FEATURE: restrict tags to be used in a category

db/migrate/20160527191614_create_category_tags.rb

@@ -0,0 +1,12 @@
+class CreateCategoryTags < ActiveRecord::Migration
+  def change
+    create_table :category_tags do |t|
+      t.references :category, null: false
+      t.references :tag,      null: false
+      t.timestamps
+    end
+
+    add_index :category_tags, [:category_id, :tag_id], name: "idx_category_tags_ix1", unique: true
+    add_index :category_tags, [:tag_id, :category_id], name: "idx_category_tags_ix2", unique: true
+  end
+end