add item set and relation

db/migrate/20170811071327_create_menu_instance_item_sets.rb

@@ -0,0 +1,10 @@
+class CreateMenuInstanceItemSets < ActiveRecord::Migration[5.1]
+  def change
+    create_table :menu_instance_item_sets do |t|
+      t.references :item_set_id, foreign_key: true
+      t.references :menu_item_instance_id, foreign_key: true
+      
+      t.timestamps
+    end
+  end
+end