Adapt ShippingMethodsController to keep disallowing delete of shipping_methods associated with orders. Added controller tests to verify behaviour

app/controllers/spree/admin/shipping_methods_controller_decorator.rb

@@ -17,14 +17,10 @@ module Spree
         collection
       end
 
-      # This method was originally written because ProductDistributions referenced shipping
-      # methods, and deleting a referenced shipping method would break all the reports that
-      # queried it.
-      # This has changed, and now all we're protecting is Orders, which is a spree resource.
-      # Do we really need to protect it ourselves? Does spree do this, or provide some means
-      # of preserving the shipping method information for past orders?
+      # Spree allows soft deletes of shipping_methods but our reports are not adapted to that.
+      # So, this method prevents the deletion (even soft) of shipping_methods that are referenced in orders.
       def do_not_destroy_referenced_shipping_methods
-        order = Order.where(:shipping_method_id => @object).first
+        order = Order.joins(shipments: :shipping_rates).where( spree_shipping_rates: { :shipping_method_id => @object } ).first
         if order
           flash[:error] = I18n.t(:shipping_method_destroy_error, number: order.number)
           redirect_to collection_url and return

spec/controllers/spree/admin/shipping_methods_controller_spec.rb

@@ -0,0 +1,31 @@
+require 'spec_helper'
+
+describe Spree::Admin::ShippingMethodsController, type: :controller do
+  include AuthenticationWorkflow
+
+  describe "shipping method not referenced by order" do
+    let(:shipping_method) { create(:shipping_method) }
+
+    scenario "is soft deleted" do
+      login_as_admin
+      expect(shipping_method.deleted_at).to be_nil
+
+      spree_delete :destroy, {"id" => shipping_method.id}
+
+      expect(shipping_method.reload.deleted_at).not_to be_nil
+    end
+  end
+
+  describe "shipping method referenced by order" do
+    let(:order) { create(:order_with_line_items) }
+
+    scenario "is not soft deleted" do
+      login_as_admin
+      expect(order.shipping_method.deleted_at).to be_nil
+
+      spree_delete :destroy, {"id" => order.shipping_method.id}
+
+      expect(order.shipping_method.reload.deleted_at).to be_nil
+    end
+  end
+end