#309

#294

Co-authored-by: miteruzo <miteruzo@naver.com>
Reviewed-on: #316

backend/app/controllers/nico_tags_controller.rb

@@ -30,15 +30,22 @@ class NicoTagsController < ApplicationController
     id = params[:id].to_i
 
     tag = Tag.find(id)
-    return head :bad_request if tag.category != 'nico'
+    return head :bad_request unless tag.nico?
 
-    linked_tag_names = params[:tags].to_s.split(' ')
+    linked_tag_names = params[:tags].to_s.split
-    linked_tags = Tag.normalise_tags(linked_tag_names, with_tagme: false)
+    linked_tags = Tag.normalise_tags(linked_tag_names, with_tagme: false,
-    return head :bad_request if linked_tags.any? { |t| t.category == 'nico' }
+                                                       with_no_deerjikist: false)
+    return head :bad_request if linked_tags.any? { |t| t.nico? }
+
+    ApplicationRecord.transaction do
+      TagVersioning.record_tag_snapshots!(linked_tags, created_by_user: current_user)
 
       tag.linked_tags = linked_tags
       tag.save!
 
+      NicoTagVersionRecorder.record!(tag:, event_type: :update, created_by_user: current_user)
+    end
+
     render json: tag.linked_tags.map { |t| TagRepr.base(t) }, status: :ok
   end
 end

backend/app/controllers/post_versions_controller.rb

@@ -0,0 +1,119 @@
+class PostVersionsController < ApplicationController
+  def index
+    post_id = params[:post].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
+
+    tag_name =
+        if tag_id
+          TagName.joins(:tag).find_by(tag: { id: tag_id })
+        end
+    return render json: { versions: [], count: 0 } if tag_id && tag_name.blank?
+
+    q = PostVersion.joins(<<~SQL.squish)
+          LEFT JOIN
+            post_versions prev
+            ON
+              prev.post_id = post_versions.post_id
+              AND prev.version_no = post_versions.version_no - 1
+        SQL
+        .select('post_versions.*', 'prev.title AS prev_title', 'prev.url AS prev_url',
+                'prev.thumbnail_base AS prev_thumbnail_base', 'prev.tags AS prev_tags',
+                'prev.original_created_from AS prev_original_created_from',
+                'prev.original_created_before AS prev_original_created_before')
+    q = q.where('post_versions.post_id = ?', post_id) if post_id
+    if tag_name
+      escaped = ActiveRecord::Base.sanitize_sql_like(tag_name.name)
+      q = q.where(("CONCAT(' ', post_versions.tags, ' ') LIKE :kw " +
+                   "OR CONCAT(' ', prev.tags, ' ') LIKE :kw"),
+                  kw: "% #{ escaped } %")
+    end
+
+    count = q.except(:select, :order, :limit, :offset).count
+
+    versions = q.order(Arel.sql('post_versions.created_at DESC, post_versions.id DESC'))
+                .limit(limit)
+                .offset(offset)
+
+    render json: { versions: serialise_versions(versions), count: }
+  end
+
+  private
+
+  def serialise_versions rows
+    user_ids = rows.map(&:created_by_user_id).compact.uniq
+    users_by_id = User.where(id: user_ids).pluck(:id, :name).to_h
+
+    rows.map do |row|
+      cur_tags = split_tags(row.tags)
+      prev_tags = split_tags(row.attributes['prev_tags'])
+
+      {
+        post_id: row.post_id,
+        version_no: row.version_no,
+        event_type: row.event_type,
+        title: {
+          current: row.title,
+          prev: row.attributes['prev_title']
+        },
+        url: {
+          current: row.url,
+          prev: row.attributes['prev_url']
+        },
+        thumbnail: {
+          current: nil,
+          prev: nil
+        },
+        thumbnail_base: {
+          current: row.thumbnail_base,
+          prev: row.attributes['prev_thumbnail_base']
+        },
+        tags: build_version_tags(cur_tags, prev_tags),
+        original_created_from: {
+          current: row.original_created_from&.iso8601,
+          prev: row.attributes['prev_original_created_from']&.iso8601
+        },
+        original_created_before: {
+          current: row.original_created_before&.iso8601,
+          prev: row.attributes['prev_original_created_before']&.iso8601
+        },
+        created_at: row.created_at.iso8601,
+        created_by_user:
+          if row.created_by_user_id
+            {
+              id: row.created_by_user_id,
+              name: users_by_id[row.created_by_user_id]
+            }
+          end
+      }
+    end
+  end
+
+  def build_version_tags(cur_tags, prev_tags)
+    (cur_tags | prev_tags).map do |name|
+      type =
+        if cur_tags.include?(name) && prev_tags.include?(name)
+          'context'
+        elsif cur_tags.include?(name)
+          'added'
+        else
+          'removed'
+        end
+
+      {
+        name:,
+        type:
+      }
+    end
+  end
+
+  def split_tags(tags)
+    tags.to_s.split(/\s+/).reject(&:blank?)
+  end
+end

backend/app/controllers/posts_controller.rb

@@ -44,7 +44,7 @@ class PostsController < ApplicationController
         filtered_posts
         .joins("LEFT JOIN (#{ pt_max_sql }) pt_max ON pt_max.post_id = posts.id")
         .reselect('posts.*', Arel.sql("#{ updated_at_all_sql } AS updated_at_all"))
-        .preload(tags: { tag_name: :wiki_page })
+        .preload(tags: [:materials, { tag_name: :wiki_page }])
         .with_attached_thumbnail
 
     q = q.where('posts.url LIKE ?', "%#{ url }%") if url
@@ -95,7 +95,7 @@ class PostsController < ApplicationController
   end
 
   def random
-    post = filtered_posts.preload(tags: { tag_name: :wiki_page })
+    post = filtered_posts.preload(tags: [:materials, { tag_name: :wiki_page }])
                          .order('RAND()')
                          .first
     return head :not_found unless post
@@ -104,7 +104,7 @@ class PostsController < ApplicationController
   end
 
   def show
-    post = Post.includes(tags: { tag_name: :wiki_page }).find_by(id: params[:id])
+    post = Post.includes(tags: [:materials, { tag_name: :wiki_page }]).find_by(id: params[:id])
     return head :not_found unless post
 
     render json: PostRepr.base(post, current_user)
@@ -128,9 +128,11 @@ class PostsController < ApplicationController
                     original_created_from:, original_created_before:)
     post.thumbnail.attach(thumbnail)
 
-    ActiveRecord::Base.transaction do
+    ApplicationRecord.transaction do
       post.save!
       tags = Tag.normalise_tags(tag_names)
+      TagVersioning.record_tag_snapshots!(tags, created_by_user: current_user)
+
       tags = Tag.expand_parent_tags(tags)
       sync_post_tags!(post, tags)
       post.resized_thumbnail!
@@ -170,10 +172,15 @@ class PostsController < ApplicationController
 
     post = Post.find(params[:id].to_i)
 
-    ActiveRecord::Base.transaction do
+    ApplicationRecord.transaction do
+      PostVersionRecorder.ensure_snapshot!(post, created_by_user: current_user)
+
       post.update!(title:, original_created_from:, original_created_before:)
-      tags = post.tags.where(category: 'nico').to_a +
+
-             Tag.normalise_tags(tag_names, with_tagme: false)
+      normalised_tags = Tag.normalise_tags(tag_names, with_tagme: false)
+      TagVersioning.record_tag_snapshots!(normalised_tags, created_by_user: current_user)
+
+      tags = post.tags.nico.to_a + normalised_tags
       tags = Tag.expand_parent_tags(tags)
       sync_post_tags!(post, tags)
       PostVersionRecorder.record!(post:, event_type: :update, created_by_user: current_user)
@@ -204,7 +211,7 @@ class PostsController < ApplicationController
     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: { tag_name: :wiki_page })
+                       tag: [:materials, { tag_name: :wiki_page }])
 
     events = []
     pts.each do |pt|

backend/app/controllers/tag_children_controller.rb

@@ -7,7 +7,16 @@ class TagChildrenController < ApplicationController
     child_id = params[:child_id]
     return head :bad_request if parent_id.blank? || child_id.blank?
 
-    Tag.find(parent_id).children << Tag.find(child_id) rescue nil
+    parent = Tag.find(parent_id)
+    child = Tag.find(child_id)
+    return head :bad_request if parent.nico? || child.nico?
+
+    ApplicationRecord.transaction do
+      TagVersioning.ensure_snapshot!(child, created_by_user: current_user)
+
+      TagImplication.find_or_create_by!(parent_tag: parent, tag: child)
+      TagVersionRecorder.record!(tag: child, event_type: :update, created_by_user: current_user)
+    end
 
     head :no_content
   end
@@ -20,7 +29,16 @@ class TagChildrenController < ApplicationController
     child_id = params[:child_id]
     return head :bad_request if parent_id.blank? || child_id.blank?
 
