add item set and relation

spec/models/item_set_spec.rb

@@ -0,0 +1,5 @@
+require 'rails_helper'
+
+RSpec.describe ItemSet, type: :model do
+  pending "add some examples to (or delete) #{__FILE__}"
+end

spec/models/menu_instance_item_set_spec.rb

@@ -0,0 +1,5 @@
+require 'rails_helper'
+
+RSpec.describe MenuInstanceItemSet, type: :model do
+  pending "add some examples to (or delete) #{__FILE__}"
+end

spec/models/menu_item_set_spec.rb

@@ -0,0 +1,5 @@
+require 'rails_helper'
+
+RSpec.describe MenuItemSet, type: :model do
+  pending "add some examples to (or delete) #{__FILE__}"
+end