backend/app/controllers/nico_tags_controller.rb

@@ -33,8 +33,8 @@ class NicoTagsController < ApplicationController
     return head :bad_request unless tag.nico?
 
     linked_tag_names = params[:tags].to_s.split
-    linked_tags = Tag.normalise_tags(linked_tag_names, with_tagme: false,
-                                                       with_no_deerjikist: false)
+    linked_tags = Tag.normalise_tags!(linked_tag_names, with_tagme: false,
+                                                        with_no_deerjikist: false)
     return head :bad_request if linked_tags.any? { |t| t.nico? }
 
     ApplicationRecord.transaction do

backend/app/controllers/posts_controller.rb

@@ -109,7 +109,7 @@ class PostsController < ApplicationController
 
     render json: PostRepr.base(post, current_user)
                          .merge(tags: build_tag_tree_for(post.tags),
-                                related: post.related(limit: 20))
+                                related: PostRepr.many(post.related(limit: 20)))
   end
 
   def create
@@ -123,28 +123,36 @@ class PostsController < ApplicationController
     tag_names = params[:tags].to_s.split
     original_created_from = params[:original_created_from]
     original_created_before = params[:original_created_before]
+    parent_post_ids = parse_parent_post_ids
 
     post = Post.new(title:, url:, thumbnail_base: nil, uploaded_user: current_user,
                     original_created_from:, original_created_before:)
-    post.thumbnail.attach(thumbnail)
+    post.thumbnail.attach(thumbnail) if thumbnail.present?
 
     ApplicationRecord.transaction do
       post.save!
-      tags = Tag.normalise_tags(tag_names)
+
+      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)
+
+      sync_parent_posts!(post, parent_post_ids)
+
       post.resized_thumbnail!
+
       PostVersionRecorder.record!(post:, event_type: :create, created_by_user: current_user)
     end
 
     post.reload
     render json: PostRepr.base(post), status: :created
-  rescue ActiveRecord::RecordInvalid
-    render json: { errors: post.errors.full_messages }, status: :unprocessable_entity
   rescue Tag::NicoTagNormalisationError
     head :bad_request
+  rescue ArgumentError => e
+    render json: { errors: [e.message] }, status: :unprocessable_entity
+  rescue ActiveRecord::RecordInvalid => e
+    render json: { errors: e.record.errors.full_messages }, status: :unprocessable_entity
   end
 
   def viewed
@@ -169,6 +177,7 @@ class PostsController < ApplicationController
     tag_names = params[:tags].to_s.split
     original_created_from = params[:original_created_from]
     original_created_before = params[:original_created_before]
+    parent_post_ids = parse_parent_post_ids
 
     post = Post.find(params[:id].to_i)
 
@@ -177,12 +186,15 @@ class PostsController < ApplicationController
 
       post.update!(title:, original_created_from:, original_created_before:)
 
-      normalised_tags = 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)
+
+      sync_parent_posts!(post, parent_post_ids)
+
       PostVersionRecorder.record!(post:, event_type: :update, created_by_user: current_user)
     end
 
@@ -190,10 +202,12 @@ class PostsController < ApplicationController
     json = post.as_json
     json['tags'] = build_tag_tree_for(post.tags)
     render json:, status: :ok
-  rescue ActiveRecord::RecordInvalid
-    render json: post.errors, status: :unprocessable_entity
   rescue Tag::NicoTagNormalisationError
     head :bad_request
+  rescue ArgumentError => e
+    render json: { errors: [e.message] }, status: :unprocessable_entity
+  rescue ActiveRecord::RecordInvalid => e
+    render json: { errors: e.record.errors.full_messages }, status: :unprocessable_entity
   end
 
   def changes
@@ -353,4 +367,41 @@ class PostsController < ApplicationController
 
     root_ids.filter_map { |id| build_node.call(id, []) }
   end
+
+  def parse_parent_post_ids
+    raise ArgumentError, 'parent_post_ids は必須です．' unless params.key?(:parent_post_ids)
+
+    params[:parent_post_ids].to_s.split.map { |token|
+      id = Integer(token, exception: false)
+      raise ArgumentError, "親投稿 Id. が不正です: #{ token }" if id.nil? || id <= 0
+
+      id
+    }.uniq
+  end
+
+  def sync_parent_posts! post, parent_post_ids
+    if parent_post_ids.include?(post.id)
+      post.errors.add(:base, '自分自身を親投稿にはできません．')
+      raise ActiveRecord::RecordInvalid, post
+    end
+
+    existing_ids = Post.where(id: parent_post_ids).pluck(:id)
+    missing_ids = parent_post_ids - existing_ids
+
+    if missing_ids.present?
+      post.errors.add(:base, "存在しない親投稿 ID があります: #{ missing_ids.join(' ') }")
+      raise ActiveRecord::RecordInvalid, post
+    end
+
+    current_ids = post.parent_posts.pluck(:id)
+
+    ids_to_add = parent_post_ids - current_ids
+    ids_to_remove = current_ids - parent_post_ids
+
+    PostImplication.where(post_id: post.id, parent_post_id: ids_to_remove).delete_all
+
+    ids_to_add.each do |parent_post_id|
+      PostImplication.create_or_find_by!(post_id: post.id, parent_post_id:)
+    end
+  end
 end

backend/app/controllers/tags_controller.rb

@@ -374,9 +374,9 @@ class TagsController < ApplicationController
   end
 
   def update_parent_tags! tag, parent_names
-    parent_tags = Tag.normalise_tags(parent_names, with_tagme: false,
-                                                   with_no_deerjikist: false,
-                                                   deny_nico: true)
+    parent_tags = Tag.normalise_tags!(parent_names, with_tagme: false,
+                                                    with_no_deerjikist: false,
+                                                    deny_nico: true)
 
     old_parent_tags = tag.parents.to_a
 

backend/app/models/post.rb

@@ -1,7 +1,6 @@
 class Post < ApplicationRecord
   require 'mini_magick'
 
-  belongs_to :parent, class_name: 'Post', optional: true, foreign_key: 'parent_id'
   belongs_to :uploaded_user, class_name: 'User', optional: true
 
   has_many :post_tags, dependent: :destroy, inverse_of: :post
@@ -13,6 +12,20 @@ class Post < ApplicationRecord
   has_many :post_similarities, dependent: :delete_all
   has_many :post_versions
 