-    Tag.find(parent_id).children.delete(Tag.find(child_id)) rescue nil
+    parent = Tag.find(parent_id)
+    child = Tag.find(child_id)
+    return head :bad_request if parent.nico? || child.nico?
+
+    ApplicationRecord.transaction do
+      TagVersioning.ensure_snapshot!(child, created_by_user: current_user)
+
+      TagImplication.find_by(parent_tag: parent, tag: child)&.destroy!
+      TagVersionRecorder.record!(tag: child, event_type: :update, created_by_user: current_user)
+    end
 
     head :no_content
   end

backend/app/controllers/tags_controller.rb

@@ -218,15 +218,21 @@ class TagsController < ApplicationController
 
     tag = Tag.find(params[:id])
 
-    if name.present?
+    if category.present? && tag.nico? != (category == 'nico')
-      tag.tag_name.update!(name:)
+      return render json: { error: 'ニコタグのカテゴリ変更はできません．' },
+                    status: :unprocessable_entity
     end
 
-    if category.present?
+    ApplicationRecord.transaction do
-      tag.update!(category:)
+      TagVersioning.ensure_snapshot!(tag, created_by_user: current_user)
+
+      tag.tag_name.update!(name:) if name.present?
+      tag.update!(category:) if category.present?
+
+      record_tag_version!(tag, event_type: :update, created_by_user: current_user)
     end
 
-    render json: TagRepr.base(tag)
+    render json: TagRepr.base(tag.reload)
   end
 
   private
@@ -244,4 +250,12 @@ class TagsController < ApplicationController
         children: tag.children.sort_by { _1.name }.map { build_tag_children(_1) },
         material: material.as_json&.merge(file:, content_type:))
   end
+
+  def record_tag_version!(tag, event_type:, created_by_user:)
+    if tag.nico?
+      NicoTagVersionRecorder.record!(tag:, event_type:, created_by_user:)
+    else
+      TagVersionRecorder.record!(tag:, event_type:, created_by_user:)
+    end
+  end
 end

backend/app/models/my_discard.rb

@@ -1,7 +1,11 @@
 module MyDiscard
   extend ActiveSupport::Concern
 
-  included { include Discard::Model }
+  included do
+    include Discard::Model
+
+    default_scope -> { kept }
+  end
 
   class_methods do
     def find_undiscard_or_create_by! attrs, &block

backend/app/models/nico_tag_version.rb

@@ -0,0 +1,7 @@
+class NicoTagVersion < ApplicationRecord
+  include VersionRecord
+
+  belongs_to :tag
+
+  validates :name, presence: true
+end

backend/app/models/post_version.rb

@@ -1,29 +1,13 @@
 class PostVersion < ApplicationRecord
-  before_update do
+  include VersionRecord
-    raise ActiveRecord::ReadOnlyRecord, '版は更新できません．'
-  end
-
-  before_destroy do
-    raise ActiveRecord::ReadOnlyRecord, '版は削除できません．'
-  end
 
   belongs_to :post
   belongs_to :parent, class_name: 'Post', optional: true
-  belongs_to :created_by_user, class_name: 'User', optional: true
 
-  enum :event_type, { create: 'create',
-                      update: 'update',
-                      discard: 'discard',
-                      restore: 'restore' }, prefix: true, validate: true
-
-  validates :version_no, presence: true, numericality: { only_integer: true, greater_than: 0 }
-  validates :event_type, presence: true, inclusion: { in: event_types.keys }
   validates :url, presence: true
 
   validate :validate_original_created_range
 
-  scope :chronological, -> { order(:version_no, :id) }
-
   private
 
   def validate_original_created_range

backend/app/models/tag.rb

@@ -8,8 +8,6 @@ class Tag < ApplicationRecord
     ;
   end
 
-  default_scope -> { kept }
-
   has_many :post_tags, inverse_of: :tag
   has_many :active_post_tags, -> { kept }, class_name: 'PostTag', inverse_of: :tag
   has_many :post_tags_with_discarded, -> { with_discarded }, class_name: 'PostTag'
@@ -36,6 +34,9 @@ class Tag < ApplicationRecord
   has_many :deerjikists, dependent: :delete_all
   has_many :materials
 
+  has_many :tag_versions
+  has_many :nico_tag_versions
+
   belongs_to :tag_name
   delegate :wiki_page, to: :tag_name
 
@@ -78,27 +79,15 @@ class Tag < ApplicationRecord
 
   def material_id = materials.first&.id
 
-  def self.tagme
+  def self.tagme = find_or_create_by_tag_name!('タグ希望', category: :meta)
-    @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)
-  end
+  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.bot
+  def self.normalise_tags tag_names, with_tagme: true,
-    @bot ||= find_or_create_by_tag_name!('bot操作', category: :meta)
+                                     with_no_deerjikist: true,
-  end
+                                     deny_nico: true
-
-  def self.no_deerjikist
-    @no_deerjikist ||= find_or_create_by_tag_name!('ニジラー情報不詳', category: :meta)
-  end
-
-  def self.video
-    @video ||= find_or_create_by_tag_name!('動画', category: :meta)
-  end
-
-  def self.niconico
-    @niconico ||= find_or_create_by_tag_name!('ニコニコ', category: :meta)
-  end
-
-  def self.normalise_tags tag_names, with_tagme: true, deny_nico: true
     if deny_nico && tag_names.any? { |n| n.downcase.start_with?('nico:') }
       raise NicoTagNormalisationError
     end
@@ -112,7 +101,7 @@ class Tag < ApplicationRecord
     end
 
     tags << Tag.tagme if with_tagme && tags.size < 10 && tags.none?(Tag.tagme)
-    tags << Tag.no_deerjikist if tags.all? { |t| !(t.deerjikist?) }
+    tags << Tag.no_deerjikist if with_no_deerjikist && tags.all? { |t| !(t.deerjikist?) }
     tags.uniq(&:id)
   end
 
@@ -150,21 +139,25 @@ class Tag < ApplicationRecord
     retry
   end
 
-  def self.merge_tags! target_tag, source_tags
+  def self.merge_tags! target_tag, source_tags, created_by_user: nil
     target_tag => Tag
 
     affected_post_ids = Set.new
 
     Tag.transaction do
+      TagVersioning.ensure_snapshot!(target_tag, created_by_user:)
+
       Array(source_tags).compact.uniq.each do |source_tag|
         source_tag => Tag
 
         next if source_tag == target_tag
 
+        TagVersioning.ensure_snapshot!(source_tag, created_by_user:)
+
         source_tag.post_tags.kept.find_each do |source_pt|
           post_id = source_pt.post_id
           affected_post_ids << post_id
-          source_pt.discard_by!(nil)
+          source_pt.discard_by!(created_by_user)
           unless PostTag.kept.exists?(post_id:, tag: target_tag)
             PostTag.create!(post_id:, tag: target_tag)
           end
@@ -176,6 +169,7 @@ class Tag < ApplicationRecord
           raise ActiveRecord::RecordInvalid.new(source_tag_name)
         end
 
+        TagVersioning.record!(source_tag, event_type: :discard, created_by_user:)
         source_tag.discard!
 
         if source_tag.nico?
@@ -184,10 +178,13 @@ class Tag < ApplicationRecord
           source_tag_name.update_columns(canonical_id: target_tag.tag_name_id,
                                          updated_at: Time.current)
         end
+
+        TagVersioning.record!(target_tag, event_type: :update, created_by_user:)
       end
 
       Post.where(id: affected_post_ids.to_a).find_each do |post|
-        PostVersionRecorder.record!(post:, event_type: :update, created_by_user: nil)
+        PostVersionRecorder.ensure_snapshot!(post, created_by_user:)
+        PostVersionRecorder.record!(post:, event_type: :update, created_by_user:)
       end
 
       # 投稿件数を再集計
@@ -197,6 +194,14 @@ class Tag < ApplicationRecord
     target_tag.reload
   end
 
+  def snapshot_aliases = tag_name.aliases.kept.order(:name).pluck(:name)
+
+  def snapshot_parent_tag_ids = parents.order(:id).pluck(:id)
+
+  def snapshot_linked_tag_names
+    linked_tags.joins(:tag_name).order('tag_names.name').pluck('tag_names.name')
+  end
+
   private
 
   def nico_tag_name_must_start_with_nico

backend/app/models/tag_name.rb

@@ -1,8 +1,6 @@
 class TagName < ApplicationRecord
   include MyDiscard
 
-  default_scope -> { kept }
-
   has_one :tag
   has_one :wiki_page
 

backend/app/models/tag_version.rb

@@ -0,0 +1,15 @@
+class TagVersion < ApplicationRecord
+  include VersionRecord
+
+  belongs_to :tag
+
+  enum :category, { deerjikist: 'deerjikist',
+                    meme: 'meme',
+                    character: 'character',
+                    general: 'general',
+                    material: 'material',
+                    meta: 'meta' }, validate: true
+
+  validates :name, presence: true
+  validates :category, presence: true
+end

backend/app/models/version_record.rb

@@ -0,0 +1,19 @@
+module VersionRecord
+  extend ActiveSupport::Concern
+
+  def readonly? = persisted?
+
+  included do
+    belongs_to :created_by_user, class_name: 'User', optional: true
+
+    enum :event_type, { create: 'create',
+                        update: 'update',
+                        discard: 'discard',
+                        restore: 'restore' }, prefix: true, validate: true
+
+    validates :version_no, presence: true, numericality: { only_integer: true, greater_than: 0 }
+    validates :event_type, presence: true
+
+    scope :chronological, -> { order(:version_no, :id) }
+  end
+end

backend/app/models/wiki_page.rb

@@ -4,8 +4,6 @@ require 'set'
 class WikiPage < ApplicationRecord
   include MyDiscard
 
-  default_scope -> { kept }
-
   has_many :wiki_revisions, dependent: :destroy
   belongs_to :created_user, class_name: 'User'
   belongs_to :updated_user, class_name: 'User'

backend/app/services/nico_tag_version_recorder.rb

@@ -0,0 +1,19 @@
+class NicoTagVersionRecorder < VersionRecorder
+  def self.record! tag:, event_type:, created_by_user:
+    new(tag:, event_type:, created_by_user:).record!
+  end
+
+  def initialize tag:, event_type:, created_by_user:
+    super(record: tag, event_type:, created_by_user:)
+  end
+
+  private
+
+  def version_class = NicoTagVersion
+  def version_association = :nico_tag_versions
+  def record_key = :tag
+
+  def snapshot_attributes
+    { name: @record.name, linked_tags: @record.snapshot_linked_tag_names.join(' ') }
+  end
+end

backend/app/services/post_version_recorder.rb

@@ -1,57 +1,31 @@
-class PostVersionRecorder
+class PostVersionRecorder < VersionRecorder
   def self.record! post:, event_type:, created_by_user:
     new(post:, event_type:, created_by_user:).record!
   end
 
   def initialize post:, event_type:, created_by_user:
-    @post = post
+    super(record: post, event_type:, created_by_user:)
-    @event_type = event_type
-    @created_by_user = created_by_user
   end
 
