backend/app/controllers/posts_controller.rb

@@ -78,6 +78,7 @@ class PostsController < ApplicationController
     if post.save
       post.resized_thumbnail!
       post.tags = Tag.normalise_tags(tag_names)
+      post.tags = Tag.expand_parent_tags(post.tags)
       render json: post.as_json(include: { tags: { only: [:id, :name, :category, :post_count] } }),
              status: :created
     else
@@ -111,6 +112,7 @@ class PostsController < ApplicationController
 
     post = Post.find(params[:id].to_i)
     tags = post.tags.where(category: 'nico').to_a + Tag.normalise_tags(tag_names)
+    tags = Tag.expand_parent_tags(tags)
     if post.update(title:, tags:, original_created_from:, original_created_before:)
       render json: post.as_json(include: { tags: { only: [:id, :name, :category, :post_count] } }),
              status: :ok

backend/app/models/tag.rb

@@ -11,6 +11,14 @@ class Tag < ApplicationRecord
                                          dependent: :destroy
   has_many :linked_nico_tags, through: :reversed_nico_tag_relations, source: :nico_tag
 
+  has_many :tag_implications, foreign_key: :parent_tag_id, dependent: :destroy
+  has_many :children, through: :tag_implications, source: :tag
+
+  has_many :reversed_tag_implications, class_name: 'TagImplication',
+                                       foreign_key: :tag_id,
+                                       dependent: :destroy
+  has_many :parents, through: :reversed_tag_implications, source: :parent_tag
+
   enum :category, { deerjikist:  'deerjikist',
                     meme:        'meme',
                     character:   'character',
@@ -61,6 +69,29 @@ class Tag < ApplicationRecord
     tags.uniq
   end
 
+  def self.expand_parent_tags tags
+    return [] if tags.blank?
+
+    seen = Set.new
+    result = []
+    stack = tags.compact.dup
+
+    until stack.empty?
+      tag = stack.pop
+      next unless tag
+
+      tag.parents.each do |parent|
+        next if seen.include?(parent.id)
+
+        seen << parent.id
+        result << parent
+        stack << parent
+      end
+    end
+
+    (result + tags).uniq { |t| t.id }
+  end
+
   private
 
   def nico_tag_name_must_start_with_nico

backend/app/models/tag_implication.rb

@@ -0,0 +1,17 @@
+class TagImplication < ApplicationRecord
+  belongs_to :tag, class_name: 'Tag'
+  belongs_to :parent_tag, class_name: 'Tag'
+
+  validates :tag_id, presence: true
+  validates :parent_tag_id, presence: true
+
+  validate :parent_tag_mustnt_be_itself
+
+  private
+
+  def parent_tag_mustnt_be_itself
+    if parent_tag == tag
+      errors.add :parent_tag_id, '親タグは子タグと同一であってはなりません．'
+    end
+  end
+end

backend/db/migrate/20251009222200_create_tag_implications.rb

@@ -0,0 +1,9 @@
+class CreateTagImplications < ActiveRecord::Migration[8.0]
+  def change
+    create_table :tag_implications do |t|
+      t.references :tag, null: false, foreign_key: { to_table: :tags }
+      t.references :parent_tag, null: false, foreign_key: { to_table: :tags }
+      t.timestamps
+    end
+  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: 2025_09_09_075500) do
+ActiveRecord::Schema[8.0].define(version: 2025_10_09_222200) 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
@@ -107,6 +107,15 @@ ActiveRecord::Schema[8.0].define(version: 2025_09_09_075500) do
     t.index ["tag_id"], name: "index_tag_aliases_on_tag_id"
   end
 
+  create_table "tag_implications", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "tag_id", null: false
+    t.bigint "parent_tag_id", null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.index ["parent_tag_id"], name: "index_tag_implications_on_parent_tag_id"
+    t.index ["tag_id"], name: "index_tag_implications_on_tag_id"
+  end
+
   create_table "tag_similarities", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "tag_id", null: false
     t.bigint "target_tag_id", null: false
@@ -176,6 +185,8 @@ ActiveRecord::Schema[8.0].define(version: 2025_09_09_075500) do
   add_foreign_key "posts", "users", column: "uploaded_user_id"
   add_foreign_key "settings", "users"
   add_foreign_key "tag_aliases", "tags"
+  add_foreign_key "tag_implications", "tags"
+  add_foreign_key "tag_implications", "tags", column: "parent_tag_id"
   add_foreign_key "tag_similarities", "tags"
   add_foreign_key "tag_similarities", "tags", column: "target_tag_id"
   add_foreign_key "user_ips", "ip_addresses"