投稿排他 (#171) (#345)

#171

#171

#171

#171

#171

#171

#171

Co-authored-by: miteruzo <miteruzo@naver.com>
Reviewed-on: #345

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: [:materials, { tag_name: :wiki_page }])
+        .preload(tags: [:deerjikists, :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: [:materials, { tag_name: :wiki_page }])
+    post = filtered_posts.preload(tags: [:deerjikists, :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: [:materials, { tag_name: :wiki_page }]).find_by(id: params[:id])
+    post = Post.includes(tags: [:deerjikists, :materials, { tag_name: :wiki_page }]).find_by(id: params[:id])
     return head :not_found unless post
 
     render json: PostRepr.base(post, current_user)
@@ -173,33 +173,68 @@ class PostsController < ApplicationController
     return head :unauthorized unless current_user
     return head :forbidden unless current_user.gte_member?
 
+    force = bool?(:force)
+    merge = bool?(:merge)
+    return head :bad_request if force && merge
+
+    base_version_no = parse_base_version_no
+    return head :bad_request if !(force) && !(base_version_no)
+
     title = params[:title].presence
     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)
+    post = nil
+    conflict_json = nil
 
     ApplicationRecord.transaction do
-      PostVersionRecorder.ensure_snapshot!(post, created_by_user: current_user)
+      post = Post.lock.find(params[:id].to_i)
 
-      post.update!(title:, original_created_from:, original_created_before:)
+      base_version = nil
+      base_snapshot = nil
+      current_snapshot = nil
+      unless force
+        base_version = post.post_versions.find_by!(version_no: base_version_no)
 
-      normalised_tags = Tag.normalise_tags!(tag_names, with_tagme: false)
-      TagVersioning.record_tag_snapshots!(normalised_tags, created_by_user: current_user)
+        base_snapshot = post_snapshot_from_version(base_version)
+        current_snapshot = post_snapshot_from_record(post)
+      end
+      incoming_snapshot = post_incoming_snapshot(title:,
+                                                 original_created_from:,
+                                                 original_created_before:,
+                                                 tag_names:,
+                                                 parent_post_ids:)
 
-      tags = post.tags.nico.to_a + normalised_tags
-      tags = Tag.expand_parent_tags(tags)
-      sync_post_tags!(post, tags)
+      snapshot_to_apply =
+          if force || post.version_no == base_version_no || current_snapshot == base_snapshot
+            incoming_snapshot
+          else
+            changes = post_snapshot_changes(base_snapshot, current_snapshot, incoming_snapshot)
+            conflicts = changes.select { |change| change[:conflict] }
 
-      sync_parent_posts!(post, parent_post_ids)
+            if merge && conflicts.empty?
+              merge_post_snapshots(base_snapshot, current_snapshot, incoming_snapshot)
+            else
+              conflict_json = post_conflict_json(post:,
+                                                 base_version_no:,
+                                                 base_snapshot:,
+                                                 current_snapshot:,
+                                                 incoming_snapshot:,
+                                                 changes:,
+                                                 conflicts:)
+              raise ActiveRecord::Rollback
+            end
+          end
 
-      PostVersionRecorder.record!(post:, event_type: :update, created_by_user: current_user)
+      apply_post_snapshot!(post, snapshot_to_apply)
     end
 
+    return render json: conflict_json, status: :conflict if conflict_json
+
     post.reload
-    json = post.as_json
+    json = PostRepr.base(post, current_user)
     json['tags'] = build_tag_tree_for(post.tags)
     render json:, status: :ok
   rescue Tag::NicoTagNormalisationError
@@ -225,7 +260,7 @@ 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: [:materials, { tag_name: :wiki_page }])
+                       tag: [:deerjikists, :materials, { tag_name: :wiki_page }])
 