-  def record!
+  def self.ensure_snapshot! post, created_by_user:
-    @post.with_lock do
+    return if post.post_versions.exists?
-      latest = @post.post_versions.order(version_no: :desc).first
-      attrs = snapshot_attributes
 
-      return latest if @event_type == :update && latest && same_snapshot?(latest, attrs)
+    record!(post:, event_type: :create, created_by_user:)
-
-      PostVersion.create!(
-          post: @post,
-          version_no: (latest&.version_no || 0) + 1,
-          event_type: @event_type,
-          title: attrs[:title],
-          url: attrs[:url],
-          thumbnail_base: attrs[:thumbnail_base],
-          tags: attrs[:tags],
-          parent: attrs[:parent],
-          original_created_from: attrs[:original_created_from],
-          original_created_before: attrs[:original_created_before],
-          created_at: Time.current,
-          created_by_user: @created_by_user)
-    end
   end
 
   private
 
-  def snapshot_attributes
+  def version_class = PostVersion
-    { title: @post.title,
+  def version_association = :post_versions
-      url: @post.url,
+  def record_key = :post
-      thumbnail_base: @post.thumbnail_base,
-      tags: @post.snapshot_tag_names.join(' '),
-      parent: @post.parent,
-      original_created_from: @post.original_created_from,
-      original_created_before: @post.original_created_before }
-  end
 
-  def same_snapshot? version, attrs
+  def snapshot_attributes
-    true &&
+    { title: @record.title,
-        version.title == attrs[:title] &&
+      url: @record.url,
-        version.url == attrs[:url] &&
+      thumbnail_base: @record.thumbnail_base,
-        version.thumbnail_base == attrs[:thumbnail_base] &&
+      tags: @record.snapshot_tag_names.join(' '),
-        version.tags == attrs[:tags] &&
+      parent_id: @record.parent_id,
-        version.parent_id == attrs[:parent]&.id &&
+      original_created_from: @record.original_created_from,
-        version.original_created_from == attrs[:original_created_from] &&
+      original_created_before: @record.original_created_before }
-        version.original_created_before == attrs[:original_created_before]
   end
 end

backend/app/services/tag_version_recorder.rb

@@ -0,0 +1,22 @@
+class TagVersionRecorder < VersionRecorder
+  def self.record! tag:, event_type:, created_by_user:
+    new(tag:, event_type:, created_by_user:).record!
+  end
+
+  def initialize tag:, event_type:, created_by_user:
+    super(record: tag, event_type:, created_by_user:)
+  end
+
+  private
+
+  def version_class = TagVersion
+  def version_association = :tag_versions
+  def record_key = :tag
+
+  def snapshot_attributes
+    { name: @record.name,
+      category: @record.category,
+      aliases: @record.snapshot_aliases.join(' '),
+      parent_tag_ids: @record.snapshot_parent_tag_ids.join(' ') }
+  end
+end

backend/app/services/tag_versioning.rb

@@ -0,0 +1,38 @@
+class TagVersioning
+  def self.record! tag, event_type:, created_by_user:
+    if tag.nico?
+      NicoTagVersionRecorder.record!(tag:, event_type:, created_by_user:)
+    else
+      TagVersionRecorder.record!(tag:, event_type:, created_by_user:)
+    end
+  end
+
+  def self.ensure_snapshot! tag, created_by_user:
+    if tag.nico?
+      return if tag.nico_tag_versions.exists?
+
+      NicoTagVersionRecorder.record!(tag:, event_type: :create, created_by_user:)
+    else
+      return if tag.tag_versions.exists?
+
+      TagVersionRecorder.record!(tag:, event_type: :create, created_by_user:)
+    end
+  end
+
+  def self.record_tag_snapshot! tag, created_by_user:
+    event_type =
+        if tag.nico?
+          tag.nico_tag_versions.exists? ? :update : :create
+        else
+          tag.tag_versions.exists? ? :update : :create
+        end
+
+    record!(tag, event_type:, created_by_user:)
+  end
+
+  def self.record_tag_snapshots! tags, created_by_user:
+    tags.each do |tag|
+      record_tag_snapshot!(tag, created_by_user:)
+    end
+  end
+end

backend/app/services/version_recorder.rb

@@ -0,0 +1,62 @@
+class VersionRecorder
+  EVENT_TYPES = ['create', 'update', 'discard', 'restore'].freeze
+
+  def initialize record:, event_type:, created_by_user:
+    @record = record
+    @event_type = event_type.to_s
+    @created_by_user = created_by_user
+
+    validate_event_type!
+  end
+
+  def record!
+    raise "#{ record_class.name } must be persisted" unless @record.persisted?
+
+    ApplicationRecord.transaction do
+      @record = record_class.unscoped.lock.find(@record.id)
+      latest = latest_version
+
+      if !(latest) && @event_type != 'create'
+        raise "#{ version_class.name } first event must be create"
+      end
+
+      if @event_type == 'create' && latest
+        raise "#{ version_class.name } create event already exists"
+      end
+
+      attrs = snapshot_attributes
+
+      return latest if @event_type == 'update' && latest && same_snapshot?(latest, attrs)
+
+      version_class.create!(base_attributes(latest).merge(record_key => @record).merge(attrs))
+    end
+  end
+
+  private
+
+  def latest_version = versions.order(version_no: :desc).first
+
+  def versions = @record.public_send(version_association)
+
+  def base_attributes latest
+    { version_no: (latest&.version_no || 0) + 1,
+      event_type: @event_type,
+      created_at: Time.current,
+      created_by_user: @created_by_user }
+  end
+
+  def same_snapshot?(version, attrs) = attrs.all? { |k, v| version.public_send(k) == v }
+
+  def validate_event_type!
+    return if EVENT_TYPES.include?(@event_type)
+
+    raise ArgumentError, "Invalid event_type: #{ @event_type }"
+  end
+
+  def version_class = raise NotImplementedError
+  def version_association = raise NotImplementedError
+  def record_key = raise NotImplementedError
+  def snapshot_attributes = raise NotImplementedError
+
+  def record_class = @record.class
+end

backend/config/routes.rb

@@ -49,6 +49,7 @@ Rails.application.routes.draw do
     collection do
       get :random
       get :changes
+      get :versions, to: 'post_versions#index'
     end
 
     member do

backend/db/migrate/20260409123700_create_post_versions.rb

@@ -2,15 +2,15 @@ require 'set'
 
 
 class CreatePostVersions < ActiveRecord::Migration[8.0]
-  class Post < ApplicationRecord
+  class Post < ActiveRecord::Base
     self.table_name = 'posts'
   end
 
-  class PostTag < ApplicationRecord
+  class PostTag < ActiveRecord::Base
     self.table_name = 'post_tags'
   end
 
-  class PostVersion < ApplicationRecord
+  class PostVersion < ActiveRecord::Base
     self.table_name = 'post_versions'
   end
 