+  has_many :parent_post_implications,
+           class_name: 'PostImplication',
+           foreign_key: :post_id,
+           dependent: :destroy,
+           inverse_of: :post
+  has_many :parents, through: :parent_post_implications, source: :parent_post
+
+  has_many :child_post_implications,
+           class_name: 'PostImplication',
+           foreign_key: :parent_post_id,
+           dependent: :destroy,
+           inverse_of: :parent_post
+  has_many :children, through: :child_post_implications, source: :post
+
   has_one_attached :thumbnail
 
   before_validation :normalise_url
@@ -22,17 +35,29 @@ class Post < ApplicationRecord
   validate :validate_original_created_range
   validate :url_must_be_http_url
 
+  def parent_posts = parents
+
+  def child_posts = children
+
+  def sibling_posts
+    parent_post_ids = parent_posts.order(:id).pluck(:id)
+
+    parent_post_ids.to_h { [_1, PostImplication.where(parent_post: _1).map(&:post)] }
+  end
+
   def as_json options = { }
-    super(options).merge({ thumbnail: thumbnail.attached? ?
-                                      Rails.application.routes.url_helpers.rails_blob_url(
-                                          thumbnail, only_path: false) :
-                                      nil })
+    super(options).merge(thumbnail: thumbnail.attached? ?
+                                    Rails.application.routes.url_helpers.rails_blob_url(
+                                        thumbnail, only_path: false) :
+                                    nil)
   rescue
     super(options).merge(thumbnail: nil)
   end
 
   def snapshot_tag_names = tags.joins(:tag_name).order('tag_names.name').pluck('tag_names.name')
 
+  def snapshot_parent_post_ids = parents.order(:id).pluck(:id)
+
   def related limit: nil
     ids = post_similarities.order(cos: :desc)
     ids = ids.limit(limit) if limit

backend/app/models/post_implication.rb

@@ -0,0 +1,19 @@
+class PostImplication < ApplicationRecord
+  self.primary_key = :post_id, :parent_post_id
+
+  belongs_to :post, inverse_of: :parent_post_implications
+  belongs_to :parent_post, class_name: 'Post', inverse_of: :child_post_implications
+
+  validates :post_id, presence: true, uniqueness: { scope: :parent_post_id }
+  validates :parent_post_id, presence: true
+
+  validate :parent_post_mustnt_be_itself
+
+  private
+
+  def parent_post_mustnt_be_itself
+    if parent_post_id == post_id
+      errors.add :parent_post_id, '親投稿に同じ投稿を設定することはできません．'
+    end
+  end
+end

backend/app/models/tag.rb

@@ -86,9 +86,9 @@ class Tag < ApplicationRecord
   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 tag_names, with_tagme: true,
-                                     with_no_deerjikist: true,
-                                     deny_nico: true
+  def self.normalise_tags! tag_names, with_tagme: true,
+                                      with_no_deerjikist: true,
+                                      deny_nico: true
     if deny_nico && tag_names.any? { |n| n.downcase.start_with?('nico:') }
       raise NicoTagNormalisationError
     end

backend/app/representations/post_repr.rb

@@ -2,7 +2,8 @@
 
 
 module PostRepr
-  BASE = { include: { tags: TagRepr::BASE, uploaded_user: UserRepr::BASE } }.freeze
+  BASE = { include: { tags: TagRepr::BASE, uploaded_user: UserRepr::BASE },
+           methods: [:parent_posts, :child_posts, :sibling_posts] }.freeze
 
   module_function
 

backend/app/services/post_version_recorder.rb

@@ -24,7 +24,7 @@ class PostVersionRecorder < VersionRecorder
       url: @record.url,
       thumbnail_base: @record.thumbnail_base,
       tags: @record.snapshot_tag_names.join(' '),
-      parent_id: @record.parent_id,
+      parent_post_ids: @record.snapshot_parent_post_ids.join(' '),
       original_created_from: @record.original_created_from,
       original_created_before: @record.original_created_before }
   end

backend/db/migrate/20260427214800_create_post_implications.rb

@@ -0,0 +1,24 @@
+class CreatePostImplications < ActiveRecord::Migration[8.0]
+  def up
+    create_table :post_implications, primary_key: [:post_id, :parent_post_id] do |t|
+      t.references :post, null: false, foreign_key: true, index: false
+      t.references :parent_post, null: false, foreign_key: { to_table: :posts }
+      t.timestamps
+
+      t.check_constraint 'post_id <> parent_post_id',
+                         name: 'chk_post_implications_no_self'
+    end
+
+    add_column :post_versions, :parent_post_ids, :text, null: false, after: :parent_id
+    remove_column :post_versions, :parent_id, :bigint
+    remove_reference :posts, :parent, foreign_key: { to_table: :posts }
+  end
+
+  def down
+    add_reference :posts, :parent, foreign_key: { to_table: :posts }, after: :thumbnail_base
+    add_column :post_versions, :parent_id, :bigint, after: :post_id
+    remove_column :post_versions, :parent_post_ids, :text
+
+    drop_table :post_implications
+  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_26_120600) do
+ActiveRecord::Schema[8.0].define(version: 2026_04_27_214800) 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
@@ -119,6 +119,15 @@ ActiveRecord::Schema[8.0].define(version: 2026_04_26_120600) do
     t.check_constraint "`version_no` > 0", name: "nico_tag_versions_version_no_positive"
   end
 
