_user: user)
-      original_version = source_tag.tag_versions.first
-
-      described_class.merge_tags!(target_tag, [source_tag], created_by_user: user)
-
-      versions = TagVersion.where(tag_id: source_tag.id).order(:version_no)
-      expect(versions.pluck(:version_no, :event_type))
-          .to eq([[1, 'create'], [2, 'discard']])
-      expect(versions.first).to eq(original_version)
-      expect(versions.last).to have_attributes(
-          name: source_name, aliases: source_alias.name, created_by_user: user)
-      expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
-
-      target_versions = target_tag.tag_versions.order(:version_no)
-      expect(target_versions.pluck(:event_type)).to eq(['create', 'update'])
-      expect(target_versions.last.aliases.split).to eq([source_name])
-    end
-
-    it 'deletes source relationships while preserving unrelated relationships' do
-      parent = create(:tag)
-      child = create(:tag)
-      nico_tag = create(:tag, :nico)
-      TagImplication.create!(tag: source_tag, parent_tag: parent)
-      TagImplication.create!(tag: child, parent_tag: source_tag)
-      kept_implication = TagImplication.create!(tag: target_tag, parent_tag: parent)
-      NicoTagRelation.create!(tag: source_tag, nico_tag:)
-      kept_relation = NicoTagRelation.create!(tag: target_tag, nico_tag:)
-      TagSimilarity.create!(tag: source_tag, target_tag:, cos: 0.5)
-      TagSimilarity.create!(tag: target_tag, target_tag: source_tag, cos: 0.5)
-      kept_similarity = TagSimilarity.create!(tag: target_tag, target_tag: parent,
-                                               cos: 0.5)
-
-      described_class.merge_tags!(target_tag, [source_tag])
-
-      expect(TagImplication.all).to contain_exactly(kept_implication)
-      expect(NicoTagRelation.all).to contain_exactly(kept_relation)
-      expect(TagSimilarity.all).to contain_exactly(kept_similarity)
-      expect(TagVersion.where(tag_id: source_tag.id).order(:version_no).last.parent_tag_ids)
-          .to eq(parent.id.to_s)
-    end
-
-    it 'preserves material history referencing the deleted source tag' do
-      source_tag.update!(category: :material)
-      target_tag.update!(category: :material)
-      material = Material.create!(tag: source_tag, url: 'https://example.com/material')
-      version = MaterialVersionRecorder.record!(
-          material:, event_type: :create, created_by_user: nil)
-      material.update!(tag: target_tag)
-
-      described_class.merge_tags!(target_tag, [source_tag])
-
-      expect(version.reload).to have_attributes(
-          tag_id: source_tag.id, tag_name: source_tag_name.name, tag_category: 'material')
-      expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
-      expect(material.reload.tag).to eq(target_tag)
-    end
-
     context 'when source_tags includes the target itself' do
       let!(:source_post_tag) { PostTag.create!(post: post_record, tag: source_tag) }
 
       it 'ignores the target in source_tags while still merging the source tag' do
         described_class.merge_tags!(target_tag, [source_tag, target_tag])
 
-        target_link = PostTag.find_by(post: post_record, tag: target_tag)
+        source_pt = PostTag.with_discarded.find(source_post_tag.id)
+        active_target = PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)
 
         expect(Tag.find(target_tag.id)).to be_present
-        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
-        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
-        expect(target_link).to be_present
+        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
+        expect(source_pt.discarded_at).to be_present
+        expect(source_pt.tag_id).to eq(source_tag.id)
+        expect(active_target).to be_present
         expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
         expect(target_tag.reload.post_count).to eq(1)
       end
@@ -360,16 +260,18 @@ RSpec.describe Tag, type: :model do
         )
       end
 
-      it 'still merges and keeps the source name as an alias without validating it' do
+      it 'still merges, but discards the source tag_name instead of aliasing it' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        target_link = PostTag.find_by(post: post_record, tag: target_tag)
+        source_pt = PostTag.with_discarded.find(source_post_tag.id)
+        active_target = PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)
+        discarded_source_tag_name = TagName.with_discarded.find(source_tag_name.id)
 
-        expect(PostTag.exists?(post: post_record, tag: source_tag)).to be(false)
-        expect(target_link).to be_present
+        expect(source_pt.discarded_at).to be_present
+        expect(source_pt.tag_id).to eq(source_tag.id)
+        expect(active_target).to be_present
 