backend/db/migrate/20260419035400_create_tag_versions.rb

@@ -0,0 +1,156 @@
+class CreateTagVersions < ActiveRecord::Migration[8.0]
+  class Tag < ActiveRecord::Base
+    self.table_name = 'tags'
+  end
+
+  class TagName < ActiveRecord::Base
+    self.table_name = 'tag_names'
+  end
+
+  class TagImplication < ActiveRecord::Base
+    self.table_name = 'tag_implications'
+  end
+
+  class TagVersion < ActiveRecord::Base
+    self.table_name = 'tag_versions'
+  end
+
+  class NicoTagVersion < ActiveRecord::Base
+    self.table_name = 'nico_tag_versions'
+  end
+
+  class NicoTagRelation < ActiveRecord::Base
+    self.table_name = 'nico_tag_relations'
+  end
+
+  def up
+    create_table :tag_versions do |t|
+      t.references :tag, null: false, foreign_key: true, index: false
+      t.integer :version_no, null: false
+      t.string :event_type, null: false
+      t.string :name, null: false
+      t.string :category, null: false
+      t.text :aliases, null: false
+      t.text :parent_tag_ids, null: false
+      t.datetime :created_at, null: false
+      t.references :created_by_user, foreign_key: { to_table: :users }, index: false
+
+      t.index [:tag_id, :version_no], unique: true
+      t.index :created_at
+      t.index [:tag_id, :created_at], order: { created_at: :desc }
+      t.index [:created_by_user_id, :created_at], order: { created_at: :desc }
+      t.check_constraint 'version_no > 0',
+                         name: 'tag_versions_version_no_positive'
+    end
+
+    create_table :nico_tag_versions do |t|
+      t.references :tag, null: false, foreign_key: true, index: false
+      t.integer :version_no, null: false
+      t.string :event_type, null: false
+      t.string :name, null: false
+      t.text :linked_tags, null: false
+      t.datetime :created_at, null: false
+      t.references :created_by_user, foreign_key: { to_table: :users }, index: false
+
+      t.index [:tag_id, :version_no], unique: true
+      t.index :created_at
+      t.index [:tag_id, :created_at], order: { created_at: :desc }
+      t.index [:created_by_user_id, :created_at], order: { created_at: :desc }
+      t.check_constraint 'version_no > 0',
+                         name: 'nico_tag_versions_version_no_positive'
+    end
+
+    TagVersion.reset_column_information
+    say_with_time 'Backfilling tag_versions' do
+      Tag.where(discarded_at: nil)
+         .where.not(category: 'nico')
+         .find_in_batches(batch_size: 500) do |tags|
+        tag_ids = tags.map(&:id)
+
+        tag_implication_rows_by_tag_id =
+            TagImplication
+            .where(tag_id: tag_ids)
+            .pluck(:tag_id, :parent_tag_id)
+            .each_with_object(Hash.new { |h, k| h[k] = [] }) do |row, h|
+              h[row[0]] << row[1]
+            end
+
+        tag_name_rows_by_tag_id =
+            TagName
+            .joins('INNER JOIN tags ON tags.tag_name_id = tag_names.id')
+            .where(tags: { id: tag_ids })
+            .pluck('tags.id', 'tag_names.name')
+            .each_with_object({ }) do |row, h|
+              h[row[0]] = row[1]
+            end
+
+        tag_alias_rows_by_tag_id =
+            TagName
+            .joins('INNER JOIN tags ON tags.tag_name_id = tag_names.canonical_id')
+            .where(tags: { id: tag_ids })
+            .where(tag_names: { discarded_at: nil })
+            .pluck('tags.id', 'tag_names.name')
+            .each_with_object(Hash.new { |h, k| h[k] = [] }) do |row, h|
+              h[row[0]] << row[1]
+            end
+
+        TagVersion.insert_all(tags.map { |tag|
+          { tag_id: tag.id,
+            version_no: 1,
+            event_type: 'create',
+            name: tag_name_rows_by_tag_id[tag.id],
+            category: tag.category,
+            aliases: tag_alias_rows_by_tag_id[tag.id].sort.join(' '),
+            parent_tag_ids: tag_implication_rows_by_tag_id[tag.id].sort.join(' '),
+            created_at: tag.created_at,
+            created_by_user_id: nil }
+        })
+      end
+    end
+
+    NicoTagVersion.reset_column_information
+    say_with_time 'Backfilling nico_tag_versions' do
+      Tag.where(discarded_at: nil, category: 'nico')
+         .find_in_batches(batch_size: 500) do |tags|
+        tag_ids = tags.map(&:id)
+
+        tag_name_rows_by_tag_id =
+            TagName
+            .joins('INNER JOIN tags ON tags.tag_name_id = tag_names.id')
+            .where(tags: { id: tag_ids })
+            .pluck('tags.id', 'tag_names.name')
+            .each_with_object({ }) do |row, h|
+              h[row[0]] = row[1]
+            end
+
+        nico_tag_relation_rows_by_tag_id =
+            NicoTagRelation
+            .joins('INNER JOIN tags nico_tags ON nico_tags.id = nico_tag_relations.nico_tag_id')
+            .joins('INNER JOIN tags linked_tags ON linked_tags.id = nico_tag_relations.tag_id')
+            .joins('INNER JOIN tag_names ON tag_names.id = linked_tags.tag_name_id')
+            .where(nico_tags: { id: tag_ids })
+            .where(linked_tags: { discarded_at: nil })
+            .where(tag_names: { discarded_at: nil })
+            .pluck('nico_tags.id', 'tag_names.name')
+            .each_with_object(Hash.new { |h, k| h[k] = [] }) do |row, h|
+              h[row[0]] << row[1]
+            end
+
+        NicoTagVersion.insert_all(tags.map { |tag|
+          { tag_id: tag.id,
+            version_no: 1,
+            event_type: 'create',
+            name: tag_name_rows_by_tag_id[tag.id],
+            linked_tags: nico_tag_relation_rows_by_tag_id[tag.id].sort.join(' '),
+            created_at: tag.created_at,
+            created_by_user_id: nil }
+        })
+      end
+    end
+  end
+
+  def down
+    drop_table :nico_tag_versions
+    drop_table :tag_versions
+  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_04_09_123700) do
+ActiveRecord::Schema[8.0].define(version: 2026_04_19_035400) 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
@@ -104,6 +104,21 @@ ActiveRecord::Schema[8.0].define(version: 2026_04_09_123700) do
     t.index ["tag_id"], name: "index_nico_tag_relations_on_tag_id"
   end
 
+  create_table "nico_tag_versions", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "tag_id", null: false
+    t.integer "version_no", null: false
+    t.string "event_type", null: false
+    t.string "name", null: false
+    t.text "linked_tags", null: false
+    t.datetime "created_at", null: false
+    t.bigint "created_by_user_id"
+    t.index ["created_at"], name: "index_nico_tag_versions_on_created_at"
+    t.index ["created_by_user_id", "created_at"], name: "index_nico_tag_versions_on_created_by_user_id_and_created_at", order: { created_at: :desc }
+    t.index ["tag_id", "created_at"], name: "index_nico_tag_versions_on_tag_id_and_created_at", order: { created_at: :desc }
+    t.index ["tag_id", "version_no"], name: "index_nico_tag_versions_on_tag_id_and_version_no", unique: true
+    t.check_constraint "`version_no` > 0", name: "nico_tag_versions_version_no_positive"
+  end
+
   create_table "post_similarities", primary_key: ["post_id", "target_post_id"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "post_id", null: false
     t.bigint "target_post_id", null: false
@@ -216,6 +231,23 @@ ActiveRecord::Schema[8.0].define(version: 2026_04_09_123700) do
     t.index ["target_tag_id"], name: "index_tag_similarities_on_target_tag_id"
   end
 
+  create_table "tag_versions", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "tag_id", null: false
+    t.integer "version_no", null: false
+    t.string "event_type", null: false
+    t.string "name", null: false
+    t.string "category", null: false
+    t.text "aliases", null: false
+    t.text "parent_tag_ids", null: false
+    t.datetime "created_at", null: false
+    t.bigint "created_by_user_id"
+    t.index ["created_at"], name: "index_tag_versions_on_created_at"
+    t.index ["created_by_user_id", "created_at"], name: "index_tag_versions_on_created_by_user_id_and_created_at", order: { created_at: :desc }
+    t.index ["tag_id", "created_at"], name: "index_tag_versions_on_tag_id_and_created_at", order: { created_at: :desc }
+    t.index ["tag_id", "version_no"], name: "index_tag_versions_on_tag_id_and_version_no", unique: true
+    t.check_constraint "`version_no` > 0", name: "tag_versions_version_no_positive"
+  end
+
   create_table "tags", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "tag_name_id", null: false
     t.string "category", default: "general", null: false
@@ -377,6 +409,8 @@ ActiveRecord::Schema[8.0].define(version: 2026_04_09_123700) 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_similarities", "posts"
   add_foreign_key "post_similarities", "posts", column: "target_post_id"
   add_foreign_key "post_tags", "posts"
@@ -394,6 +428,8 @@ ActiveRecord::Schema[8.0].define(version: 2026_04_09_123700) 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"
   add_foreign_key "theatre_comments", "users"

backend/lib/tasks/sync_nico.rake

@@ -115,6 +115,10 @@ namespace :nico do
       datum['tags'].each do |raw|
         name = TagNameSanitisationRule.sanitise("nico:#{ raw }")
         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)
+
         desired_nico_tag_based_ids << tag.id
 
         # 新たに記載される外部タグと連携される内部タグを記載
@@ -149,6 +153,7 @@ namespace :nico do
       if post_created
         PostVersionRecorder.record!(post:, event_type: :create, created_by_user: nil)
       elsif post_changed || kept_tag_ids != desired_all_tag_ids.to_set
+        PostVersionRecorder.ensure_snapshot!(post, created_by_user: nil)
         PostVersionRecorder.record!(post:, event_type: :update, created_by_user: nil)
       end
     end

backend/spec/models/version_record_spec.rb

@@ -0,0 +1,74 @@
+require 'rails_helper'
+
+RSpec.describe VersionRecord, type: :model do
+  let!(:tag) { create(:tag, name: 'version_record_tag') }
+  let!(:nico_tag) { create(:tag, :nico, name: 'nico:version_record_tag') }
+
+  it 'makes TagVersion read only after create' do
+    version = TagVersion.create!(
+      tag: tag,
+      version_no: 1,
+      event_type: 'create',
+      name: tag.name,
+      category: tag.category,
+      aliases: '',
+      parent_tag_ids: '',
+      created_at: Time.current,
+      created_by_user: nil
+    )
+
+    expect {
+      version.update!(name: 'changed')
+    }.to raise_error(ActiveRecord::ReadOnlyRecord)
+  end
+
+  it 'prevents TagVersion destroy' do
+    version = TagVersion.create!(
+      tag: tag,
+      version_no: 1,
+      event_type: 'create',
+      name: tag.name,
+      category: tag.category,
+      aliases: '',
+      parent_tag_ids: '',
+      created_at: Time.current,
+      created_by_user: nil
+    )
+
+    expect {
+      version.destroy!
+    }.to raise_error(ActiveRecord::ReadOnlyRecord)
+  end
+
+  it 'makes NicoTagVersion read only after create' do
+    version = NicoTagVersion.create!(
+      tag: nico_tag,
+      version_no: 1,
+      event_type: 'create',
+      name: nico_tag.name,
+      linked_tags: '',
+      created_at: Time.current,
+      created_by_user: nil
+    )
+
+    expect {
+      version.update!(name: 'nico:changed')
+    }.to raise_error(ActiveRecord::ReadOnlyRecord)
+  end
+
+  it 'prevents NicoTagVersion destroy' do
+    version = NicoTagVersion.create!(
+      tag: nico_tag,
+      version_no: 1,
+      event_type: 'create',
+      name: nico_tag.name,
+      linked_tags: '',
+      created_at: Time.current,
+      created_by_user: nil
+    )
+
+    expect {
+      version.destroy!
+    }.to raise_error(ActiveRecord::ReadOnlyRecord)
+  end
+end

backend/spec/requests/nico_tags_spec.rb

@@ -14,6 +14,7 @@ RSpec.describe 'NicoTags', type: :request do
 
   describe 'PATCH /tags/nico/:id' do
     let(:member) { create(:user, :member) }
+    let(:admin) { create(:user, :admin) }
     let(:nico_tag) { create(:tag, :nico) }
 