+  create_table "post_implications", primary_key: ["post_id", "parent_post_id"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "post_id", null: false
+    t.bigint "parent_post_id", null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["parent_post_id"], name: "index_post_implications_on_parent_post_id"
+    t.check_constraint "`post_id` <> `parent_post_id`", name: "chk_post_implications_no_self"
+  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
@@ -155,13 +164,12 @@ ActiveRecord::Schema[8.0].define(version: 2026_04_26_120600) do
     t.string "url", limit: 768, null: false
     t.string "thumbnail_base", limit: 2000
     t.text "tags", null: false
-    t.bigint "parent_id"
+    t.text "parent_post_ids", null: false
     t.datetime "original_created_from"
     t.datetime "original_created_before"
     t.datetime "created_at", null: false
     t.bigint "created_by_user_id"
     t.index ["created_by_user_id"], name: "index_post_versions_on_created_by_user_id"
-    t.index ["parent_id"], name: "index_post_versions_on_parent_id"
     t.index ["post_id", "version_no"], name: "index_post_versions_on_post_id_and_version_no", unique: true
     t.index ["post_id"], name: "index_post_versions_on_post_id"
     t.check_constraint "`event_type` in (_utf8mb4'create',_utf8mb4'update',_utf8mb4'discard',_utf8mb4'restore')", name: "post_versions_event_type_valid"
@@ -172,13 +180,11 @@ ActiveRecord::Schema[8.0].define(version: 2026_04_26_120600) do
     t.string "title"
     t.string "url", limit: 768, null: false
     t.string "thumbnail_base", limit: 2000
-    t.bigint "parent_id"
     t.bigint "uploaded_user_id"
     t.datetime "created_at", null: false
     t.datetime "original_created_from"
     t.datetime "original_created_before"
     t.datetime "updated_at", null: false
-    t.index ["parent_id"], name: "index_posts_on_parent_id"
     t.index ["uploaded_user_id"], name: "index_posts_on_uploaded_user_id"
     t.index ["url"], name: "index_posts_on_url", unique: true
   end
@@ -428,6 +434,8 @@ ActiveRecord::Schema[8.0].define(version: 2026_04_26_120600) do
   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_tags", "posts"
@@ -435,9 +443,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_04_26_120600) do
   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", "posts", column: "parent_id"
   add_foreign_key "post_versions", "users", column: "created_by_user_id"
-  add_foreign_key "posts", "posts", column: "parent_id"
   add_foreign_key "posts", "users", column: "uploaded_user_id"
   add_foreign_key "settings", "users"
   add_foreign_key "tag_implications", "tags"

backend/spec/models/post_implication_spec.rb

@@ -0,0 +1,51 @@
+require 'rails_helper'
+
+RSpec.describe PostImplication, type: :model do
+  let!(:post_record) do
+    Post.create!(
+      title: 'post',
+      url: 'https://example.com/post-implication-post'
+    )
+  end
+
+  let!(:parent_post) do
+    Post.create!(
+      title: 'parent post',
+      url: 'https://example.com/post-implication-parent'
+    )
+  end
+
+  it 'is valid with post and parent_post' do
+    implication = described_class.new(
+      post: post_record,
+      parent_post:
+    )
+
+    expect(implication).to be_valid
+  end
+
+  it 'does not allow same post as parent_post' do
+    implication = described_class.new(
+      post: post_record,
+      parent_post: post_record
+    )
+
+    expect(implication).not_to be_valid
+    expect(implication.errors[:parent_post_id]).to be_present
+  end
+
+  it 'does not allow duplicate pair' do
+    described_class.create!(
+      post: post_record,
+      parent_post:
+    )
+
+    duplicate = described_class.new(
+      post: post_record,
+      parent_post:
+    )
+
+    expect(duplicate).not_to be_valid
+    expect(duplicate.errors[:post_id]).to be_present
+  end
+end

backend/spec/models/post_version_spec.rb

@@ -19,7 +19,7 @@ RSpec.describe PostVersion, type: :model do
       url: post_record.url,
       thumbnail_base: post_record.thumbnail_base,
       tags: post_record.snapshot_tag_names.join(' '),
-      parent: post_record.parent,
+      parent_post_ids: post_record.snapshot_parent_post_ids.join(' '),
       original_created_from: post_record.original_created_from,
       original_created_before: post_record.original_created_before,
       created_at: Time.current,

backend/spec/models/tag_spec.rb

@@ -161,7 +161,7 @@ RSpec.describe Tag, type: :model do
         url: post.url,
         thumbnail_base: post.thumbnail_base,
         tags: snapshot_tags(post),
-        parent: post.parent,
+        parent_post_ids: post.snapshot_parent_post_ids.join(' '),
         original_created_from: post.original_created_from,
         original_created_before: post.original_created_before,
         created_at: Time.current,

backend/spec/requests/posts_spec.rb

@@ -15,6 +15,31 @@ RSpec.describe 'Posts API', type: :request do
     Rack::Test::UploadedFile.new(StringIO.new('dummy'), 'image/jpeg', original_filename: 'dummy.jpg')
   end
 
+  def post_write_params params = { }
+    { parent_post_ids: '' }.merge(params)
+  end
+
+  def create_parent_post! title:, url:
+    Post.create!(title:, url:)
+  end
+
+  def create_post_version_for! post
+    PostVersion.create!(
+      post:,
+      version_no: 1,
+      event_type: 'create',
+      title: post.title,
+      url: post.url,
+      thumbnail_base: post.thumbnail_base,
+      tags: post.snapshot_tag_names.join(' '),
+      parent_post_ids: post.snapshot_parent_post_ids.join(' '),
+      original_created_from: post.original_created_from,
+      original_created_before: post.original_created_before,
+      created_at: post.created_at,
+      created_by_user: post.uploaded_user
+    )
+  end
+
   let!(:tag_name) { TagName.create!(name: 'spec_tag') }
   let!(:tag)      { Tag.create!(tag_name: tag_name, category: :general) }
 
@@ -457,6 +482,65 @@ RSpec.describe 'Posts API', type: :request do
         expect(json).to have_key('viewed')
         expect([true, false]).to include(json['viewed'])
       end
+
+      context 'when post has parent, child, and sibling posts' do
+        let!(:parent_post) do
+          create_parent_post!(
+            title: 'shared parent post',
+            url: 'https://example.com/shared-parent-post'
+          )
+        end
+
+        let!(:child_post) do
+          Post.create!(
+            title: 'child post',
+            url: 'https://example.com/show-child-post'
+          )
+        end
+
+        let!(:sibling_post) do
+          Post.create!(
+            title: 'sibling post',
+            url: 'https://example.com/show-sibling-post'
+          )
+        end
+
+        before do
+          PostImplication.create!(
+            post: post_record,
+            parent_post:
+          )
+
+          PostImplication.create!(
+            post: child_post,
+            parent_post: post_record
+          )
+
+          PostImplication.create!(
+            post: sibling_post,
+            parent_post:
+          )
+        end
+
+        it 'returns parent_posts, child_posts, and sibling_posts' do
+          get "/posts/#{post_record.id}"
+
+          expect(response).to have_http_status(:ok)
+
+          parent_ids = json.fetch('parent_posts').map { |p| p.fetch('id') }
+          child_ids = json.fetch('child_posts').map { |p| p.fetch('id') }
+
+          expect(parent_ids).to include(parent_post.id)
+          expect(child_ids).to include(child_post.id)
+
+          sibling_posts_by_parent = json.fetch('sibling_posts')
+          siblings = sibling_posts_by_parent.fetch(parent_post.id.to_s)
+
+          sibling_ids = siblings.map { |p| p.fetch('id') }
+          expect(sibling_ids).to include(post_record.id)
+          expect(sibling_ids).to include(sibling_post.id)
+        end
+      end
     end
 
