I was banging my head against the wall today trying to add a before_save filter to a model that was getting saved in a nested form. It seems that only the top level model’s callbacks will be called.
So say you have save some kind of Order with many LineItems all at once (via the Order form), only Order callbacks will get called. If you need to manipulate LineItems in a callback, then add a callback to Order and loop through your LineItems.
Doesn’t seem intuitive to me, but there may be a reason for doing it that way.