     it '401 when not logged in' do
@@ -34,5 +35,59 @@ RSpec.describe 'NicoTags', type: :request do
       patch "/tags/nico/#{non_nico.id}", params: { tags: 'a b' }
       expect(response).to have_http_status(:bad_request)
     end
+
+    it '200 and updates linked tags while recording tag versions' do
+      sign_in_as(admin)
+
+      nico_tag_name = TagName.create!(name: 'nico:nico_tags_spec_source')
+      nico_tag = Tag.create!(tag_name: nico_tag_name, category: :nico)
+
+      linked_a_name = TagName.create!(name: 'nico_linked_a')
+      linked_a = Tag.create!(tag_name: linked_a_name, category: :general)
+
+      linked_b_name = TagName.create!(name: 'nico_linked_b')
+      linked_b = Tag.create!(tag_name: linked_b_name, category: :general)
+
+      TagVersioning.ensure_snapshot!(nico_tag, created_by_user: admin)
+
+      expect {
+        patch "/tags/nico/#{nico_tag.id}", params: {
+          tags: "  #{linked_a.name}\n#{linked_b.name}  "
+        }
+      }.to change(TagVersion, :count).by(2)
+       .and change(NicoTagVersion, :count).by(1)
+
+      expect(response).to have_http_status(:ok)
+
+      names = json.map { |t| t['name'] }
+      expect(names).to match_array(['nico_linked_a', 'nico_linked_b'])
+
+      linked_versions = TagVersion.where(tag: [linked_a, linked_b]).order(:tag_id)
+      expect(linked_versions.map(&:event_type)).to eq(['create', 'create'])
+      expect(linked_versions.map(&:created_by_user_id)).to all(eq(admin.id))
+
+      versions = nico_tag.reload.nico_tag_versions.order(:version_no)
+      expect(versions.map(&:event_type)).to eq(['create', 'update'])
+      expect(versions.last.linked_tags.split).to match_array([
+        'nico_linked_a',
+        'nico_linked_b'
+      ])
+      expect(versions.last.created_by_user_id).to eq(admin.id)
+    end
+
+    it '400 when linked tag normalises to nico tag' do
+      sign_in_as(member)
+
+      other_nico = create(:tag, :nico, name: 'nico:linked_ng')
+      TagName.create!(name: 'linked_ng_alias', canonical: other_nico.tag_name)
+
+      TagVersioning.ensure_snapshot!(nico_tag, created_by_user: member)
+
+      expect {
+        patch "/tags/nico/#{nico_tag.id}", params: { tags: 'linked_ng_alias' }
+      }.not_to change(NicoTagVersion, :count)
+
+      expect(response).to have_http_status(:bad_request)
+    end
   end
 end

backend/spec/requests/posts_spec.rb

@@ -1,8 +1,8 @@
-include ActiveSupport::Testing::TimeHelpers
-
 require 'rails_helper'
 require 'set'
 
+include ActiveSupport::Testing::TimeHelpers
+
 RSpec.describe 'Posts API', type: :request do
   # create / update で thumbnail.attach は走るが、
   # resized_thumbnail! が MiniMagick 依存でコケやすいので request spec ではスタブしとくのが無難。
@@ -756,6 +756,218 @@ RSpec.describe 'Posts API', type: :request do
     end
   end
 
+  describe 'GET /posts/versions' do
+    let(:member) { create(:user, :member, name: 'version member') }
+
+    let(:t_v1) { Time.zone.local(2020, 1, 1, 12, 0, 0) }
+    let(:t_v2) { Time.zone.local(2020, 1, 2, 12, 0, 0) }
+    let(:t_other) { Time.zone.local(2020, 1, 3, 12, 0, 0) }
+
+    let(:oc_from) { Time.zone.local(2019, 12, 31, 0, 0, 0) }
+    let(:oc_before) { Time.zone.local(2020, 1, 1, 0, 0, 0) }
+
+    let!(:tag_name2) { TagName.create!(name: 'spec_tag_2') }
+    let!(:tag2) { Tag.create!(tag_name: tag_name2, category: :general) }
+
+    def snapshot_tags(post)
+      post.snapshot_tag_names.join(' ')
+    end
+
+    def create_post_version!(post, version_no:, event_type:, created_by_user:, created_at:)
+      PostVersion.create!(
+        post: post,
+        version_no: version_no,
+        event_type: event_type,
+        title: post.title,
+        url: post.url,
+        thumbnail_base: post.thumbnail_base,
+        tags: snapshot_tags(post),
+        parent: post.parent,
+        original_created_from: post.original_created_from,
+        original_created_before: post.original_created_before,
+        created_at: created_at,
+        created_by_user: created_by_user
+      )
+    end
+
+    let!(:v1) do
+      travel_to(t_v1) do
+        create_post_version!(
+          post_record,
+          version_no: 1,
+          event_type: 'create',
+          created_by_user: member,
+          created_at: t_v1
+        )
+      end
+    end
+
+    let!(:v2) do
+      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',
+        original_created_from: oc_from,
+        original_created_before: oc_before
+      )
+
+      travel_to(t_v2) do
+        create_post_version!(
+          post_record.reload,
+          version_no: 2,
+          event_type: 'update',
+          created_by_user: member,
+          created_at: t_v2
+        )
+      end
+    end
+
+    let!(:other_post_version) do
+      other_post = Post.create!(
+        title: 'other versioned post',
+        url: 'https://example.com/other-versioned'
+      )
+      PostTag.create!(post: other_post, tag: tag)
+
+      travel_to(t_other) do
+        create_post_version!(
+          other_post,
+          version_no: 1,
+          event_type: 'create',
+          created_by_user: member,
+          created_at: t_other
+        )
+      end
+    end
+
+    it 'returns versions for the specified post in reverse chronological order' do
+      get '/posts/versions', params: { post: post_record.id }
+
+      expect(response).to have_http_status(:ok)
+      expect(json).to include('versions', 'count')
+      expect(json.fetch('count')).to eq(2)
+
+      versions = json.fetch('versions')
+      expect(versions.map { |v| v['post_id'] }.uniq).to eq([post_record.id])
+      expect(versions.map { |v| v['version_no'] }).to eq([2, 1])
+
+      latest = versions.first
+      expect(latest).to include(
+        'post_id' => post_record.id,
+        'version_no' => 2,
+        'event_type' => 'update',
+        'created_by_user' => {
+          'id' => member.id,
+          'name' => member.name
+        }
+      )
+
+      expect(latest.fetch('title')).to eq(
+        'current' => 'updated spec post',
+        'prev' => 'spec post'
+      )
+      expect(latest.fetch('url')).to eq(
+        'current' => 'https://example.com/spec',
+        'prev' => 'https://example.com/spec'
+      )
+      expect(latest.fetch('thumbnail')).to eq(
+        'current' => nil,
+        'prev' => nil
+      )
+      expect(latest.fetch('thumbnail_base')).to eq(
+        'current' => nil,
+        'prev' => nil
+      )
+      expect(latest.fetch('tags')).to include(
+        { 'name' => 'spec_tag_2', 'type' => 'added' },
+        { 'name' => 'spec_tag', 'type' => 'removed' }
+      )
+      expect(latest.fetch('original_created_from')).to eq(
+        'current' => oc_from.iso8601,
+        'prev' => nil
+      )
+      expect(latest.fetch('original_created_before')).to eq(
+        'current' => oc_before.iso8601,
+        'prev' => nil
+      )
+      expect(latest.fetch('created_at')).to eq(t_v2.iso8601)
+
+      first = versions.second
+      expect(first).to include(
+        'post_id' => post_record.id,
+        'version_no' => 1,
+        'event_type' => 'create',
+        'created_by_user' => {
+          'id' => member.id,
+          'name' => member.name
+        }
+      )
+      expect(first.fetch('title')).to eq(
+        'current' => 'spec post',
+        'prev' => nil
+      )
+      expect(first.fetch('tags')).to include(
+        { 'name' => 'spec_tag', 'type' => 'added' }
+      )
+      expect(first.fetch('created_at')).to eq(t_v1.iso8601)
+    end
+
+    it 'filters versions by tag when the current snapshot includes the tag' do
+      get '/posts/versions', params: { post: post_record.id, tag: tag2.id }
+
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('count')).to eq(1)
+
+      versions = json.fetch('versions')
+      expect(versions.size).to eq(1)
+      expect(versions[0]['post_id']).to eq(post_record.id)
+      expect(versions[0]['version_no']).to eq(2)
+      expect(versions[0]['tags']).to include(
+        { 'name' => 'spec_tag_2', 'type' => 'added' }
+      )
+    end
+
+    it 'filters versions by tag when the tag exists in either current or previous snapshot' do
+      get '/posts/versions', params: { post: post_record.id, tag: tag.id }
+
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('count')).to eq(2)
+
+      versions = json.fetch('versions')
+      expect(versions.map { |v| v['post_id'] }).to all(eq(post_record.id))
+      expect(versions.map { |v| v['version_no'] }).to eq([2, 1])
+
+      latest = versions[0]
+      first = versions[1]
+
+      expect(latest['tags']).to include(
+        { 'name' => 'spec_tag', 'type' => 'removed' }
+      )
+      expect(first['tags']).to include(
+        { 'name' => 'spec_tag', 'type' => 'added' }
+      )
+    end
+
+    it 'returns empty when tag does not exist' do
+      get '/posts/versions', params: { tag: 999_999_999 }
+
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('versions')).to eq([])
+      expect(json.fetch('count')).to eq(0)
+    end
+
+    it 'clamps page and limit to at least 1' do
+      get '/posts/versions', params: { post: post_record.id, page: 0, limit: 0 }
+
+      expect(response).to have_http_status(:ok)
+      expect(json.fetch('count')).to eq(2)
+
+      versions = json.fetch('versions')
+      expect(versions.size).to eq(1)
+      expect(versions[0]['version_no']).to eq(2)
+    end
+  end
+
   describe 'POST /posts/:id/viewed' do
     let(:user) { create(:user) }
 
@@ -870,15 +1082,16 @@ RSpec.describe 'Posts API', type: :request do
 
     it 'does not create a new version on PUT /posts/:id when snapshot is unchanged' do
       sign_in_as(member)
-      create_post_version_for!(post_record)
 
