FEATURE: ability to restrict tags to categories using groups

db/migrate/20160606204319_create_category_tag_groups.rb

@@ -0,0 +1,11 @@
+class CreateCategoryTagGroups < ActiveRecord::Migration
+  def change
+    create_table :category_tag_groups do |t|
+      t.references :category,  null: false
+      t.references :tag_group, null: false
+      t.timestamps
+    end
+
+    add_index :category_tag_groups, [:category_id, :tag_group_id], name: "idx_category_tag_groups_ix1", unique: true
+  end
+end