     events = []
     pts.each do |pt|
@@ -404,4 +439,205 @@ class PostsController < ApplicationController
       PostImplication.create_or_find_by!(post_id: post.id, parent_post_id:)
     end
   end
+
+  def parse_base_version_no
+    version_no = Integer(params[:base_version_no], exception: false)
+    if version_no&.positive?
+      version_no
+    else
+      nil
+    end
+  end
+
+  def post_snapshot_from_version version
+    { title: version.title,
+      original_created_from: snapshot_time(version.original_created_from),
+      original_created_before: snapshot_time(version.original_created_before),
+      tag_names: editable_tag_names_from_version(version),
+      parent_post_ids: snapshot_parent_post_ids_from_version(version) }
+  end
+
+  def editable_tag_names_from_version version
+    version.tags.to_s.split.reject { |name| name.downcase.start_with?('nico:') }.sort
+  end
+
+  def post_snapshot_from_record post
+    { title: post.title,
+      original_created_from: snapshot_time(post.original_created_from),
+      original_created_before: snapshot_time(post.original_created_before),
+      tag_names: editable_tag_names_from_post(post),
+      parent_post_ids: post.parent_posts.order(:id).pluck(:id) }
+  end
+
+  def editable_tag_names_from_post post
+    post.tags.not_nico.joins(:tag_name).order('tag_names.name').pluck('tag_names.name')
+  end
+
+  def post_incoming_snapshot title:, original_created_from:, original_created_before:,
+                             tag_names:, parent_post_ids:
+    { title:,
+      original_created_from: snapshot_time(original_created_from),
+      original_created_before: snapshot_time(original_created_before),
+      tag_names: incoming_tag_names_for_snapshot(tag_names),
+      parent_post_ids: parent_post_ids.sort }
+  end
+
+  def snapshot_parent_post_ids_from_version version
+    if version.respond_to?(:parent_post_ids)
+      version.parent_post_ids.to_s.split.map { |id| id.to_i }.sort
+    elsif version.respond_to?(:parent_id) && version.parent_id
+      [version.parent_id]
+    else
+      []
+    end
+  end
+
+  def snapshot_time value
+    return nil if value.blank?
+
+    value = Time.zone.parse(value.to_s) if value in String
+    value&.in_time_zone&.iso8601(6)
+  rescue ArgumentError, TypeError
+    value.to_s
+  end
+
+  def incoming_tag_names_for_snapshot raw_tag_names
+    tags = Tag.normalise_tags!(raw_tag_names, with_tagme: false)
+
+    Tag.expand_parent_tags(tags).map(&:name).uniq.sort
+  end
+
+  def post_conflict_json post:, base_version_no:, base_snapshot:,
+                         current_snapshot:, incoming_snapshot:, changes:, conflicts:
+    { error: 'conflict',
+      message: '競合が発生しました．',
+      post_id: post.id,
+      base_version_no:,
+      current_version_no: post.version_no,
+      base: base_snapshot,
+      current: current_snapshot,
+      mine: incoming_snapshot,
+      changes:,
+      conflicts:,
+      mergeable: conflicts.empty? }
+  end
+
+  def post_snapshot_changes base_snapshot, current_snapshot, incoming_snapshot
+    [scalar_snapshot_change(:title, 'タイトル',
+                            base_snapshot, current_snapshot, incoming_snapshot),
+     scalar_snapshot_change(:original_created_from, 'オリジナルの作成日時（以降）',
+                            base_snapshot, current_snapshot, incoming_snapshot),
+     scalar_snapshot_change(:original_created_before, 'オリジナルの作成日時（より前）',
+                            base_snapshot, current_snapshot, incoming_snapshot),
+     set_snapshot_change(:tag_names, 'タグ',
+                         base_snapshot, current_snapshot, incoming_snapshot),
+     set_snapshot_change(:parent_post_ids, '親投稿',
+                         base_snapshot, current_snapshot, incoming_snapshot)].compact
+  end
+
+  def scalar_snapshot_change field, label, base_snapshot, current_snapshot, incoming_snapshot
+    base = base_snapshot[field]
+    current = current_snapshot[field]
+    mine = incoming_snapshot[field]
+
+    return nil if current == base && mine == base
+
+    { field:, label:, base:, current:, mine:,
+      changed_by_current: current != base,
+      changed_by_me: mine != base,
+      conflict: scalar_snapshot_conflict?(base, current, mine) }
+  end
+
+  def scalar_snapshot_conflict? base, current, mine
+    current != base && mine != base && current != mine
+  end
+
+  def set_snapshot_change field, label, base_snapshot, current_snapshot, incoming_snapshot
+    base = base_snapshot[field].to_a
+    current = current_snapshot[field].to_a
+    mine = incoming_snapshot[field].to_a
+
+    added_by_current = current - base
+    removed_by_current = base - current
+    added_by_me = mine - base
+    removed_by_me = base - mine
+
+    if (added_by_current.empty? &&
+        removed_by_current.empty? &&
+        added_by_me.empty? &&
+        removed_by_me.empty?)
+      return nil
+    end
+
+    { field:, label:, base:, current:, mine:, added_by_current:, removed_by_current:,
+      added_by_me:, removed_by_me:,
+      changed_by_current: added_by_current.present? || removed_by_current.present?,
+      changed_by_me: added_by_me.present? || removed_by_me.present?,
+      conflict: set_snapshot_conflict?(added_by_current:,
+                                       removed_by_current:,
+                                       added_by_me:,
+                                       removed_by_me:) }
+  end
+
+  def set_snapshot_conflict? added_by_current:, removed_by_current:,
+                             added_by_me:, removed_by_me:
+    (added_by_current & removed_by_me).present? || (removed_by_current & added_by_me).present?
+  end
+
+  def apply_post_snapshot! post, snapshot
+    PostVersionRecorder.ensure_snapshot!(post, created_by_user: current_user)
+
+    post.update!(title: snapshot[:title],
+                 original_created_from: snapshot[:original_created_from],
+                 original_created_before: snapshot[:original_created_before])
+
+    editable_tags = Tag.normalise_tags!(snapshot[:tag_names], with_tagme: false)
+    TagVersioning.record_tag_snapshots!(editable_tags, created_by_user: current_user)
+
+    readonly_tags = post.tags.nico.to_a
+
+    tags = readonly_tags + editable_tags
+    tags = Tag.expand_parent_tags(tags)
+
+    sync_post_tags!(post, tags)
+    sync_parent_posts!(post, snapshot[:parent_post_ids])
+
+    PostVersionRecorder.record!(post:, event_type: :update, created_by_user: current_user)
+  end
+
+  def merge_post_snapshots base_snapshot, current_snapshot, incoming_snapshot
+    [:title, :original_created_from, :original_created_before].map {
+      [_1, merge_scalar_snapshot_value(base_snapshot[_1],
+                                       current_snapshot[_1],
+                                       incoming_snapshot[_1])]
+    }.to_h.merge([:tag_names, :parent_post_ids].map {
+      [_1, merge_set_snapshot_value(base_snapshot[_1],
+                                    current_snapshot[_1],
+                                    incoming_snapshot[_1])]
+    }.to_h)
+  end
+
+  def merge_scalar_snapshot_value base, current, mine
+    return mine if current == base
+    return current if mine == base || current == mine
+
+    raise ArgumentError, '競合してゐる項目はマージできません．'
+  end
+
+  def merge_set_snapshot_value base, current, mine
+    base = base.to_a
+    current = current.to_a
+    mine = mine.to_a
+
+    added_by_current = current - base
+    removed_by_current = base - current
+    added_by_me = mine - base
+    removed_by_me = base - mine
+
+    merged = base + added_by_current + added_by_me
+    merged -= removed_by_current
+    merged -= removed_by_me
+
+    merged.uniq.sort
+  end
 end