-      expect do
+      PostTag.create!(post: post_record, tag: Tag.no_deerjikist)
+      create_post_version_for!(post_record.reload)
+
+      expect {
         put "/posts/#{post_record.id}", params: {
           title: post_record.title,
           tags: 'spec_tag'
         }
-      end.not_to change(PostVersion, :count)
+      }.not_to change(PostVersion, :count)
-
       expect(response).to have_http_status(:ok)
 
       version = post_record.reload.post_versions.order(:version_no).last
@@ -918,4 +1131,52 @@ RSpec.describe 'Posts API', type: :request do
       expect(response).to have_http_status(:unprocessable_entity)
     end
   end
+
+  describe 'tag versioning from post write actions' do
+    let(:member) { create(:user, :member) }
+
+    it 'creates tag snapshot for normalised tags on POST /posts' do
+      sign_in_as(member)
+
+      expect {
+        post '/posts', params: {
+          title: 'tag versioned post',
+          url: 'https://example.com/tag-versioned-post',
+          tags: 'spec_tag',
+          thumbnail: dummy_upload
+        }
+      }.to change { tag.reload.tag_versions.count }.by(1)
+
+      expect(response).to have_http_status(:created)
+
+      version = tag.reload.tag_versions.order(:version_no).last
+      expect(version.version_no).to eq(1)
+      expect(version.event_type).to eq('create')
+      expect(version.name).to eq('spec_tag')
+      expect(version.category).to eq('general')
+      expect(version.created_by_user_id).to eq(member.id)
+    end
+
+    it 'creates tag snapshot for normalised tags on PUT /posts/:id' do
+      sign_in_as(member)
+
+      tag_name2 = TagName.create!(name: 'spec_tag_2')
+      tag2 = Tag.create!(tag_name: tag_name2, category: :general)
+
+      expect {
+        put "/posts/#{post_record.id}", params: {
+          title: 'updated title',
+          tags: 'spec_tag_2'
+        }
+      }.to change { tag2.reload.tag_versions.count }.by(1)
+
+      expect(response).to have_http_status(:ok)
+
+      version = tag2.reload.tag_versions.order(:version_no).last
+      expect(version.version_no).to eq(1)
+      expect(version.event_type).to eq('create')
+      expect(version.name).to eq('spec_tag_2')
+      expect(version.created_by_user_id).to eq(member.id)
+    end
+  end
 end

backend/spec/requests/tag_children_spec.rb

@@ -58,15 +58,47 @@ RSpec.describe "TagChildren", type: :request do
       end
     end
 
-    context "when Tag.find raises (invalid ids) it still returns 204" do
+    context "when Tag.find raises (invalid ids)" do
       before { stub_current_user(admin) }
 
       let(:parent_id) { -1 }
       let(:child_id) { -1 }
 
-      it "returns 204 (rescue nil)" do
+      it "returns 404" do
         do_request
-        expect(response).to have_http_status(:no_content)
+        expect(response).to have_http_status(:not_found)
+      end
+    end
+
+    context 'when parent is nico' do
+      before { stub_current_user(admin) }
+
+      let!(:parent) { create(:tag, :nico, name: 'nico:parent_ng') }
+      let(:parent_id) { parent.id }
+      let(:child_id) { child.id }
+
+      it 'returns 400 and does not create relation' do
+        expect {
+          do_request
+        }.not_to change(TagImplication, :count)
+
+        expect(response).to have_http_status(:bad_request)
+      end
+    end
+
+    context 'when child is nico' do
+      before { stub_current_user(admin) }
+
+      let!(:child) { create(:tag, :nico, name: 'nico:child_ng') }
+      let(:parent_id) { parent.id }
+      let(:child_id) { child.id }
+
+      it 'returns 400 and does not create relation' do
+        expect {
+          do_request
+        }.not_to change(TagImplication, :count)
+
+        expect(response).to have_http_status(:bad_request)
       end
     end
   end
@@ -116,17 +148,57 @@ RSpec.describe "TagChildren", type: :request do
 
         expect(response).to have_http_status(:no_content)
       end
+
+      it 'records create and update versions for child tag' do
+        expect {
+          do_request
+        }.to change(TagVersion, :count).by(2)
+
+        expect(response).to have_http_status(:no_content)
+
+        versions = child.reload.tag_versions.order(:version_no)
+        expect(versions.map(&:event_type)).to eq(['create', 'update'])
+        expect(versions.first.parent_tag_ids.split).to include(parent.id.to_s)
+        expect(versions.second.parent_tag_ids).to eq('')
+        expect(versions.second.created_by_user_id).to eq(admin.id)
+      end
     end
 
-    context "when Tag.find raises (invalid ids) it still returns 204" do
+    context "when Tag.find raises (invalid ids)" do
       before { stub_current_user(admin) }
 
       let(:parent_id) { -1 }
       let(:child_id) { -1 }
 
-      it "returns 204 (rescue nil)" do
+      it "returns 404" do
         do_request
-        expect(response).to have_http_status(:no_content)
+        expect(response).to have_http_status(:not_found)
+      end
+    end
+
+    context 'when parent is nico' do
+      before { stub_current_user(admin) }
+
+      let!(:parent) { create(:tag, :nico, name: 'nico:parent_ng_delete') }
+      let(:parent_id) { parent.id }
+      let(:child_id) { child.id }
+
+      it 'returns 400' do
+        do_request
+        expect(response).to have_http_status(:bad_request)
+      end
+    end
+
+    context 'when child is nico' do
+      before { stub_current_user(admin) }
+
+      let!(:child) { create(:tag, :nico, name: 'nico:child_ng_delete') }
+      let(:parent_id) { parent.id }
+      let(:child_id) { child.id }
+
+      it 'returns 400' do
+        do_request
+        expect(response).to have_http_status(:bad_request)
       end
     end
   end

backend/spec/requests/tags_spec.rb

@@ -364,9 +364,70 @@ RSpec.describe 'Tags API', type: :request do
         expect(response.status).to be_in([404, 500])
       end
 
-      it 'バリデーションで update! が失敗したら（通常は 422 か 500）' do
+      it 'nico category への変更は 422 を返す' do
         patch "/tags/#{tag.id}", params: { name: 'new', category: 'nico' }
-        expect(response.status).to be_in([422, 500])
+
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(tag.reload.name).to eq('spec_tag')
+        expect(tag.category).to eq('general')
+      end
+
+      it 'creates initial and update tag versions when name and category change' do
+        expect {
+          patch "/tags/#{tag.id}", params: { name: 'new_tag_name', category: 'meme' }
+        }.to change(TagVersion, :count).by(2)
+
+        expect(response).to have_http_status(:ok)
+
+        versions = tag.reload.tag_versions.order(:version_no)
+
+        expect(versions.map(&:event_type)).to eq(['create', 'update'])
+
+        expect(versions.first.name).to eq('spec_tag')
+        expect(versions.first.category).to eq('general')
+        expect(versions.first.aliases.split).to include('unko')
+
+        expect(versions.second.name).to eq('new_tag_name')
+        expect(versions.second.category).to eq('meme')
+        expect(versions.second.created_by_user_id).to eq(member_user.id)
+      end
+
+      it 'returns 422 when changing normal tag category to nico' do
+        expect {
+          patch "/tags/#{tag.id}", params: { category: 'nico' }
+        }.not_to change(TagVersion, :count)
+
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(tag.reload.category).to eq('general')
+      end
+
+      it 'creates nico tag versions when updating nico tag name' do
+        nico_tag_name = TagName.create!(name: 'nico:tags_spec_source')
+        nico_tag = Tag.create!(tag_name: nico_tag_name, category: :nico)
+
+        expect {
+          patch "/tags/#{nico_tag.id}", params: { name: 'nico:tags_spec_renamed' }
+        }.to change(NicoTagVersion, :count).by(2)
+
+        expect(response).to have_http_status(:ok)
+
+        versions = nico_tag.reload.nico_tag_versions.order(:version_no)
+        expect(versions.map(&:event_type)).to eq(['create', 'update'])
+        expect(versions.first.name).to eq('nico:tags_spec_source')
+        expect(versions.second.name).to eq('nico:tags_spec_renamed')
+        expect(versions.second.created_by_user_id).to eq(member_user.id)
+      end
+
+      it 'returns 422 when changing nico tag category to normal category' do
+        nico_tag_name = TagName.create!(name: 'nico:category_change_ng')
+        nico_tag = Tag.create!(tag_name: nico_tag_name, category: :nico)
+
+        expect {
+          patch "/tags/#{nico_tag.id}", params: { category: 'general' }
+        }.not_to change(NicoTagVersion, :count)
+
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(nico_tag.reload.category).to eq('nico')
       end
     end
   end