     context 'when post does not exist' do
@@ -475,25 +559,28 @@ RSpec.describe 'Posts API', type: :request do
 
     it '401 when not logged in' do
       sign_out
-      post '/posts', params: { title: 't', url: 'https://example.com/x', tags: 'a', thumbnail: dummy_upload }
+      post '/posts', params: post_write_params(title: 't', url: 'https://example.com/x', tags: 'a',
+                                               thumbnail: dummy_upload)
+
       expect(response).to have_http_status(:unauthorized)
     end
 
     it '403 when not member' do
       sign_in_as(create(:user, role: 'guest'))
-      post '/posts', params: { title: 't', url: 'https://example.com/x', tags: 'a', thumbnail: dummy_upload }
+      post '/posts', params: post_write_params(title: 't', url: 'https://example.com/x', tags: 'a',
+                                               thumbnail: dummy_upload)
       expect(response).to have_http_status(:forbidden)
     end
 
     it '201 and creates post + tags when member' do
       sign_in_as(member)
 
-      post '/posts', params: {
+      post '/posts', params: post_write_params(
         title: 'new post',
         url: 'https://example.com/new',
         tags: 'spec_tag', # 既存タグ名を投げる
         thumbnail: dummy_upload
-      }
+      )
 
       expect(response).to have_http_status(:created)
       expect(json).to include('id', 'title', 'url')
@@ -507,12 +594,12 @@ RSpec.describe 'Posts API', type: :request do
     it '201 and creates post + tags when member and tags have aliases' do
       sign_in_as(member)
 
-      post '/posts', params: {
+      post '/posts', params: post_write_params(
         title: 'new post',
         url: 'https://example.com/new',
         tags: 'manko', # 既存タグ名を投げる
         thumbnail: dummy_upload
-      }
+      )
 
       expect(response).to have_http_status(:created)
       expect(json).to include('id', 'title', 'url')
@@ -533,13 +620,14 @@ RSpec.describe 'Posts API', type: :request do
       it 'return 400' do
         sign_in_as(member)
 
-        post '/posts', params: {
-            title: 'new post',
-            url: 'https://example.com/nico_tag',
-            tags: 'nico:nico_tag',
-            thumbnail: dummy_upload }
+        post '/posts', params: post_write_params(
+          title: 'new post',
+          url: 'https://example.com/nico-tag-post',
+          tags: 'nico:nico_tag',
+          thumbnail: dummy_upload
+        )
 
-        expect(response).to have_http_status(:bad_request)
+        expect(response).to have_http_status(:bad_request), response.body
       end
     end
 
@@ -547,11 +635,11 @@ RSpec.describe 'Posts API', type: :request do
       it 'returns 422' do
         sign_in_as(member)
 
-        post '/posts', params: {
+        post '/posts', params: post_write_params(
             title: 'new post',
             url: '   ',
             tags: 'spec_tag', # 既存タグ名を投げる
-            thumbnail: dummy_upload }
+            thumbnail: dummy_upload)
 
         expect(response).to have_http_status(:unprocessable_entity)
       end
@@ -561,14 +649,154 @@ RSpec.describe 'Posts API', type: :request do
       it 'returns 422' do
         sign_in_as(member)
 
-        post '/posts', params: {
-          title: 'new post',
-          url: 'ぼざクリタグ広場',
-          tags: 'spec_tag', # 既存タグ名を投げる
+        post '/posts', params: post_write_params(
+            title: 'new post',
+            url: 'ぼざクリタグ広場',
+            tags: 'spec_tag', # 既存タグ名を投げる
+            thumbnail: dummy_upload)
+
+        expect(response).to have_http_status(:unprocessable_entity)
+      end
+    end
+
+    context 'when parent_post_ids is provided' do
+      let!(:parent_post_1) do
+        create_parent_post!(
+          title: 'parent post 1',
+          url: 'https://example.com/parent-post-1'
+        )
+      end
+
+      let!(:parent_post_2) do
+        create_parent_post!(
+          title: 'parent post 2',
+          url: 'https://example.com/parent-post-2'
+        )
+      end
+
+      it 'creates post implications for parent posts' do
+        sign_in_as(member)
+
+        expect {
+          post '/posts', params: {
+              title: 'child post',
+              url: 'https://example.com/child-post',
+              tags: 'spec_tag',
+              parent_post_ids: "#{parent_post_1.id} #{parent_post_2.id}",
+              thumbnail: dummy_upload }
+        }.to change(PostImplication, :count).by(2)
+
+        expect(response).to have_http_status(:created)
+
+        created_post = Post.find(json.fetch('id'))
+        expect(created_post.parent_posts.order(:id).pluck(:id)).to eq(
+          [parent_post_1.id, parent_post_2.id].sort
+        )
+
+        expect(PostImplication.exists?(
+          post_id: created_post.id,
+          parent_post_id: parent_post_1.id
+        )).to be(true)
+
+        expect(PostImplication.exists?(
+          post_id: created_post.id,
+          parent_post_id: parent_post_2.id
+        )).to be(true)
+      end
+
+      it 'deduplicates parent_post_ids' do
+        sign_in_as(member)
+
+        expect {
+          post '/posts', params: post_write_params(
+            title: 'dedup child post',
+            url: 'https://example.com/dedup-child-post',
+            tags: 'spec_tag',
+            parent_post_ids: "#{parent_post_1.id} #{parent_post_1.id}",
+            thumbnail: dummy_upload
+          )
+        }.to change(PostImplication, :count).by(1)
+
+        expect(response).to have_http_status(:created)
+
+        created_post = Post.find(json.fetch('id'))
+        expect(created_post.parent_posts.pluck(:id)).to eq([parent_post_1.id])
+      end
+
+      it 'records parent_post_ids in post version' do
+        sign_in_as(member)
+
+        post '/posts', params: post_write_params(
+          title: 'versioned child post',
+          url: 'https://example.com/versioned-child-post',
+          tags: 'spec_tag',
+          parent_post_ids: "#{parent_post_1.id} #{parent_post_2.id}",
           thumbnail: dummy_upload
-        }
+        )
+
+        expect(response).to have_http_status(:created)
+
+        created_post = Post.find(json.fetch('id'))
+        version = PostVersion.find_by!(post: created_post, version_no: 1)
+
+        expect(version.parent_post_ids.split.map(&:to_i)).to eq(
+          [parent_post_1.id, parent_post_2.id].sort
+        )
+      end
+    end
+
+    context 'when parent_post_ids is missing' do
+      it 'returns 422' do
+        sign_in_as(member)
+
+        expect {
+          post '/posts', params: {
+              title: 'missing parent_post_ids',
+              url: 'https://example.com/missing-parent-post-ids',
+              tags: 'spec_tag',
+              thumbnail: dummy_upload }
+        }.not_to change(Post, :count)
+
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(json.fetch('errors')).to be_present
+      end
+    end
+
+    context 'when parent_post_ids includes invalid token' do
+      it 'returns 422 and does not create post' do
+        sign_in_as(member)
+
+        expect {
+          post '/posts', params: post_write_params(
+            title: 'invalid parent ids',
+            url: 'https://example.com/invalid-parent-ids',
+            tags: 'spec_tag',
+            parent_post_ids: 'abc',
+            thumbnail: dummy_upload
+          )
+        }.not_to change(Post, :count)
 