-        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
-        expect(source_tag_name.reload.canonical_id).to eq(target_tag.tag_name_id)
+        expect(Tag.with_discarded.find(source_tag.id)).to be_discarded
         expect(target_tag.reload.post_count).to eq(1)
       end
     end
@@ -386,72 +288,36 @@ RSpec.describe Tag, type: :model do
             message: 'init')
       end
 
-      it 'rolls back earlier deletions, links, and history when a later source has a wiki' do
-        earlier_source = create(:tag)
-        earlier_name = earlier_source.tag_name
-        source_section = create(:post_tag_section, post: post_record,
-                                                   tag: source_tag,
-                                                   begin_ms: 1000, end_ms: 2000)
-
+      it 'rolls back the transaction' do
         expect {
-          described_class.merge_tags!(target_tag, [earlier_source, source_tag])
+          described_class.merge_tags!(target_tag, [source_tag])
         }.to raise_error(ActiveRecord::RecordInvalid)
 
-        expect(Tag.unscoped.exists?(earlier_source.id)).to be(true)
-        expect(earlier_name.reload.canonical_id).to be_nil
-        expect(TagVersion.where(tag_id: [earlier_source.id, source_tag.id, target_tag.id]))
-            .to be_empty
-        expect(Tag.unscoped.exists?(source_tag.id)).to be(true)
-        expect(TagName.unscoped.exists?(source_tag_name.id)).to be(true)
-        expect(source_post_tag.reload.tag_id).to eq(source_tag.id)
-        expect(source_post_tag.sections).to contain_exactly(source_section)
-        expect(PostTag.find_by(post: post_record, tag: target_tag)).to be_nil
-        expect(source_tag.reload.post_count).to eq(1)
+        expect(Tag.with_discarded.find(source_tag.id)).not_to be_discarded
+        expect(TagName.with_discarded.find(source_tag_name.id)).not_to be_discarded
+        expect(PostTag.kept.find(source_post_tag.id).tag_id).to eq(source_tag.id)
+        expect(PostTag.kept.find_by(post_id: post_record.id, tag_id: target_tag.id)).to be_nil
         expect(source_tag_name.reload.canonical_id).to be_nil
         expect(target_tag.reload.post_count).to eq(0)
       end
     end
 
     context 'when merging a nico source tag' do
-      let!(:target_tag) do
-        create(:tag, category: :nico, tag_name: create(:tag_name, name: 'nico:foo'))
-      end
-      let!(:source_tag) do
-        create(:tag, category: :nico, tag_name: create(:tag_name, name: 'nico:bar'))
-      end
+      let!(:target_tag) { create(:tag, category: :nico, name: 'nico:foo') }
+      let!(:source_tag) { create(:tag, category: :nico, name: 'nico:bar') }
       let!(:source_tag_name_id) { source_tag.tag_name_id }
 
-      it 'deletes the source tag and name instead of keeping an alias' do
+      it 'discards the source tag_name instead of aliasing it' do
         described_class.merge_tags!(target_tag, [source_tag])
 
-        expect(Tag.unscoped.exists?(source_tag.id)).to be(false)
-        expect(TagName.unscoped.exists?(source_tag_name_id)).to be(false)
+        discarded_source_tag = Tag.with_discarded.find(source_tag.id)
+        discarded_source_tag_name = TagName.with_discarded.find(source_tag_name_id)
+
+        expect(discarded_source_tag).to be_discarded
+        expect(discarded_source_tag_name).to be_discarded
+        expect(discarded_source_tag_name.canonical_id).to be_nil
         expect(target_tag.reload.post_count).to eq(0)
       end
-
-      it 'keeps nico history while deleting source links and allows recreating the name' do
-        linked_tag = create(:tag)
-        NicoTagRelation.create!(nico_tag: source_tag, tag: linked_tag)
-        kept_relation = NicoTagRelation.create!(nico_tag: target_tag, tag: linked_tag)
-        user = create_member_user!
-        source_name = source_tag.name
-
-        described_class.merge_tags!(target_tag, [source_tag], created_by_user: user)
-
-        expect(NicoTagRelation.all).to contain_exactly(kept_relation)
-        versions = NicoTagVersion.where(tag_id: source_tag.id).order(:version_no)
-        expect(versions.pluck(:version_no, :event_type))
-            .to eq([[1, 'create'], [2, 'discard']])
-        expect(versions.last).to have_attributes(
-            name: source_name, linked_tags: linked_tag.name, created_by_user: user)
-
-        recreated = described_class.find_or_create_by_tag_name!(source_name, category: :nico)
-
-        expect(recreated.id).not_to eq(source_tag.id)
-        expect(recreated.tag_name_id).not_to eq(source_tag_name_id)
-        expect(recreated.nico_tag_versions).to be_empty
-        expect(versions.reload.size).to eq(2)
-      end
     end
 