backend/app/models/post.rb

@@ -28,6 +28,8 @@ class Post < ApplicationRecord
 
   has_one_attached :thumbnail
 
+  attribute :version_no, :integer, default: 1
+
   before_validation :normalise_url
 
   validates :url, presence: true, uniqueness: true

backend/app/models/tag.rb

@@ -40,6 +40,8 @@ class Tag < ApplicationRecord
   belongs_to :tag_name
   delegate :wiki_page, to: :tag_name
 
+  attribute :version_no, :integer, default: 1
+
   delegate :name, to: :tag_name, allow_nil: true
   validates :tag_name, presence: true
 

backend/app/models/wiki_page.rb

@@ -15,6 +15,8 @@ class WikiPage < ApplicationRecord
 
   has_many :wiki_versions
 
+  attribute :version_no, :integer, default: 1
+
   belongs_to :tag_name
   validates :tag_name, presence: true
   validates :body, presence: true

backend/app/services/version_recorder.rb

@@ -16,19 +16,20 @@ class VersionRecorder
       @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
+      validate_version_sequence!(latest)
 
       attrs = snapshot_attributes
 
-      return latest if @event_type == 'update' && latest && same_snapshot?(latest, attrs)
+      if @event_type == 'update' && latest && same_snapshot?(latest, attrs)
+        return latest
+      end
 
-      version_class.create!(base_attributes(latest).merge(record_key => @record).merge(attrs))
+      version = version_class.create!(
+          base_attributes(latest).merge(record_key => @record).merge(attrs))
+
+      update_record_version_no!(version.version_no)
+
+      version
     end
   end
 
@@ -45,7 +46,31 @@ class VersionRecorder
       created_by_user: @created_by_user }
   end
 
-  def same_snapshot?(version, attrs) = attrs.all? { |k, v| version.public_send(k) == v }
+  def update_record_version_no! version_no
+    @record.update_columns(version_no:)
+    @record.version_no = version_no
+  end
+
+  def validate_version_sequence! latest
+    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
+
+    return unless latest
+
+    if @record.version_no != latest.version_no
+      raise ("#{ record_class.name }##{ @record.id } version_no is #{ @record.version_no }, " +
+             "but latest #{ version_class.name } version_no is #{ latest.version_no }")
+    end
+  end
+
+  def same_snapshot? version, attrs
+    attrs.all? { |k, v| version.public_send(k) == v }
+  end
 
   def validate_event_type!
     return if EVENT_TYPES.include?(@event_type)

backend/db/migrate/20260507124000_add_version_no_to_posts.rb

@@ -0,0 +1,27 @@
+class AddVersionNoToPosts < ActiveRecord::Migration[8.0]
+  def up
+    add_column :posts, :version_no, :integer
+
+    execute <<~SQL
+      UPDATE
+        posts
+      SET
+        version_no = (
+          SELECT
+            MAX(version_no)
+          FROM
+            post_versions
+          WHERE
+            post_id = posts.id)
+    SQL
+
+    change_column_null :posts, :version_no, false
+
+    add_check_constraint :posts, 'version_no > 0', name: 'chk_posts_version_no_positive'
+  end
+
+  def down
+    remove_check_constraint :posts, name: 'chk_posts_version_no_positive'
+    remove_column :posts, :version_no
+  end
+end