         expect(response).to have_http_status(:unprocessable_entity)
+        expect(json.fetch('errors')).to be_present
+      end
+    end
+
+    context 'when parent_post_ids includes nonexistent post id' do
+      it 'returns 422 and does not create post implication' do
+        sign_in_as(member)
+
+        expect {
+          post '/posts', params: post_write_params(
+            title: 'missing parent post',
+            url: 'https://example.com/missing-parent-post',
+            tags: 'spec_tag',
+            parent_post_ids: '999999999',
+            thumbnail: dummy_upload
+          )
+        }.not_to change(PostImplication, :count)
+
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(json.fetch('errors')).to be_present
       end
     end
   end
@@ -578,13 +806,13 @@ RSpec.describe 'Posts API', type: :request do
 
     it '401 when not logged in' do
       sign_out
-      put "/posts/#{post_record.id}", params: { title: 'updated', tags: 'spec_tag' }
+      put "/posts/#{post_record.id}", params: post_write_params(title: 'updated', tags: 'spec_tag')
       expect(response).to have_http_status(:unauthorized)
     end
 
     it '403 when not member' do
       sign_in_as(create(:user, role: 'guest'))
-      put "/posts/#{post_record.id}", params: { title: 'updated', tags: 'spec_tag' }
+      put "/posts/#{post_record.id}", params: post_write_params(title: 'updated', tags: 'spec_tag')
       expect(response).to have_http_status(:forbidden)
     end
 
@@ -595,10 +823,9 @@ RSpec.describe 'Posts API', type: :request do
       tn2 = TagName.create!(name: 'spec_tag_2')
       Tag.create!(tag_name: tn2, category: :general)
 
-      put "/posts/#{post_record.id}", params: {
-        title: 'updated title',
-        tags: 'spec_tag_2'
-      }
+      put "/posts/#{post_record.id}", params: post_write_params(
+          title: 'updated title',
+          tags: 'spec_tag_2')
 
       expect(response).to have_http_status(:ok)
       expect(json).to have_key('tags')
@@ -619,11 +846,178 @@ RSpec.describe 'Posts API', type: :request do
       it 'return 400' do
         sign_in_as(member)
 
-        put "/posts/#{ post_record.id }", params: {
+        put "/posts/#{post_record.id}", params: post_write_params(
+          title: 'updated title',
+          tags: 'nico:nico_tag'
+        )
+
+        expect(response).to have_http_status(:bad_request), response.body
+      end
+    end
+
+    context 'when parent_post_ids is provided' do
+      let!(:old_parent_post) do
+        create_parent_post!(
+          title: 'old parent post',
+          url: 'https://example.com/old-parent-post'
+        )
+      end
+
+      let!(:new_parent_post_1) do
+        create_parent_post!(
+          title: 'new parent post 1',
+          url: 'https://example.com/new-parent-post-1'
+        )
+      end
+
+      let!(:new_parent_post_2) do
+        create_parent_post!(
+          title: 'new parent post 2',
+          url: 'https://example.com/new-parent-post-2'
+        )
+      end
+
+      before do
+        PostImplication.create!(
+          post: post_record,
+          parent_post: old_parent_post
+        )
+      end
+
+      it 'replaces parent posts' do
+        sign_in_as(member)
+
+        put "/posts/#{post_record.id}", params: post_write_params(
+          title: 'updated title',
+          tags: 'spec_tag',
+          parent_post_ids: "#{new_parent_post_1.id} #{new_parent_post_2.id}"
+        )
+
+        expect(response).to have_http_status(:ok)
+
+        expect(post_record.reload.parent_posts.order(:id).pluck(:id)).to eq(
+          [new_parent_post_1.id, new_parent_post_2.id].sort
+        )
+
+        expect(PostImplication.exists?(
+          post_id: post_record.id,
+          parent_post_id: old_parent_post.id
+        )).to be(false)
+      end
+
+      it 'clears parent posts when parent_post_ids is blank' do
+        sign_in_as(member)
+
+        put "/posts/#{post_record.id}", params: post_write_params(
+          title: 'updated title',
+          tags: 'spec_tag',
+          parent_post_ids: ''
+        )
+
+        expect(response).to have_http_status(:ok)
+        expect(post_record.reload.parent_posts).to be_empty
+      end
+
+      it 'records changed parent_post_ids in post version' do
+        sign_in_as(member)
+        create_post_version_for!(post_record.reload)
+
+        put "/posts/#{post_record.id}", params: post_write_params(
+          title: 'updated title',
+          tags: 'spec_tag',
+          parent_post_ids: "#{new_parent_post_1.id} #{new_parent_post_2.id}"
+        )
+
+        expect(response).to have_http_status(:ok)
+
+        version = post_record.reload.post_versions.order(:version_no).last
+
+        expect(version.version_no).to eq(2)
+        expect(version.parent_post_ids.split.map(&:to_i)).to eq(
+          [new_parent_post_1.id, new_parent_post_2.id].sort
+        )
+      end
+    end
+
+    context 'when parent_post_ids is missing' do
+      it 'returns 422' do
+        sign_in_as(member)
+
+        put "/posts/#{post_record.id}", params: {
             title: 'updated title',
-            tags: 'nico:nico_tag' }
+            tags: 'spec_tag' }
 