     def snapshot_tags(post)
@@ -499,15 +365,12 @@ RSpec.describe Tag, type: :model do
         expect(latest.event_type).to eq('update')
         expect(latest.created_by_user).to be_nil
         expect(latest.tags).to eq(snapshot_tags(post_record.reload))
-        expect(latest.tags_json.map { |item| item.fetch('id') }).to eq([target_tag.id])
-        expect(affected_versions.first.tags_json.map { |item| item.fetch('id') })
-            .to eq([source_tag.id])
 
         expect(unaffected_post.reload.post_versions.count).to eq(1)
       end
     end
 
-    context 'when the source tag has no post_tags' do
+    context 'when the source tag has no active post_tags' do
       let!(:another_post) do
         Post.create!(url: 'https://example.com/posts/3', title: 'another post')
       end

backend/spec/requests/posts_spec.rb

@@ -148,15 +148,12 @@ RSpec.describe 'Posts API', type: :request do
 
     it 'keeps children and sections keys in non-detail tag responses' do
       PostTagSection.create!(post: hit_post, tag:, begin_ms: 1_000, end_ms: nil)
-      deprecated_tag = create(:tag, deprecated_at: Time.current)
-      create(:post_tag, post: hit_post, tag: deprecated_tag)
 
       get '/posts'
 
       expect(response).to have_http_status(:ok)
 
       hit_json = json.fetch('posts').find { |post| post['id'] == hit_post.id }
-      expect(hit_json.fetch('tags').map { |item| item.fetch('id') }).to eq([tag.id])
       tag_json = hit_json.fetch('tags').find { |item| item['name'] == 'spec_tag' }
 
       expect(tag_json.fetch('children')).to eq([])
@@ -165,26 +162,6 @@ RSpec.describe 'Posts API', type: :request do
       ])
     end
 
-    it 'preloads tag details and sections as the number of posts grows' do
-      5.times do
-        link = create(:post_tag, post: create(:post, uploaded_user: user))
-        create(:post_tag_section, post: link.post, tag: link.tag,
-                                  begin_ms: 1000, end_ms: 2000)
-      end
-      get '/posts', params: { limit: 1 }
-
-      one_post_queries = count_sql_queries do
-        get '/posts', params: { limit: 1 }
-      end
-      many_post_queries = count_sql_queries do
-        get '/posts', params: { limit: 20 }
-      end
-
-      expect(response).to have_http_status(:ok)
-      expect(json.fetch('posts').size).to eq(8)
-      expect(many_post_queries).to be <= one_post_queries
-    end
-
     context 'when q is provided' do
       it 'filters posts by q (hit case)' do
         get '/posts', params: { tags: 'spec_tag' }
@@ -483,71 +460,6 @@ RSpec.describe 'Posts API', type: :request do
       end
     end
 