backend/db/migrate/20260507211600_add_version_no_to_tags.rb

@@ -0,0 +1,37 @@
+class AddVersionNoToTags < ActiveRecord::Migration[8.0]
+  def up
+    add_column :tags, :version_no, :integer
+
+    execute <<~SQL
+      UPDATE
+        tags
+      SET
+        version_no = (
+          CASE category
+          WHEN 'nico' THEN
+            (SELECT
+               MAX(version_no)
+             FROM
+               nico_tag_versions
+             WHERE
+               tag_id = tags.id)
+          ELSE
+            (SELECT
+               MAX(version_no)
+             FROM
+               tag_versions
+             WHERE
+               tag_id = tags.id)
+          END)
+    SQL
+
+    change_column_null :tags, :version_no, false
+
+    add_check_constraint :tags, 'version_no > 0', name: 'chk_tags_version_no_positive'
+  end
+
+  def down
+    remove_check_constraint :tags, name: 'chk_tags_version_no_positive'
+    remove_column :tags, :version_no
+  end
+end

backend/db/migrate/20260507213300_add_version_no_to_wiki_pages.rb

@@ -0,0 +1,27 @@
+class AddVersionNoToWikiPages < ActiveRecord::Migration[8.0]
+  def up
+    add_column :wiki_pages, :version_no, :integer
+
+    execute <<~SQL
+      UPDATE
+        wiki_pages
+      SET
+        version_no = (
+          SELECT
+            MAX(version_no)
+          FROM
+            wiki_versions
+          WHERE
+            wiki_page_id = wiki_pages.id)
+    SQL
+
+    change_column_null :wiki_pages, :version_no, false
+
+    add_check_constraint :wiki_pages, 'version_no > 0', name: 'chk_wiki_pages_version_no_positive'
+  end
+
+  def down
+    remove_check_constraint :wiki_pages, name: 'chk_wiki_pages_version_no_positive'
+    remove_column :wiki_pages, :version_no
+  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_05_01_153900) do
+ActiveRecord::Schema[8.0].define(version: 2026_05_07_213300) 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
@@ -186,8 +186,10 @@ ActiveRecord::Schema[8.0].define(version: 2026_05_01_153900) do
     t.datetime "original_created_from"
     t.datetime "original_created_before"
     t.datetime "updated_at", null: false
+    t.integer "version_no", null: false
     t.index ["uploaded_user_id"], name: "index_posts_on_uploaded_user_id"
     t.index ["url"], name: "index_posts_on_url", unique: true
+    t.check_constraint "`version_no` > 0", name: "chk_posts_version_no_positive"
   end
 
   create_table "settings", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
@@ -262,8 +264,10 @@ ActiveRecord::Schema[8.0].define(version: 2026_05_01_153900) do
     t.datetime "updated_at", null: false
     t.integer "post_count", default: 0, null: false
     t.datetime "discarded_at"
+    t.integer "version_no", null: false
     t.index ["discarded_at"], name: "index_tags_on_discarded_at"
     t.index ["tag_name_id"], name: "index_tags_on_tag_name_id", unique: true
+    t.check_constraint "`version_no` > 0", name: "chk_tags_version_no_positive"
   end
 
   create_table "theatre_comments", primary_key: ["theatre_id", "no"], charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
@@ -369,10 +373,12 @@ ActiveRecord::Schema[8.0].define(version: 2026_05_01_153900) do
     t.datetime "updated_at", null: false
     t.datetime "discarded_at"
     t.integer "next_asset_no", default: 1, null: false
+    t.integer "version_no", null: false
     t.index ["created_user_id"], name: "index_wiki_pages_on_created_user_id"
     t.index ["discarded_at"], name: "index_wiki_pages_on_discarded_at"
     t.index ["tag_name_id"], name: "index_wiki_pages_on_tag_name_id", unique: true
     t.index ["updated_user_id"], name: "index_wiki_pages_on_updated_user_id"
+    t.check_constraint "`version_no` > 0", name: "chk_wiki_pages_version_no_positive"
   end
 
   create_table "wiki_revision_lines", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|

backend/spec/requests/posts_spec.rb

@@ -10,6 +10,10 @@ RSpec.describe 'Posts API', type: :request do
     allow_any_instance_of(Post).to receive(:resized_thumbnail!).and_return(true)
   end
 
