[Rails] 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.

https://gist.github.com/847748

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