Touch ShippingMethod when it's assigned to a new distributor

app/models/distributor_shipping_method.rb

@@ -1,5 +1,5 @@
 class DistributorShippingMethod < ActiveRecord::Base
   self.table_name = "distributors_shipping_methods"
-  belongs_to :shipping_method, class_name: Spree::ShippingMethod
+  belongs_to :shipping_method, class_name: Spree::ShippingMethod, touch: true
   belongs_to :distributor, class_name: Enterprise, touch: true
 end

spec/models/spree/shipping_method_spec.rb

@@ -109,5 +109,15 @@ module Spree
         expect(shipping_method.include?(address)).to be true
       end
     end
+
+    describe "touches" do
+      let!(:distributor) { create(:distributor_enterprise) }
+      let!(:shipping_method) { create(:shipping_method) }
+      let(:add_distributor) { shipping_method.distributors << distributor }
+
+      it "is touched when applied to a distributor" do
+        expect{ add_distributor }.to change { shipping_method.reload.updated_at}
+      end
+    end
   end
 end