backend/spec/tasks/nico_sync_spec.rb

@@ -214,4 +214,108 @@ RSpec.describe "nico:sync" do
     expect(version.event_type).to eq('create')
     expect(version.tags).to eq(snapshot_tags(post.reload))
   end
+
+  it '新規 nico tag に nico tag version を作る' do
+    Tag.bot
+    Tag.tagme
+    Tag.niconico
+    Tag.video
+    Tag.no_deerjikist
+
+    stub_python([{
+      'code' => 'sm9',
+      'title' => 't',
+      'tags' => ['AAA'],
+      'uploaded_at' => '2026-01-01 12:34:56'
+    }])
+
+    allow(URI).to receive(:open).and_return(StringIO.new('<html></html>'))
+
+    expect {
+      run_rake_task('nico:sync')
+    }.to change(NicoTagVersion, :count).by(1)
+
+    nico_tag = Tag.joins(:tag_name).find_by!(tag_names: { name: 'nico:AAA' })
+    version = nico_tag.nico_tag_versions.order(:version_no).last
+
+    expect(version.version_no).to eq(1)
+    expect(version.event_type).to eq('create')
+    expect(version.name).to eq('nico:AAA')
+    expect(version.created_by_user).to be_nil
+  end
+
+  it '既存 post に version が無い場合は create snapshot を補う' do
+    post = Post.create!(
+      title: 'old',
+      url: 'https://www.nicovideo.jp/watch/sm9',
+      uploaded_user: nil
+    )
+
+    kept_general = create_tag!('spec_kept_without_version', category: 'general')
+    PostTag.create!(post: post, tag: kept_general)
+
+    Tag.bot
+    Tag.tagme
+    Tag.no_deerjikist
+
+    stub_python([{
+      'code' => 'sm9',
+      'title' => 'changed title',
+      'tags' => ['AAA'],
+      'uploaded_at' => '2026-01-01 12:34:56'
+    }])
+
+    allow(URI).to receive(:open).and_return(StringIO.new('<html></html>'))
+
+    expect {
+      run_rake_task('nico:sync')
+    }.to change { post.reload.post_versions.count }.by(1)
+
+    versions = post.reload.post_versions.order(:version_no)
+
+    expect(versions.map(&:event_type)).to eq(['create'])
+    expect(versions.first.title).to eq('changed title')
+    expect(versions.first.tags).to eq(snapshot_tags(post.reload))
+  end
+
+  it '既存 version がある post には update version を作る' do
+    post = Post.create!(
+      title: 'old',
+      url: 'https://www.nicovideo.jp/watch/sm9',
+      uploaded_user: nil
+    )
+
+    kept_general = create_tag!('spec_kept_with_version', category: 'general')
+    PostTag.create!(post: post, tag: kept_general)
+
+    PostVersionRecorder.record!(
+      post: post,
+      event_type: :create,
+      created_by_user: nil
+    )
+
+    Tag.bot
+    Tag.tagme
+    Tag.no_deerjikist
+
+    stub_python([{
+      'code' => 'sm9',
+      'title' => 'changed title',
+      'tags' => ['AAA'],
+      'uploaded_at' => '2026-01-01 12:34:56'
+    }])
+
+    allow(URI).to receive(:open).and_return(StringIO.new('<html></html>'))
+
+    expect {
+      run_rake_task('nico:sync')
+    }.to change { post.reload.post_versions.count }.by(1)
+
+    versions = post.reload.post_versions.order(:version_no)
+
+    expect(versions.map(&:event_type)).to eq(['create', 'update'])
+    expect(versions.first.title).to eq('old')
+    expect(versions.second.title).to eq('changed title')
+    expect(versions.second.tags).to eq(snapshot_tags(post.reload))
+  end
 end

frontend/src/components/DraggableDroppableTagRow.tsx

@@ -90,7 +90,9 @@ export default (({ tag, nestLevel, pathKey, parentTagId, suppressClickRef, sp }:
 	  className={cn ('rounded select-none', over && 'ring-2 ring-offset-2')}
 	  {...attributes}
 	  {...listeners}>
-	<motion.div layoutId={`tag-${ sp ? 'sp-' : '' }${ tag.id }`}>
+	<motion.div
+	    transition={{ layout: { duration: .2, ease: 'easeOut' } }}
+	    layoutId={`tag-${ sp ? 'sp-' : '' }${ tag.id }`}>
 	  <TagLink tag={tag} nestLevel={nestLevel}/>
 	</motion.div>
       </div>)

frontend/src/components/PostEditForm.tsx

@@ -62,7 +62,7 @@ export default (({ post, onSave }: Props) => {
 	  <Label>タイトル</Label>
 	  <input type="text"
 		 className="w-full border rounded p-2"
-		 value={title}
+		 value={title ?? ''}
 		 onChange={ev => setTitle (ev.target.value)}/>
 	</div>
 

frontend/src/components/TagDetailSidebar.tsx

@@ -313,7 +313,9 @@ export default (({ post, sp }: Props) => {
 	  {CATEGORIES.map ((cat: Category) => ((tags[cat] ?? []).length > 0 || dragging) && (
 	      <div className="my-3" key={cat}>
 		<SubsectionTitle>
-		  <motion.div layoutId={`tag-${ sp ? 'sp-' : '' }${ cat }`}>
+		  <motion.div
+		      layoutId={`tag-${ sp ? 'sp-' : '' }${ cat }`}
+		      transition={{ layout: { duration: .2, ease: 'easeOut' } }}>
 		    {CATEGORY_NAMES[cat]}
 		  </motion.div>
 		</SubsectionTitle>
@@ -325,7 +327,9 @@ export default (({ post, sp }: Props) => {
 		</ul>
 	      </div>))}
 	  {post && (
-	      <motion.div layoutId={`post-info-${ sp }`}>
+	      <motion.div
+		  layoutId={`post-info-${ sp }`}
+		  transition={{ layout: { duration: .2, ease: 'easeOut' } }}>
 		<SectionTitle>情報</SectionTitle>
 		<ul>
 		  <li>Id.: {post.id}</li>

frontend/src/lib/posts.ts

@@ -1,6 +1,6 @@
 import { apiDelete, apiGet, apiPost } from '@/lib/api'
 
-import type { FetchPostsParams, Post, PostTagChange } from '@/types'
+import type { FetchPostsParams, Post, PostVersion } from '@/types'
 
 
 export const fetchPosts = async (
@@ -29,15 +29,15 @@ export const fetchPost = async (id: string): Promise<Post> => await apiGet (`/po
 
 
 export const fetchPostChanges = async (
-    { id, tag, page, limit }: {
+    { post, tag, page, limit }: {
-	id?:    string
+	post?:  string
 	tag?:   string
 	page:   number
 	limit:  number },
 ): Promise<{
-    changes:  PostTagChange[]
+    versions: PostVersion[]
     count:    number }> =>
-	await apiGet ('/posts/changes', { params: { ...(id && { id }),
+	await apiGet ('/posts/versions', { params: { ...(post && { post }),
 						     ...(tag && { tag }),
 						     page, limit } })
 

frontend/src/lib/queryKeys.ts

@@ -5,7 +5,7 @@ export const postsKeys = {
     index:    (p: FetchPostsParams) => ['posts', 'index', p] as const,
     show:     (id: string) => ['posts', id] as const,
     related:  (id: string) => ['related', id] as const,
-    changes:  (p: { id?: string; tag?: string; page: number; limit: number }) =>
+    changes:  (p: { post?: string; tag?: string; page: number; limit: number }) =>
 		  ['posts', 'changes', p] as const }
 
 export const tagsKeys = {

frontend/src/pages/posts/PostDetailPage.tsx

@@ -96,7 +96,7 @@ export default (({ user }: Props) => {
       <div className="md:flex md:flex-1 overflow-y-auto md:overflow-y-hidden">
 	<Helmet>
 	  {(post?.thumbnail || post?.thumbnailBase) && (
-	      <meta name="thumbnail" content={post.thumbnail || post.thumbnailBase}/>)}
+	      <meta name="thumbnail" content={post.thumbnail! || post.thumbnailBase!}/>)}
 	  {post && <title>{`${ post.title || post.url } | ${ SITE_TITLE }`}</title>}
 	</Helmet>
 
@@ -116,7 +116,7 @@ export default (({ user }: Props) => {
 			initial={{ opacity: 1 }}
 			animate={{ opacity: 0 }}
 			transition={{ duration: .2, ease: 'easeOut' }}>
-		      <img src={post.thumbnail || post.thumbnailBase}
+		      <img src={post.thumbnail || post.thumbnailBase || undefined}
 			   alt={post.title || post.url}
 			   title={post.title || post.url || undefined}
 			   className="object-cover w-full h-full"/>

frontend/src/pages/posts/PostHistoryPage.tsx

@@ -1,4 +1,4 @@
-import { useQuery } from '@tanstack/react-query'
+import { useQuery, useQueryClient } from '@tanstack/react-query'
 import { motion } from 'framer-motion'
 import { useEffect } from 'react'
 import { Helmet } from 'react-helmet-async'
@@ -9,15 +9,30 @@ import PrefetchLink from '@/components/PrefetchLink'
 import PageTitle from '@/components/common/PageTitle'
 import Pagination from '@/components/common/Pagination'
 import MainArea from '@/components/layout/MainArea'
+import { toast } from '@/components/ui/use-toast'
 import { SITE_TITLE } from '@/config'
+import { apiPut } from '@/lib/api'
 import { fetchPostChanges } from '@/lib/posts'
 import { postsKeys, tagsKeys } from '@/lib/queryKeys'
 import { fetchTag } from '@/lib/tags'
-import { cn, dateString } from '@/lib/utils'
+import { cn, dateString, originalCreatedAtString } from '@/lib/utils'
 
 import type { FC } from 'react'
 
 
+const renderDiff = (diff: { current: string | null; prev: string | null }) => (
+    <>
+      {(diff.prev && diff.prev !== diff.current) && (
+	  <>
+	    <del className="text-red-600 dark:text-red-400">
+	      {diff.prev}
+	    </del>
+	    {diff.current && <br/>}
+	  </>)}
+      {diff.current}
+    </>)
+
+
 export default (() => {
   const location = useLocation ()
   const query = new URLSearchParams (location.search)
@@ -36,15 +51,17 @@ export default (() => {
       : { data: null }
 
   const { data, isLoading: loading } = useQuery ({
-      queryKey: postsKeys.changes ({ ...(id && { id }),
+      queryKey: postsKeys.changes ({ ...(id && { post: id }),
 				     ...(tagId && { tag: tagId }),
 				     page, limit }),
-      queryFn:  () => fetchPostChanges ({ ...(id && { id }),
+      queryFn:  () => fetchPostChanges ({ ...(id && { post: id }),
 					  ...(tagId && { tag: tagId }),
 					  page, limit }) })
-  const changes = data?.changes ?? []
+  const changes = data?.versions ?? []
   const totalPages = data ? Math.ceil (data.count / limit) : 0
 