-    context 'when update times include version history' do
-      let(:t0) { Time.zone.parse('2020-01-01 12:00:00') }
-      let(:t1) { t0 + 1.day }
-      let(:t2) { t0 + 2.days }
-      let(:t3) { t0 + 3.days }
-      let!(:history_post) do
-        create(:post, url: 'https://example.com/version-time/history',
-                      created_at: t0, updated_at: t0)
-      end
-      let!(:plain_post) do
-        create(:post, url: 'https://example.com/version-time/plain',
-                      created_at: t1, updated_at: t1)
-      end
-      let!(:newer_post) do
-        create(:post, url: 'https://example.com/version-time/newer',
-                      created_at: t0, updated_at: t3)
-      end
-
-      before do
-        link = create(:post_tag, post: history_post, tag:)
-        travel_to(t0) do
-          PostVersionRecorder.record!(post: history_post,
-                                      event_type: :create, created_by_user: nil)
-          PostVersionRecorder.record!(post: newer_post,
-                                      event_type: :create, created_by_user: nil)
-        end
-        travel_to(t2) do
-          link.destroy!
-          PostVersionRecorder.record!(post: history_post,
-                                      event_type: :update, created_by_user: nil)
-        end
-        create(:post_tag, post: plain_post, tag:, created_at: t3)
-      end
-
-      ['asc', 'desc'].each do |direction|
-        it "sorts by the later of post update and latest version time (#{ direction })" do
-          get '/posts', params: { url: '/version-time/', order: "updated_at:#{ direction }" }
-
-          expect(response).to have_http_status(:ok)
-          expected_ids = [plain_post.id, history_post.id, newer_post.id]
-          expected_ids.reverse! if direction == 'desc'
-          expect(json.fetch('posts').map { |item| item.fetch('id') }).to eq(expected_ids)
-          expect(json.fetch('count')).to eq(3)
-
-          times = json.fetch('posts').to_h do |item|
-            [item.fetch('id'), Time.zone.parse(item.fetch('updated_at'))]
-          end
-          expect(times).to eq({ plain_post.id => t1,
-                               history_post.id => t2,
-                               newer_post.id => t3 })
-          expect(history_post.reload.updated_at).to eq(t0)
-        end
-      end
-
-      it 'filters inclusively by the latest version time after a tag is deleted' do
-        get '/posts', params: { url: '/version-time/',
-                               updated_from: t2.iso8601,
-                               updated_to: t2.iso8601 }
-
-        expect(response).to have_http_status(:ok)
-        expect(json.fetch('posts').map { |item| item.fetch('id') }).to eq([history_post.id])
-        expect(json.fetch('count')).to eq(1)
-      end
-    end
-
     context 'when original_created_from/original_created_to are provided' do
       # 注意: controller の現状ロジックに合わせてる
       # original_created_from は `original_created_before > ?`
@@ -1252,7 +1164,9 @@ RSpec.describe 'Posts API', type: :request do
 
     context 'when nico tag already exists in tags' do
       before do
-        Tag.find_or_create_by_tag_name!('nico:nico_tag', category: :nico)
+        Tag.find_undiscard_or_create_by!(
+            tag_name: TagName.find_undiscard_or_create_by!(name: 'nico:nico_tag'),
+            category: :nico)
       end
 
       it 'returns 422 with tag field errors' do
@@ -1505,11 +1419,9 @@ RSpec.describe 'Posts API', type: :request do
 
     it '200 and updates title + resync tags when member' do
       sign_in_as(member)
-      create(:post_tag_section, post: post_record, tag:,
-                                begin_ms: 1000, end_ms: 2000)
 
       tn2 = TagName.create!(name: 'spec_tag_2')
-      replacement_tag = Tag.create!(tag_name: tn2, category: :general)
+      Tag.create!(tag_name: tn2, category: :general)
 
       put "/posts/#{post_record.id}", params: post_update_params(
           post_record,
@@ -1522,38 +1434,6 @@ RSpec.describe 'Posts API', type: :request do
 
       names = json['tags'].map { |n| n['name'] }
       expect(names).to include('spec_tag_2')
-      expect(names).not_to include('spec_tag')
-      expect(PostTag.exists?(post: post_record, tag:)).to be(false)
-      expect(PostTagSection.exists?(post: post_record, tag:)).to be(false)
-      expect(tag.reload.post_count).to eq(0)
-      expect(replacement_tag.reload.post_count).to eq(1)
-
-      versions = post_record.post_versions.order(:version_no)
-      expect(versions.first.tags_json).to include(
-          a_hash_including('id' => tag.id,
-                           'sections' => [{ 'begin_ms' => 1000, 'end_ms' => 2000 }]))
-      expect(versions.last.tags_json.map { |item| item.fetch('id') })
-          .not_to include(tag.id)
-    end
-
-    it 'can add a removed tag again and records both changes' do
-      sign_in_as(member)
-
-      put "/posts/#{ post_record.id }", params: post_update_params(post_record, tags: '')
-      expect(response).to have_http_status(:ok)
-      expect(PostTag.exists?(post: post_record, tag:)).to be(false)
-
-      put "/posts/#{ post_record.id }", params: post_update_params(
-          post_record, tags: 'spec_tag')
-
-      expect(response).to have_http_status(:ok)
-      expect(PostTag.where(post: post_record, tag:).count).to eq(1)
-      expect(PostTag.find_by!(post: post_record, tag:).created_user).to eq(member)
-      expect(tag.reload.post_count).to eq(1)
-      snapshots = post_record.post_versions.order(:version_no).map do |version|
-        version.tags_json.map { |item| item.fetch('id') }
-      end
-      expect(snapshots.map { |ids| ids.include?(tag.id) }).to eq([true, false, true])
     end
 
