add pivot table for menu

db/migrate/20170810121704_create_item_sets.rb

@@ -0,0 +1,9 @@
+class CreateItemSets < ActiveRecord::Migration[5.1]
+  def change
+  	create_table :item_sets do |t|
+      t.string :name, :null => false
+      t.integer :min_selectable_qty
+      t.integer :max_selectable_qty
+      t.timestamps
+  end
+end

db/migrate/20170811121704_create_menu_instance_item_sets.rb

@@ -0,0 +1,8 @@
+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

db/migrate/20170811121704_create_menu_item_sets.rb

@@ -0,0 +1,8 @@
+class CreateMenuItemSets < ActiveRecord::Migration[5.1]
+  def change
+  	create_table :menu_item_sets do |t|
+      t.references :item_set_id, foreign_key: true
+      t.references :menu_item_id, foreign_key: true
+      t.timestamps
+  end
+end