+  const qc = useQueryClient ()
+
   useEffect (() => {
     document.querySelector ('table')?.scrollIntoView ({ behavior: 'smooth' })
   }, [location.search])
@@ -65,76 +82,171 @@ export default (() => {
 
 	{loading ? 'Loading...' : (
 	    <>
-	      <table className="table-auto w-full border-collapse">
+	      <div className="overflow-x-auto">
+		<table className="w-full min-w-[1200px] table-fixed border-collapse">
+		  <colgroup>
+		    {/* 投稿 */}
+		    <col className="w-64"/>
+		    {/* 版 */}
+		    <col className="w-40"/>
+		    {/* タイトル */}
+		    <col className="w-96"/>
+		    {/* URL */}
+		    <col className="w-96"/>
+		    {/* タグ */}
+		    <col className="w-[48rem]"/>
+		    {/* オリジナルの投稿日時 */}
+		    <col className="w-96"/>
+		    {/* 更新日時 */}
+		    <col className="w-64"/>
+		    {/* (差戻ボタン) */}
+		    <col className="w-20"/>
+		  </colgroup>
+
 		  <thead className="border-b-2 border-black dark:border-white">
 		    <tr>
 		      <th className="p-2 text-left">投稿</th>
-		    <th className="p-2 text-left">変更</th>
+		      <th className="p-2 text-left">版</th>
-		    <th className="p-2 text-left">日時</th>
+		      <th className="p-2 text-left">タイトル</th>
+		      <th className="p-2 text-left">URL</th>
+		      <th className="p-2 text-left">タグ</th>
+		      <th className="p-2 text-left">オリジナルの投稿日時</th>
+		      <th className="p-2 text-left">更新日時</th>
+		      <th className="p-2"/>
 		    </tr>
 		  </thead>
+
 		  <tbody>
 		    {changes.map ((change, i) => {
-		    const withPost = i === 0 || change.post.id !== changes[i - 1].post.id
+		      const withPost = i === 0 || change.postId !== changes[i - 1].postId
 		      if (withPost)
 			{
 			  rowsCnt = 1
 			  for (let j = i + 1;
 			       (j < changes.length
-			      && change.post.id === changes[j].post.id);
+				&& change.postId === changes[j].postId);
 			       ++j)
 			    ++rowsCnt
 			}
 
-		    let layoutId: string | undefined = `page-${ change.post.id }`
+		      let layoutId: string | undefined = `page-${ change.postId }`
 		      if (layoutIds.includes (layoutId))
 			layoutId = undefined
 		      else
 			layoutIds.push (layoutId)
 
 		      return (
-			<tr key={`${ change.timestamp }-${ change.post.id }-${ change.tag?.id }`}
+			  <tr key={`${ change.postId }.${ change.versionNo }`}
 			      className={cn ('even:bg-gray-100 dark:even:bg-gray-700',
 					     withPost && 'border-t')}>
 			    {withPost && (
 				<td className="align-top p-2 bg-white dark:bg-[#242424] border-r"
 				    rowSpan={rowsCnt}>
-				<PrefetchLink to={`/posts/${ change.post.id }`}>
+				  <PrefetchLink to={`/posts/${ change.postId }`}>
 				    <motion.div
 					layoutId={layoutId}
-				      transition={{ type: 'spring',
+					transition={{ layout: { duration: .2, ease: 'easeOut' } }}>
-						    stiffness: 500,
+				      <img src={change.thumbnail.current
-						    damping: 40,
+						|| change.thumbnailBase.current
-						    mass: .5 }}>
-				    <img src={change.post.thumbnail
-					      || change.post.thumbnailBase
 						|| undefined}
-					 alt={change.post.title || change.post.url}
+					   alt={change.title.current || change.url.current}
-					 title={change.post.title || change.post.url || undefined}
+					   title={change.title.current || change.url.current || undefined}
 					   className="w-40"/>
 				    </motion.div>
 				  </PrefetchLink>
 				</td>)}
+			    <td className="p-2">{change.postId}.{change.versionNo}</td>
+			    <td className="p-2 break-all">{renderDiff (change.title)}</td>
+			    <td className="p-2 break-all">{renderDiff (change.url)}</td>
 			    <td className="p-2">
-			    {change.tag
+			      {change.tags.map ((tag, i) => (
-			     ? <TagLink tag={change.tag} withWiki={false} withCount={false}/>
+				  tag.type === 'added'
-			     : '(マスタ削除済のタグ) '}
+				  ? (
-			    {`を${ change.changeType === 'add' ? '記載' : '消除' }`}
+				      <ins
+					  key={i}
+					  className="mr-2 text-green-600 dark:text-green-400">
+					{tag.name}
+				      </ins>)
+				  : (
+				      tag.type === 'removed'
+				      ? (
+					  <del
+					      key={i}
+					      className="mr-2 text-red-600 dark:text-red-400">
+					    {tag.name}
+					  </del>)
+				      : (
+					  <span key={i} className="mr-2">
+					    {tag.name}
+					  </span>))))}
 			    </td>
 			    <td className="p-2">
-			    {change.user
+			      {change.versionNo === 1
+			       ? originalCreatedAtString (change.originalCreatedFrom.current,
+							  change.originalCreatedBefore.current)
+			       : renderDiff ({
+				  current: originalCreatedAtString (
+					       change.originalCreatedFrom.current,
+					       change.originalCreatedBefore.current),
+				  prev: originalCreatedAtString (
+					    change.originalCreatedFrom.prev,
+					    change.originalCreatedBefore.prev) })}
+			    </td>
+			    <td className="p-2">
+			      {change.createdByUser
 			       ? (
-				<PrefetchLink to={`/users/${ change.user.id }`}>
+				  <PrefetchLink to={`/users/${ change.createdByUser.id }`}>
-				  {change.user.name}
+				    {change.createdByUser.name
+				     || `名もなきニジラー（#${ change.createdByUser.id }）`}
 				  </PrefetchLink>)
 			       : 'bot 操作'}
 			      <br/>
-			    {dateString (change.timestamp)}
+			      {dateString (change.createdAt)}
+			    </td>
+			    <td className="p-2">
+			      <a
+				  href="#"
+				  onClick={async e => {
+				    e.preventDefault ()
+
+				    if (!(confirm (
+					`『${ change.title.current
+					      || change.url.current }』を版 ${
+					change.versionNo } に差戻します．\nよろしいですか?`)))
+				      return
+
+				    try
+				    {
+				      await apiPut (
+					  `/posts/${ change.postId }`,
+					  { title: change.title.current,
+					    tags: change.tags
+						  .filter (t => t.type !== 'removed')
+						  .map (t => t.name)
+						  .filter (t => t.slice (0, 5) !== 'nico:')
+						  .join (' '),
+					    original_created_from:
+						change.originalCreatedFrom.current,
+					    original_created_before:
+						change.originalCreatedBefore.current })
+
+				      qc.invalidateQueries ({ queryKey: postsKeys.root })
+				      qc.invalidateQueries ({ queryKey: tagsKeys.root })
+				      toast ({ description: '差戻しました．' })
+				    }
+				    catch
+				    {
+				      toast ({ description: '差戻に失敗……' })
+				    }
+				  }}>
+				復元
+			      </a>
 			    </td>
 			  </tr>)
 		    })}
 		  </tbody>
 		</table>
+	      </div>
 
 	      <Pagination page={page} totalPages={totalPages}/>
 	    </>)}

frontend/src/pages/posts/PostSearchPage.tsx

@@ -289,7 +289,7 @@ export default (() => {
 		    {results.map (row => (
 			<tr key={row.id} className="even:bg-gray-100 dark:even:bg-gray-700">
 			  <td className="p-2">
-			    <PrefetchLink to={`/posts/${ row.id }`} title={row.title}>
+			    <PrefetchLink to={`/posts/${ row.id }`} title={row.title || undefined}>
 			      <motion.div
 				  layoutId={`page-${ row.id }`}
 				  transition={{ type: 'spring',
@@ -304,7 +304,7 @@ export default (() => {
 			    </PrefetchLink>
 			  </td>
 			  <td className="p-2 truncate">
-			    <PrefetchLink to={`/posts/${ row.id }`} title={row.title}>
+			    <PrefetchLink to={`/posts/${ row.id }`} title={row.title || undefined}>
 			      {row.title}
 			    </PrefetchLink>
 			  </td>

frontend/src/types.ts

@@ -117,9 +117,9 @@ export type NiconicoViewerHandle = {
 export type Post = {
     id:                     number
     url:                    string
-    title:                  string
+    title:                  string | null
-    thumbnail:              string
+    thumbnail:              string | null
-    thumbnailBase:          string
+    thumbnailBase:          string | null
     tags:                   Tag[]
     viewed:                 boolean
     related:                Post[]
@@ -127,7 +127,7 @@ export type Post = {
     originalCreatedBefore:  string | null
     createdAt:              string
     updatedAt:              string
-    uploadedUser:           { id: number; name: string } | null }
+    uploadedUser:           { id: number; name: string | null } | null }
 
 export type PostTagChange = {
     post:       Post
@@ -136,6 +136,20 @@ export type PostTagChange = {
     changeType: 'add' | 'remove'
     timestamp:  string }
 
+export type PostVersion = {
+    postId:                 number
+    versionNo:              number
+    eventType:              'create' | 'update' | 'discard' | 'restore'
+    title:                  { current: string | null; prev: string | null }
+    url:                    { current: string; prev: string | null }
+    thumbnail:              { current: string | null; prev: string | null }
+    thumbnailBase:          { current: string | null; prev: string | null }
+    tags:                   { name: string; type: 'context' | 'added' | 'removed' }[]
+    originalCreatedFrom:    { current: string | null; prev: string | null }
+    originalCreatedBefore:  { current: string | null; prev: string | null }
+    createdAt:              string
+    createdByUser:          { id: number; name: string | null } | null }
+
 export type SubMenuComponentItem = {
     component:  ReactNode
     visible:    boolean }