     it 'rejects a deprecated tag specified directly' do
@@ -1578,7 +1458,9 @@ RSpec.describe 'Posts API', type: :request do
 
     context 'when nico tag already exists in tags' do
       before do
-        Tag.find_or_create_by_tag_name!('nico:nico_tag', category: :nico)
+        Tag.find_undiscard_or_create_by!(
+            tag_name: TagName.find_undiscard_or_create_by!(name: 'nico:nico_tag'),
+            category: :nico)
       end
 
       it 'returns 422 with tag field errors' do
@@ -1997,29 +1879,123 @@ RSpec.describe 'Posts API', type: :request do
       expect(response).to have_http_status(:not_found)
     end
 
-    it 'returns viewed state and current tags with their sections' do
-      create(:post_tag_section, post: post_record, tag:,
-                                begin_ms: 1000, end_ms: nil)
-      deprecated_tag = create(:tag, deprecated_at: Time.current)
-      create(:post_tag, post: post_record, tag: deprecated_tag)
-
+    it '200 and returns viewed boolean' do
       get '/posts/random'
-
       expect(response).to have_http_status(:ok)
       expect(json).to have_key('viewed')
       expect([true, false]).to include(json['viewed'])
-      expect(json.fetch('tags')).to contain_exactly(
-          a_hash_including('id' => tag.id,
-                           'children' => [],
-                           'sections' => [{ 'begin_ms' => 1000, 'end_ms' => nil }]))
     end
   end
 
   describe 'GET /posts/changes' do
-    it 'returns 404 for the retired history endpoint' do
-      get '/posts/changes'
+    let(:member) { create(:user, :member) }
 
-      expect(response).to have_http_status(:not_found)
+    it 'returns add/remove events (history) for a post' do
+      # add
+      tn2 = TagName.create!(name: 'spec_tag2')
+      tag2 = Tag.create!(tag_name: tn2, category: :general)
+      pt = PostTag.create!(post: post_record, tag: tag2, created_user: member)
+
+      # remove (discard)
+      pt.discard_by!(member)
+
+      get '/posts/changes', params: { id: post_record.id }
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include('changes', 'count')
+      expect(json['changes']).to be_an(Array)
+      expect(json['count']).to be >= 2
+
+      types = json['changes'].map { |e| e['change_type'] }.uniq
+      expect(types).to include('add')
+      expect(types).to include('remove')
+    end
+
+    it 'filters history by tag' do
+      tn2 = TagName.create!(name: 'history_tag_hit')
+      tag2 = Tag.create!(tag_name: tn2, category: :general)
+
+      tn3 = TagName.create!(name: 'history_tag_miss')
+      tag3 = Tag.create!(tag_name: tn3, category: :general)
+
+      other_post = Post.create!(
+        title: 'other post',
+        url: 'https://example.com/history-other'
+      )
+
+      # hit: add
+      PostTag.create!(post: post_record, tag: tag2, created_user: member)
+
+      # hit: add + remove
+      pt2 = PostTag.create!(post: other_post, tag: tag2, created_user: member)
+      pt2.discard_by!(member)
+
+      # miss: add + remove
+      pt3 = PostTag.create!(post: post_record, tag: tag3, created_user: member)
+      pt3.discard_by!(member)
+
+      get '/posts/changes', params: { tag: tag2.id }
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include('changes', 'count')
+      expect(json['count']).to eq(3)
+
+      changes = json.fetch('changes')
+
+      expect(changes.map { |e| e.dig('tag', 'id') }.uniq).to eq([tag2.id])
+      expect(changes.map { |e| e['change_type'] }).to match_array(%w[add add remove])
+      expect(changes.map { |e| e.dig('post', 'id') }).to match_array([
+        post_record.id,
+        other_post.id,
+        other_post.id
+      ])
+    end
+
+    it 'filters history by post and tag together' do
+      tn2 = TagName.create!(name: 'history_tag_combo_hit')
+      tag2 = Tag.create!(tag_name: tn2, category: :general)
+
+      tn3 = TagName.create!(name: 'history_tag_combo_miss')
+      tag3 = Tag.create!(tag_name: tn3, category: :general)
+
+      other_post = Post.create!(
+        title: 'other combo post',
+        url: 'https://example.com/history-combo-other'
+      )
+
+      # hit
+      PostTag.create!(post: post_record, tag: tag2, created_user: member)
+
+      # miss by post
+      pt2 = PostTag.create!(post: other_post, tag: tag2, created_user: member)
+      pt2.discard_by!(member)
+
+      # miss by tag
+      pt3 = PostTag.create!(post: post_record, tag: tag3, created_user: member)
+      pt3.discard_by!(member)
+
+      get '/posts/changes', params: { id: post_record.id, tag: tag2.id }
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include('changes', 'count')
+      expect(json['count']).to eq(1)
+
+      changes = json.fetch('changes')
+      expect(changes.size).to eq(1)
+      expect(changes[0]['change_type']).to eq('add')
+      expect(changes[0].dig('post', 'id')).to eq(post_record.id)
+      expect(changes[0].dig('tag', 'id')).to eq(tag2.id)
+    end
+
+    it 'returns empty history when tag does not match' do
+      tn2 = TagName.create!(name: 'history_tag_no_hit')
+      tag2 = Tag.create!(tag_name: tn2, category: :general)
+
+      get '/posts/changes', params: { tag: tag2.id }
+
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('changes')).to eq([])
+      expect(json.fetch('count')).to eq(0)
     end
   end
 