-        expect(response).to have_http_status(:bad_request)
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(json.fetch('errors')).to be_present
+      end
+    end
+
+    context 'when parent_post_ids includes invalid token' do
+      it 'returns 422 and does not change parent posts' do
+        sign_in_as(member)
+
+        parent_post = create_parent_post!(
+          title: 'valid parent post',
+          url: 'https://example.com/valid-parent-post'
+        )
+
+        PostImplication.create!(
+          post: post_record,
+          parent_post:
+        )
+
+        put "/posts/#{post_record.id}", params: post_write_params(
+          title: 'updated title',
+          tags: 'spec_tag',
+          parent_post_ids: 'abc'
+        )
+
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(post_record.reload.parent_posts.pluck(:id)).to eq([parent_post.id])
+      end
+    end
+
+    context 'when parent_post_ids includes nonexistent post id' do
+      it 'returns 422 and does not change parent posts' do
+        sign_in_as(member)
+
+        parent_post = create_parent_post!(
+          title: 'existing parent post',
+          url: 'https://example.com/existing-parent-post'
+        )
+
+        PostImplication.create!(
+          post: post_record,
+          parent_post:
+        )
+
+        put "/posts/#{post_record.id}", params: post_write_params(
+          title: 'updated title',
+          tags: 'spec_tag',
+          parent_post_ids: '999999999'
+        )
+
+        expect(response).to have_http_status(:unprocessable_entity)
+        expect(post_record.reload.parent_posts.pluck(:id)).to eq([parent_post.id])
+      end
+    end
+
+    context 'when parent_post_ids includes self id' do
+      it 'returns 422 and does not create self implication' do
+        sign_in_as(member)
+
+        put "/posts/#{post_record.id}", params: post_write_params(
+          title: 'updated title',
+          tags: 'spec_tag',
+          parent_post_ids: post_record.id.to_s
+        )
+
+        expect(response).to have_http_status(:unprocessable_entity)
+
+        expect(PostImplication.exists?(
+          post_id: post_record.id,
+          parent_post_id: post_record.id
+        )).to be(false)
       end
     end
   end
@@ -773,20 +1167,20 @@ RSpec.describe 'Posts API', type: :request do
       post.snapshot_tag_names.join(' ')
     end
 
-    def create_post_version!(post, version_no:, event_type:, created_by_user:, created_at:)
+    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,
+        post:,
+        version_no:,
+        event_type:,
         title: post.title,
         url: post.url,
         thumbnail_base: post.thumbnail_base,
         tags: snapshot_tags(post),
-        parent: post.parent,
+        parent_post_ids: post.snapshot_parent_post_ids.join(' '),
         original_created_from: post.original_created_from,
         original_created_before: post.original_created_before,
-        created_at: created_at,
-        created_by_user: created_by_user
+        created_at:,
+        created_by_user:
       )
     end
 
@@ -1015,33 +1409,15 @@ RSpec.describe 'Posts API', type: :request do
       post.snapshot_tag_names.join(' ')
     end
 
-    def create_post_version_for!(post)
-      PostVersion.create!(
-        post: post,
-        version_no: 1,
-        event_type: 'create',
-        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: post.created_at,
-        created_by_user: post.uploaded_user
-      )
-    end
-
     it 'creates version 1 on POST /posts' do
       sign_in_as(member)
 
       expect do
-        post '/posts', params: {
-          title: 'versioned post',
-          url: 'https://example.com/versioned-post',
-          tags: 'spec_tag',
-          thumbnail: dummy_upload
-        }
+        post '/posts', params: post_write_params(
+            title: 'versioned post',
+            url: 'https://example.com/versioned-post',
+            tags: 'spec_tag',
+            thumbnail: dummy_upload)
       end.to change(PostVersion, :count).by(1)
 
       expect(response).to have_http_status(:created)
@@ -1064,10 +1440,9 @@ RSpec.describe 'Posts API', type: :request do
       Tag.create!(tag_name: tag_name2, category: :general)
 
       expect do
-        put "/posts/#{post_record.id}", params: {
-          title: 'updated title',
-          tags: 'spec_tag_2'
-        }
+        put "/posts/#{post_record.id}", params: post_write_params(
+            title: 'updated title',
+            tags: 'spec_tag_2')
       end.to change(PostVersion, :count).by(1)
 
       expect(response).to have_http_status(:ok)
@@ -1087,10 +1462,9 @@ RSpec.describe 'Posts API', type: :request do
       create_post_version_for!(post_record.reload)
 
       expect {
-        put "/posts/#{post_record.id}", params: {
-          title: post_record.title,
-          tags: 'spec_tag'
-        }
+        put "/posts/#{post_record.id}", params: post_write_params(
+            title: post_record.title,
+            tags: 'spec_tag')
       }.not_to change(PostVersion, :count)
       expect(response).to have_http_status(:ok)
 
@@ -1104,12 +1478,11 @@ RSpec.describe 'Posts API', type: :request do
       sign_in_as(member)
 
       expect do
-        post '/posts', params: {
-          title: 'invalid post',
-          url: 'ぼざクリタグ広場',
-          tags: 'spec_tag',
-          thumbnail: dummy_upload
-        }
+        post '/posts', params: post_write_params(
+            title: 'invalid post',
+            url: 'ぼざクリタグ広場',
+            tags: 'spec_tag',
+            thumbnail: dummy_upload)
       end.not_to change(PostVersion, :count)
 
       expect(response).to have_http_status(:unprocessable_entity)
@@ -1120,12 +1493,11 @@ RSpec.describe 'Posts API', type: :request do
       create_post_version_for!(post_record)
 