+  def create_nico_tag!(name)
+    Tag.find_or_create_by_tag_name!(name, category: :nico)
+  end
+
   def dummy_upload
     # 中身は何でもいい（加工処理はスタブしてる）
     Rack::Test::UploadedFile.new(StringIO.new('dummy'), 'image/jpeg', original_filename: 'dummy.jpg')
@@ -23,21 +27,34 @@ RSpec.describe 'Posts API', type: :request do
     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
-    )
+  def create_post_version_for!(post)
+    version =
+        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)
+
+    post.update_columns(version_no: version.version_no) if post.has_attribute?(:version_no)
+    post.version_no = version.version_no if post.respond_to?(:version_no=)
+
+    version
+  end
+
+  def post_update_params(post, params = { })
+    base_version =
+        post.post_versions.order(version_no: :desc).first ||
+        create_post_version_for!(post.reload)
+
+    post_write_params({ base_version_no: base_version.version_no }.merge(params))
   end
 
   let!(:tag_name) { TagName.create!(name: 'spec_tag') }
@@ -806,24 +823,26 @@ RSpec.describe 'Posts API', type: :request do
 
     it '401 when not logged in' do
       sign_out
-      put "/posts/#{post_record.id}", params: post_write_params(title: 'updated', tags: 'spec_tag')
+      put "/posts/#{post_record.id}", params: post_update_params(
+          post_record, 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: post_write_params(title: 'updated', tags: 'spec_tag')
+      put "/posts/#{post_record.id}", params: post_update_params(
+          post_record, title: 'updated', tags: 'spec_tag')
       expect(response).to have_http_status(:forbidden)
     end
 
     it '200 and updates title + resync tags when member' do
       sign_in_as(member)
 
-      # 追加で別タグも作って、更新時に入れ替わることを見る
       tn2 = TagName.create!(name: 'spec_tag_2')
       Tag.create!(tag_name: tn2, category: :general)
 
-      put "/posts/#{post_record.id}", params: post_write_params(
+      put "/posts/#{post_record.id}", params: post_update_params(
+          post_record,
           title: 'updated title',
           tags: 'spec_tag_2')
 
@@ -831,7 +850,6 @@ RSpec.describe 'Posts API', type: :request do
       expect(json).to have_key('tags')
       expect(json['tags']).to be_an(Array)
 
-      # show と同様、update 後レスポンスもツリー形式
       names = json['tags'].map { |n| n['name'] }
       expect(names).to include('spec_tag_2')
     end
@@ -846,10 +864,10 @@ RSpec.describe 'Posts API', type: :request do
       it 'return 400' do
         sign_in_as(member)
 
-        put "/posts/#{post_record.id}", params: post_write_params(
-          title: 'updated title',
-          tags: 'nico:nico_tag'
-        )
+        put "/posts/#{post_record.id}", params: post_update_params(
+            post_record,
+            title: 'updated title',
+            tags: 'nico:nico_tag')
 
         expect(response).to have_http_status(:bad_request), response.body
       end
@@ -887,11 +905,11 @@ RSpec.describe 'Posts API', type: :request do
       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}"
-        )
+        put "/posts/#{post_record.id}", params: post_update_params(
+            post_record,
+            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)
 
@@ -908,7 +926,8 @@ RSpec.describe 'Posts API', type: :request do
       it 'clears parent posts when parent_post_ids is blank' do
         sign_in_as(member)
 
-        put "/posts/#{post_record.id}", params: post_write_params(
+        put "/posts/#{post_record.id}", params: post_update_params(
+          post_record,
           title: 'updated title',
           tags: 'spec_tag',
           parent_post_ids: ''
@@ -922,7 +941,8 @@ RSpec.describe 'Posts API', type: :request do
         sign_in_as(member)
         create_post_version_for!(post_record.reload)
 
-        put "/posts/#{post_record.id}", params: post_write_params(
+        put "/posts/#{post_record.id}", params: post_update_params(
+          post_record,
           title: 'updated title',
           tags: 'spec_tag',
           parent_post_ids: "#{new_parent_post_1.id} #{new_parent_post_2.id}"
@@ -943,7 +963,10 @@ RSpec.describe 'Posts API', type: :request do
       it 'returns 422' do
         sign_in_as(member)
 
+        base_version = create_post_version_for!(post_record.reload)
+
         put "/posts/#{post_record.id}", params: {
+            base_version_no: base_version.version_no,
             title: 'updated title',
             tags: 'spec_tag' }
 
@@ -966,7 +989,8 @@ RSpec.describe 'Posts API', type: :request do
           parent_post:
         )
 
-        put "/posts/#{post_record.id}", params: post_write_params(
+        put "/posts/#{post_record.id}", params: post_update_params(
+          post_record,
           title: 'updated title',
           tags: 'spec_tag',
           parent_post_ids: 'abc'
@@ -991,7 +1015,8 @@ RSpec.describe 'Posts API', type: :request do
           parent_post:
         )
 