@@ -2071,7 +2047,7 @@ RSpec.describe 'Posts API', type: :request do
     end
 
     let!(:v2) do
-      post_record.post_tags.find_by!(tag: tag).destroy!
+      post_record.post_tags.kept.find_by!(tag: tag).discard_by!(member)
       PostTag.create!(post: post_record, tag: tag2, created_user: member)
       post_record.update!(
         title: 'updated spec post',

backend/spec/requests/tags_deerjikists_spec.rb

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

backend/spec/services/youtube/sync_spec.rb

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

backend/spec/tasks/nico_sync_spec.rb

@@ -8,7 +8,8 @@ RSpec.describe 'nico:sync' do
   end
 
   def create_tag!(name, category:)
-    Tag.find_or_create_by_tag_name!(name, category:)
+    tn = TagName.find_undiscard_or_create_by!(name: name.to_s.strip)
+    Tag.find_undiscard_or_create_by!(tag_name_id: tn.id) { |t| t.category = category }
   end
 
   def link_nico_to_tag!(nico_tag, tag)
@@ -107,7 +108,7 @@ RSpec.describe 'nico:sync' do
     expect(calls).to eq(2)
   end
 
-  it '古い nico tag の関連を物理削除し、変更前後の履歴を version に残す' do
+  it '既存 post にあった古い nico tag は active から外され、履歴として discard される' do
     post = Post.create!(
       title: 'old',
       url: 'https://www.nicovideo.jp/watch/sm9',
@@ -116,8 +117,8 @@ RSpec.describe 'nico:sync' do
 
     # 旧nicoタグ（今回の同期結果に含まれない）
     old_nico = create_tag!('nico:OLD', category: 'nico')
-    PostTag.create!(post:, tag: old_nico)
-    create_post_version_for!(post)
+    old_pt = PostTag.create!(post: post, tag: old_nico)
+    expect(old_pt.discarded_at).to be_nil
 
     # 今回は NEW のみ欲しい
     new_nico = create_tag!('nico:NEW', category: 'nico')
@@ -131,17 +132,9 @@ RSpec.describe 'nico:sync' do
 
     run_rake_task('nico:sync')
 
-    expect(PostTag.exists?(post:, tag: old_nico)).to be(false)
-    expect(old_nico.reload.post_count).to eq(0)
-    expect(new_nico.reload.post_count).to eq(1)
-
-    versions = post.post_versions.order(:version_no)
-    expect(versions.first.tags_json.map { |item| item.fetch('id') })
-        .to include(old_nico.id)
-    expect(versions.last.tags_json.map { |item| item.fetch('id') })
-        .to include(new_nico.id)
-    expect(versions.last.tags_json.map { |item| item.fetch('id') })
-        .not_to include(old_nico.id)
+    # OLD は active から外れる（discarded_at が入る）
+    old_pts = PostTag.where(post_id: post.id, tag_id: old_nico.id).order(:id).to_a
+    expect(old_pts.last.discarded_at).to be_present
 
     # NEW は active にいる
     post.reload