       expect do
-        put "/posts/#{post_record.id}", params: {
-          title: 'updated title',
-          tags: 'spec_tag',
-          original_created_from: Time.zone.local(2020, 1, 2, 0, 0, 0).iso8601,
-          original_created_before: Time.zone.local(2020, 1, 1, 0, 0, 0).iso8601
-        }
+        put "/posts/#{post_record.id}", params: post_write_params(
+            title: 'updated title',
+            tags: 'spec_tag',
+            original_created_from: Time.zone.local(2020, 1, 2, 0, 0, 0).iso8601,
+            original_created_before: Time.zone.local(2020, 1, 1, 0, 0, 0).iso8601)
       end.not_to change(PostVersion, :count)
 
       expect(response).to have_http_status(:unprocessable_entity)
@@ -1139,12 +1511,11 @@ RSpec.describe 'Posts API', type: :request 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
-        }
+        post '/posts', params: post_write_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)
@@ -1164,10 +1535,9 @@ RSpec.describe 'Posts API', type: :request do
       tag2 = Tag.create!(tag_name: tag_name2, category: :general)
 
       expect {
-        put "/posts/#{post_record.id}", params: {
-          title: 'updated title',
-          tags: 'spec_tag_2'
-        }
+        put "/posts/#{post_record.id}", params: post_write_params(
+            title: 'updated title',
+            tags: 'spec_tag_2')
       }.to change { tag2.reload.tag_versions.count }.by(1)
 
       expect(response).to have_http_status(:ok)

backend/spec/tasks/nico_sync_spec.rb

@@ -104,7 +104,7 @@ RSpec.describe "nico:sync" do
       url: post.url,
       thumbnail_base: post.thumbnail_base,
       tags: snapshot_tags(post),
-      parent: post.parent,
+      parent_post_ids: post.snapshot_parent_post_ids.join(' '),
       original_created_from: post.original_created_from,
       original_created_before: post.original_created_before,
       created_at: Time.current,

frontend/src/components/PostEditForm.tsx

@@ -4,6 +4,7 @@ import PostFormTagsArea from '@/components/PostFormTagsArea'
 import PostOriginalCreatedTimeField from '@/components/PostOriginalCreatedTimeField'
 import Label from '@/components/common/Label'
 import { Button } from '@/components/ui/button'
+import { toast } from '@/components/ui/use-toast'
 import { apiPut } from '@/lib/api'
 
 import type { FC } from 'react'
@@ -35,20 +36,34 @@ export default (({ post, onSave }: Props) => {
       useState<string | null> (post.originalCreatedBefore)
   const [originalCreatedFrom, setOriginalCreatedFrom] =
       useState<string | null> (post.originalCreatedFrom)
-  const [title, setTitle] = useState (post.title)
+  const [parentPostIds, setParentPostIds] =
+      useState ((post.parentPosts ?? []).map (p => p.id).join (' '))
   const [tags, setTags] = useState<string> ('')
+  const [title, setTitle] = useState (post.title)
 
   const handleSubmit = async () => {
-    const data = await apiPut<Post> (
-	`/posts/${ post.id }`,
-	{ title, tags, original_created_from: originalCreatedFrom,
-	  original_created_before: originalCreatedBefore },
-	{ headers: { 'Content-Type': 'multipart/form-data' } })
-    onSave ({ ...post,
-	      title:                  data.title,
-	      tags:                   data.tags,
-	      originalCreatedFrom:    data.originalCreatedFrom,
-	      originalCreatedBefore:  data.originalCreatedBefore } as Post)
+    try
+    {
+      const data = await apiPut<Post> (
+	  `/posts/${ post.id }`,
+	  { title, tags, parent_post_ids: parentPostIds,
+	    original_created_from: originalCreatedFrom,
+	    original_created_before: originalCreatedBefore },
+	  { headers: { 'Content-Type': 'multipart/form-data' } })
+      onSave ({ ...post,
+		title:                  data.title,
+		tags:                   data.tags,
+		parentPosts:            data.parentPosts,
+		childPosts:             data.childPosts,
+		siblingPosts:           data.siblingPosts,
+		originalCreatedFrom:    data.originalCreatedFrom,
+		originalCreatedBefore:  data.originalCreatedBefore } as Post)
+      toast ({ description: '更新しました．' })
+    }
+    catch
+    {
+      toast ({ description: '更新はできなかったよ……' })
+    }
   }
 
   useEffect (() => {
@@ -66,6 +81,16 @@ export default (({ post, onSave }: Props) => {
 		 onChange={ev => setTitle (ev.target.value)}/>
 	</div>
 
+	{/* 親投稿 */}
+	<div>
+	  <Label>親投稿</Label>
+	  <input
+	      type="text"
+	      value={parentPostIds}
+	      onChange={e => setParentPostIds (e.target.value)}
+	      className="w-full border p-2 rounded"/>
+	</div>
+
 	{/* タグ */}
 	<PostFormTagsArea tags={tags} setTags={setTags}/>
 

frontend/src/components/PostList.tsx

@@ -3,6 +3,7 @@ import { useRef } from 'react'
 import { useLocation } from 'react-router-dom'
 
 import PrefetchLink from '@/components/PrefetchLink'
+import { cn } from '@/lib/utils'
 import { useSharedTransitionStore } from '@/stores/sharedTransitionStore'
 
 import type { FC, MouseEvent } from 'react'
@@ -39,8 +40,10 @@ export default (({ posts, onClick }: Props) => {
 		<motion.div
 		    ref={cardRef}
 		    layoutId={layoutId}
-		    className="w-full h-full overflow-hidden rounded-xl shadow
-			       transform-gpu will-change-transform"
+		    className={cn ('w-full h-full overflow-hidden rounded-xl shadow',
+				   'transform-gpu will-change-transform',
+				   (post.childPosts ?? []).length > 0 && 'outline-4 outline-green-500',
+				   (post.parentPosts ?? []).length > 0 && 'ring-4 ring-yellow-500')}
 		    whileHover={{ scale: 1.02 }}
 		    onLayoutAnimationStart={() => {
 		      if (!(cardRef.current))

frontend/src/pages/posts/PostDetailPage.tsx

@@ -21,7 +21,7 @@ import ServiceUnavailable from '@/pages/ServiceUnavailable'
 
 import type { FC } from 'react'
 