-        put "/posts/#{post_record.id}", params: post_write_params(
+        put "/posts/#{post_record.id}", params: post_update_params(
+          post_record,
           title: 'updated title',
           tags: 'spec_tag',
           parent_post_ids: '999999999'
@@ -1006,7 +1031,8 @@ RSpec.describe 'Posts API', type: :request do
       it 'returns 422 and does not create self implication' do
         sign_in_as(member)
 
-        put "/posts/#{post_record.id}", params: post_write_params(
+        put "/posts/#{post_record.id}", params: post_update_params(
+          post_record,
           title: 'updated title',
           tags: 'spec_tag',
           parent_post_ids: post_record.id.to_s
@@ -1020,6 +1046,221 @@ RSpec.describe 'Posts API', type: :request do
         )).to be(false)
       end
     end
+
+    context 'with optimistic locking' do
+      let!(:no_deerjikist_tag) { Tag.no_deerjikist }
+
+      before do
+        PostTag.create!(post: post_record, tag: no_deerjikist_tag)
+      end
+
+      it '400 when base_version_no is missing without force' do
+        sign_in_as(member)
+
+        put "/posts/#{post_record.id}", params: post_write_params(
+            title: 'updated title',
+            tags: 'spec_tag')
+
+        expect(response).to have_http_status(:bad_request)
+      end
+
+      it '400 when force and merge are both true' do
+        sign_in_as(member)
+
+        put "/posts/#{post_record.id}", params: post_write_params(
+            title: 'updated title',
+            tags: 'spec_tag',
+            force: '1',
+            merge: '1')
+
+        expect(response).to have_http_status(:bad_request)
+      end
+
+      it '409 when scalar fields are changed both by current and incoming updates' do
+        sign_in_as(member)
+
+        base_version = create_post_version_for!(post_record.reload)
+
+        post_record.update!(title: 'updated by other user')
+        PostVersionRecorder.record!(
+            post: post_record.reload,
+            event_type: :update,
+            created_by_user: member)
+
+        put "/posts/#{post_record.id}", params: post_write_params(
+            base_version_no: base_version.version_no,
+            title: 'updated by me',
+            tags: "spec_tag #{Tag.no_deerjikist.name}")
+
+        expect(response).to have_http_status(:conflict)
+
+        expect(json.fetch('error')).to eq('conflict')
+        expect(json.fetch('base_version_no')).to eq(base_version.version_no)
+        expect(json.fetch('current_version_no')).to eq(2)
+        expect(json.fetch('mergeable')).to be(false)
+
+        conflict_fields = json.fetch('conflicts').map { |change| change.fetch('field') }
+        expect(conflict_fields).to include('title')
+
+        expect(post_record.reload.title).to eq('updated by other user')
+      end
+
+      it 'returns 409 with mergeable true when stale tag changes do not conflict but merge is not requested' do
+        sign_in_as(member)
+
+        base_version = create_post_version_for!(post_record.reload)
+
+        current_tag = Tag.find_or_create_by_tag_name!('current_added_tag', category: :general)
+        PostTag.create!(post: post_record, tag: current_tag, created_user: member)
+
+        PostVersionRecorder.record!(
+            post: post_record.reload,
+            event_type: :update,
+            created_by_user: member)
+
+        put "/posts/#{post_record.id}", params: post_write_params(
+            base_version_no: base_version.version_no,
+            title: post_record.title,
+            tags: "spec_tag #{Tag.no_deerjikist.name} incoming_added_tag")
+
+        expect(response).to have_http_status(:conflict)
+
+        expect(json.fetch('mergeable')).to be(true)
+
+        tag_change = json.fetch('changes').find { |change| change.fetch('field') == 'tag_names' }
+        expect(tag_change).to be_present
+        expect(tag_change.fetch('conflict')).to be(false)
+        expect(tag_change.fetch('added_by_current')).to include('current_added_tag')
+        expect(tag_change.fetch('added_by_me')).to include('incoming_added_tag')
+      end
+
+      it 'merges non-conflicting stale tag changes when merge is true' do
+        sign_in_as(member)
+
+        base_version = create_post_version_for!(post_record.reload)
+
+        current_tag = Tag.find_or_create_by_tag_name!('current_merge_tag', category: :general)
+        PostTag.create!(post: post_record, tag: current_tag, created_user: member)
