How do you display validation errors when using Association Callbacks?

I’m trying to get a validation error to appear on my form. I’m adding the error to my model inside of a method called from a ‘before_add’ association callback (see prevent_user_from_having_more_than_one). From what I understand about Association Callbacks, you raise an exception so that you trigger a database rollback so the invalid data is reversed out of the database. And that works for me…it will not let me associate more than one group to a user with a role of ‘user’.

The problem is that I can’t get that error to show up in the view (on the form) even though I explicitly set the error on the model in that :before_add method. So what am I doing wrong? Thanks for any help.