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 +1,27 @@
+class AddUniqueIndexToTagImplications < ActiveRecord::Migration[8.0]
+  def up
+    execute <<~SQL
+      DELETE
+        ti1
+      FROM
+        tag_implications ti1
+        INNER JOIN
+          tag_implications ti2
+          ON
+            ti1.tag_id = ti2.tag_id
+            AND ti1.parent_tag_id = ti2.parent_tag_id
+            AND ti1.id > ti2.id
+      ;
+    SQL
+
+    add_index :tag_implications, [:tag_id, :parent_tag_id],
+              unique: true,
+              name: 'index_tag_implications_on_tag_id_and_parent_tag_id'
+  end
+
+  def down
+    # NOTE: 重複削除は復元されなぃ．
+    remove_index :tag_implications,
+                 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
 

frontend/src/components/TagDetailSidebar.tsx

@@ -27,7 +27,9 @@ const renderTagTree = (
       </li>)
 
   return [self,
-	  ...(tag.children?.flatMap (child => renderTagTree (child, nestLevel + 1, key)) ?? [])]
+	  ...(tag.children
+	      ?.sort ((a, b) => a.name < b.name ? -1 : 1)
+	      .flatMap (child => renderTagTree (child, nestLevel + 1, key)) ?? [])]
 }