+
+        PostVersionRecorder.record!(
+            post: post_record.reload,
+            event_type: :update,
+            created_by_user: member)
+
+        put "/posts/#{post_record.id}", params: post_write_params(
+            base_version_no: base_version.version_no,
+            title: post_record.title,
+            tags: "spec_tag #{Tag.no_deerjikist.name} incoming_merge_tag",
+            merge: '1')
+
+        expect(response).to have_http_status(:ok)
+
+        names = post_record.reload.tags.map(&:name)
+
+        expect(names).to include('spec_tag')
+        expect(names).to include(Tag.no_deerjikist.name)
+        expect(names).to include('current_merge_tag')
+        expect(names).to include('incoming_merge_tag')
+      end
+
+      it 'does not conflict when only nico tags changed after the base version' do
+        sign_in_as(member)
+
+        base_version = create_post_version_for!(post_record.reload)
+
+        nico_tag = create_nico_tag!('nico:optimistic_lock_nico')
+        PostTag.create!(post: post_record, tag: nico_tag, created_user: member)
+
+        PostVersionRecorder.record!(
+            post: post_record.reload,
+            event_type: :update,
+            created_by_user: member)
+
+        expect(post_record.reload.version_no).to eq(2)
+
+        put "/posts/#{post_record.id}", params: post_write_params(
+            base_version_no: base_version.version_no,
+            title: post_record.title,
+            tags: "spec_tag #{ Tag.no_deerjikist.name }")
+
+        expect(response).to have_http_status(:ok)
+
+        names = post_record.reload.tags.map(&:name)
+
+        expect(names).to include('spec_tag')
+        expect(names).to include(Tag.no_deerjikist.name)
+        expect(names).to include(nico_tag.name)
+      end
+
+      it 'keeps nico tags even when they are not included in PUT tags' do
+        sign_in_as(member)
+
+        nico_tag = create_nico_tag!('nico:readonly_update_nico')
+        PostTag.create!(post: post_record, tag: nico_tag, created_user: member)
+
+        base_version = create_post_version_for!(post_record.reload)
+
+        put "/posts/#{post_record.id}", params: post_write_params(
+            base_version_no: base_version.version_no,
+            title: 'updated title',
+            tags: "spec_tag #{ Tag.no_deerjikist.name }")
+
+        expect(response).to have_http_status(:ok)
+
+        names = post_record.reload.tags.map(&:name)
+
+        expect(names).to include('spec_tag')
+        expect(names).to include(Tag.no_deerjikist.name)
+        expect(names).to include(nico_tag.name)
+      end
+
+      it 'allows non-nico tags linked from nico tags to be removed by normal post update' do
+        sign_in_as(member)
+
+        nico_tag = create_nico_tag!('nico:relation_source')
+        linked_tag = Tag.find_or_create_by_tag_name!('relation_linked_tag', category: :general)
+
+        NicoTagRelation.create!(nico_tag:, tag: linked_tag)
+        PostTag.create!(post: post_record, tag: nico_tag, created_user: member)
+        PostTag.create!(post: post_record, tag: linked_tag, created_user: member)
+
+        base_version = create_post_version_for!(post_record.reload)
+
+        put "/posts/#{post_record.id}", params: post_write_params(
+            base_version_no: base_version.version_no,
+            title: post_record.title,
+            tags: "spec_tag #{ Tag.no_deerjikist.name }")
+
+        expect(response).to have_http_status(:ok)
+
+        names = post_record.reload.tags.map(&:name)
+
+        expect(names).to include(nico_tag.name)
+        expect(names).to include('spec_tag')
+        expect(names).to include(Tag.no_deerjikist.name)
+        expect(names).not_to include(linked_tag.name)
+      end
+
+      it 'force-updates stale posts without base_version_no' do
+        sign_in_as(member)
+
+        create_post_version_for!(post_record.reload)
+
+        post_record.update!(title: 'updated by other user')
+        PostVersionRecorder.record!(
+            post: post_record.reload,
+            event_type: :update,
+            created_by_user: member)
+
+        put "/posts/#{post_record.id}", params: post_write_params(
+            title: 'forced title',
+            tags: "spec_tag #{Tag.no_deerjikist.name}",
+            force: '1')
+
+        expect(response).to have_http_status(:ok)
+        expect(post_record.reload.title).to eq('forced title')
+      end
+    end
   end
 
