Add missing specs for shipping methods decorator

spec/controllers/spree/admin/shipping_methods_controller_decorator_spec.rb

@@ -0,0 +1,37 @@
+require 'spec_helper'
+
+describe Spree::Admin::ShippingMethodsController do
+  include AuthenticationWorkflow
+
+  describe '#destroy' do
+    let!(:shipping_method) { create(:shipping_method) }
+
+    before { login_as_admin }
+
+    context 'when the shipping method is in use' do
+      before { create(:order, shipping_method: shipping_method) }
+
+      it 'does not allow to destroy the shipping method' do
+        expect { delete :destroy, id: shipping_method.id }
+          .not_to change(Spree::ShippingMethod, :count)
+      end
+
+      it 'shows a flash error message' do
+        spree_delete :destroy, id: shipping_method.id
+        expect(flash[:error]).to match('That shipping method cannot be deleted')
+      end
+
+      it 'redirects to the collection url' do
+        expect(spree_delete(:destroy, id: shipping_method.id))
+          .to redirect_to('/admin/shipping_methods')
+      end
+    end
+
+    context 'when the shipping method is not in use' do
+      it 'allows to destroy the shipping method' do
+        expect { delete :destroy, id: shipping_method.id }
+          .to change(Spree::ShippingMethod, :count)
+      end
+    end
+  end
+end