-import type { NiconicoViewerHandle, User } from '@/types'
+import type { NiconicoViewerHandle, Post, User } from '@/types'
 
 type Props = { user: User | null }
 
@@ -108,6 +108,34 @@ export default (({ user }: Props) => {
 	  {post
 	   ? (
 	      <>
+		{(post.childPosts ?? []).length > 0 && (
+		    <div className="mb-4 bg-green-200 dark:bg-green-800 text-sm p-2 rounded-md">
+		      <p>この投稿には {post.childPosts!.length} 件の子投稿があります．</p>
+		      <PostList posts={[{ ...post, childPosts: [{ } as Post] },
+					...post.childPosts!.map (p => ({
+					    ...p, parentPosts: [{ } as Post] }))]}/>
+		    </div>
+		)}
+		{(post.parentPosts ?? []).map (pp => {
+		  const siblings = post.siblingPosts?.[String (pp.id) as `${ number }`]
+		  if (!(siblings))
+		    return
+
+		  return (
+		      <div
+			  key={pp.id}
+			  className="mb-4 bg-yellow-200 dark:bg-yellow-800 text-sm p-2 rounded-md">
+			<p>
+			  この投稿には 1 件の親投稿{
+			      siblings.length > 1
+			      && `と ${ siblings.length - 1 } 件の姉妹投稿`}があります．
+			</p>
+			<PostList posts={[{ ...pp, childPosts: [{ } as Post] },
+					  ...siblings.map (p => ({
+					      ...p, parentPosts: [{ } as Post] }))]}/>
+		      </div>)
+		})}
+
 		{(post.thumbnail || post.thumbnailBase) && (
 		    <motion.div
 			layoutId={`page-${ id }`}
@@ -146,7 +174,6 @@ export default (({ user }: Props) => {
 					       (prev: any) => newPost ?? prev)
 			      qc.invalidateQueries ({ queryKey: postsKeys.root })
 			      qc.invalidateQueries ({ queryKey: tagsKeys.root })
-			      toast ({ description: '更新しました．' })
 			    }}/>
 		      </Tab>)}
 		</TabGroup>

frontend/src/pages/posts/PostHistoryPage.tsx

@@ -95,6 +95,8 @@ export default (() => {
 		    <col className="w-96"/>
 		    {/* タグ */}
 		    <col className="w-[48rem]"/>
+		    {/* TODO: 親投稿 */}
+		    {/* <col className="w-[48rem]"/> */}
 		    {/* オリジナルの投稿日時 */}
 		    <col className="w-96"/>
 		    {/* 更新日時 */}
@@ -110,6 +112,8 @@ export default (() => {
 		      <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>
+		      {/* TODO: 親投稿の履歴 */}
+		      {/* <th className="p-2 text-left">親投稿</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"/>
@@ -180,6 +184,29 @@ export default (() => {
 					    {tag.name}
 					  </span>))))}
 			    </td>
+			    {/* TODO: 親投稿の履歴 */}
+			    {/* <td className="p-2">
+			      {change.parentPosts.map ((pp, i) => (
+				  pp.type === 'added'
+				  ? (
+				      <ins
+					  key={i}
+					  className="mr-2 text-green-600 dark:text-green-400">
+					{pp.title}
+				      </ins>)
+				  : (
+				      pp.type === 'removed'
+				      ? (
+					  <del
+					      key={i}
+					      className="mr-2 text-red-600 dark:text-red-400">
+					    {pp.title}
+					  </del>)
+				      : (
+					  <span key={i} className="mr-2">
+					    {pp.title}
+					  </span>))))}
+			    </td> */}
 			    <td className="p-2">
 			      {change.versionNo === 1
 			       ? originalCreatedAtString (change.originalCreatedFrom.current,
@@ -225,6 +252,11 @@ export default (() => {
 						  .map (t => t.name)
 						  .filter (t => t.slice (0, 5) !== 'nico:')
 						  .join (' '),
+					    parent_post_ids:
+						(change.parentPosts ?? [])
+						.filter (p => p.type !== 'removed')
+						.map (p => p.id)
+						.join (' '),
 					    original_created_from:
 						change.originalCreatedFrom.current,
 					    original_created_before:

frontend/src/pages/posts/PostNewPage.tsx

@@ -29,6 +29,7 @@ export default (({ user }: Props) => {
 
   const [originalCreatedBefore, setOriginalCreatedBefore] = useState<string | null> (null)
   const [originalCreatedFrom, setOriginalCreatedFrom] = useState<string | null> (null)
+  const [parentPostIds, setParentPostIds] = useState ('')
   const [tags, setTags] = useState ('')
   const [thumbnailAutoFlg, setThumbnailAutoFlg] = useState (true)
   const [thumbnailFile, setThumbnailFile] = useState<File | null> (null)
@@ -46,6 +47,7 @@ export default (({ user }: Props) => {
     formData.append ('title', title)
     formData.append ('url', url)
     formData.append ('tags', tags)
+    formData.append ('parent_post_ids', parentPostIds)
     if (thumbnailFile)
       formData.append ('thumbnail', thumbnailFile)
     if (originalCreatedFrom)
@@ -177,6 +179,16 @@ export default (({ user }: Props) => {
 		     className="mt-2 max-h-48 rounded border"/>)}
 	  </div>
 
+	  {/* 親投稿 */}
+	  <div>
+	    <Label>親投稿</Label>
+	    <input
+		type="text"
+		value={parentPostIds}
+		onChange={e => setParentPostIds (e.target.value)}
+		className="w-full border p-2 rounded"/>
+	  </div>
+
 	  {/* タグ */}
 	  <PostFormTagsArea tags={tags} setTags={setTags}/>
 

frontend/src/types.ts

@@ -121,6 +121,9 @@ export type Post = {
     thumbnail:              string | null
     thumbnailBase:          string | null
     tags:                   Tag[]
+    parentPosts?:           Post[]
+    childPosts?:            Post[]
+    siblingPosts?:          Record<`${ number }`, Post[]>
     viewed:                 boolean
     related:                Post[]
     originalCreatedFrom:    string | null
@@ -144,7 +147,11 @@ export type PostVersion = {
     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' }[]
+    tags:                   { name: string
+			      type: 'context' | 'added' | 'removed' }[]
+    parentPosts:            { id: number
+			      title: string
+			      type: 'context' | 'added' | 'removed' }[]
     originalCreatedFrom:    { current: string | null; prev: string | null }
     originalCreatedBefore:  { current: string | null; prev: string | null }
     createdAt:              string