ActiveRecord: save_only_valid_attributes ?

Hello,

I've been going through the Rails code today, looking for a method
that will allow me to save an invalid record, but only update
attributes that do not have errors attached. (i.e.
model.errors.on(:attribute) is nil).

Any ideas on how to accomplish this would be appreciated. If this
isn't a part of Rails, I think it would be used enough to be a good
idea for a patch.

Thanks,
Brady