How do I recalculate a field if another field was changed in a form

And where do I put that code? In the model or the controller? I was thinking in the controller update function, is that the right place?

Also, how do I know the old value of a field? The $request variable only contains the new value.

This is for creating a new password when the username is changed.

(Yes I know passwords should not be kept in cleartext in a database, but this is for provisioned devices, so there's no other way)

via Chebli Mohamed