#174 完了

backend/app/models/tag_implication.rb

@@ -2,7 +2,7 @@ class TagImplication < ApplicationRecord
   belongs_to :tag, class_name: 'Tag'
   belongs_to :parent_tag, class_name: 'Tag'
 
-  validates :tag_id, presence: true
+  validates :tag_id, presence: true, uniqueness: { scope: :parent_tag_id }
   validates :parent_tag_id, presence: true
 
   validate :parent_tag_mustnt_be_itself

backend/db/migrate/20251210123200_add_unique_index_to_tag_implications.rb

@@ -0,0 +1,7 @@
+class AddUniqueIndexToTagImplications < ActiveRecord::Migration[8.0]
+  def change
+    add_index :tag_implications, [:tag_id, :parent_tag_id],
+              unique: true,
+              name: 'index_tag_implications_on_tag_id_and_parent_tag_id'
+  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_10_09_222200) do
+ActiveRecord::Schema[8.0].define(version: 2025_12_10_123200) 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
@@ -120,6 +120,7 @@ ActiveRecord::Schema[8.0].define(version: 2025_10_09_222200) do
     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", "parent_tag_id"], name: "index_tag_implications_on_tag_id_and_parent_tag_id", unique: true
     t.index ["tag_id"], name: "index_tag_implications_on_tag_id"
   end