   describe 'GET /posts/random' do
@@ -1434,13 +1675,14 @@ RSpec.describe 'Posts API', type: :request do
 
     it 'creates next version on PUT /posts/:id when snapshot changes' do
       sign_in_as(member)
-      create_post_version_for!(post_record)
+      base_version = create_post_version_for!(post_record)
 
       tag_name2 = TagName.create!(name: 'spec_tag_2')
       Tag.create!(tag_name: tag_name2, category: :general)
 
       expect do
         put "/posts/#{post_record.id}", params: post_write_params(
+            base_version_no: base_version.version_no,
             title: 'updated title',
             tags: 'spec_tag_2')
       end.to change(PostVersion, :count).by(1)
@@ -1459,13 +1701,15 @@ RSpec.describe 'Posts API', type: :request do
       sign_in_as(member)
 
       PostTag.create!(post: post_record, tag: Tag.no_deerjikist)
-      create_post_version_for!(post_record.reload)
+      base_version = create_post_version_for!(post_record.reload)
 
       expect {
         put "/posts/#{post_record.id}", params: post_write_params(
+            base_version_no: base_version.version_no,
             title: post_record.title,
             tags: 'spec_tag')
       }.not_to change(PostVersion, :count)
+
       expect(response).to have_http_status(:ok)
 
       version = post_record.reload.post_versions.order(:version_no).last
@@ -1490,10 +1734,11 @@ RSpec.describe 'Posts API', type: :request do
 
     it 'does not create a version when PUT /posts/:id is invalid' do
       sign_in_as(member)
-      create_post_version_for!(post_record)
+      base_version = create_post_version_for!(post_record)
 
       expect do
         put "/posts/#{post_record.id}", params: post_write_params(
+            base_version_no: base_version.version_no,
             title: 'updated title',
             tags: 'spec_tag',
             original_created_from: Time.zone.local(2020, 1, 2, 0, 0, 0).iso8601,
@@ -1507,46 +1752,22 @@ RSpec.describe 'Posts API', type: :request do
   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: 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)
-
-      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)
 
+      base_version = create_post_version_for!(post_record.reload)
+
       tag_name2 = TagName.create!(name: 'spec_tag_2')
       tag2 = Tag.create!(tag_name: tag_name2, category: :general)
 
       expect {
         put "/posts/#{post_record.id}", params: post_write_params(
+            base_version_no: base_version.version_no,
             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)
+      expect(response).to have_http_status(:ok), response.body
     end
   end
 end

backend/spec/requests/tag_versions_spec.rb

@@ -26,17 +26,22 @@ RSpec.describe 'TagVersions API', type: :request do
     created_by_user:,
     created_at:
   )
-    TagVersion.create!(
-      tag: tag,
-      version_no: version_no,
-      event_type: event_type,
-      name: name,
-      category: category,
-      aliases: Array(aliases).join(' '),
-      parent_tag_ids: Array(parent_tags).map(&:id).join(' '),
-      created_by_user: created_by_user,
-      created_at: created_at
-    )
+    version =
+        TagVersion.create!(
+            tag: tag,
+            version_no: version_no,
+            event_type: event_type,
+            name: name,
+            category: category,
+            aliases: Array(aliases).join(' '),
+            parent_tag_ids: Array(parent_tags).map(&:id).join(' '),
+            created_by_user: created_by_user,
+            created_at: created_at)
+
+    tag.update_columns(version_no: version_no) if tag.has_attribute?(:version_no)
+    tag.version_no = version_no if tag.respond_to?(:version_no=)
+
+    version
   end
 
   let!(:v1) do

backend/spec/services/version_recorder_spec.rb

@@ -0,0 +1,85 @@
+require 'rails_helper'
+
+RSpec.describe VersionRecorder do
+  let(:member) { create(:user, :member) }
+
+  let(:post_record) do
+    Post.create!(
+        title: 'version recorder post',
+        url: 'https://example.com/version-recorder-post')
+  end
+
+  it 'updates record version_no when creating the first version' do
+    version =
+        PostVersionRecorder.record!(
+            post: post_record,
+            event_type: :create,
+            created_by_user: member)
+
+    expect(version.version_no).to eq(1)
+    expect(post_record.reload.version_no).to eq(1)
+  end
+
+  it 'updates record version_no when creating the next version' do
+    PostVersionRecorder.record!(
+        post: post_record,
+        event_type: :create,
+        created_by_user: member)
+
+    post_record.update!(title: 'updated version recorder post')
+
+    version =
+        PostVersionRecorder.record!(
+            post: post_record.reload,
+            event_type: :update,
+            created_by_user: member)
+
+    expect(version.version_no).to eq(2)
+    expect(post_record.reload.version_no).to eq(2)
+  end
+
+  it 'does not create a new version or advance version_no when snapshot is unchanged' do
+    first =
+        PostVersionRecorder.record!(
+            post: post_record,
+            event_type: :create,
+            created_by_user: member)
+
+    expect {
+      version =
+          PostVersionRecorder.record!(
+              post: post_record.reload,
+              event_type: :update,
+              created_by_user: member)
+
+      expect(version).to eq(first)
+    }.not_to change(PostVersion, :count)
+
+    expect(post_record.reload.version_no).to eq(1)
+  end
+
+  it 'raises when record version_no is older than the latest version' do
+    PostVersionRecorder.record!(
+        post: post_record,
+        event_type: :create,
+        created_by_user: member)
+
+    post_record.update!(title: 'updated once')
+
+    PostVersionRecorder.record!(
+        post: post_record.reload,
+        event_type: :update,
+        created_by_user: member)
+
+    post_record.update_columns(version_no: 1)
+
+    post_record.update!(title: 'updated with stale version_no')
+
+    expect {
+      PostVersionRecorder.record!(
+          post: post_record.reload,
+          event_type: :update,
+          created_by_user: member)
+    }.to raise_error(RuntimeError, /version_no/)
+  end
+end