unit testing.. assert model.errors.invalid?(:field)

seems like the above line always evals to false.. if the field is
required and missing, it evals to false. if the field is required and
present, it evals to false. if the field is not required and present
(or not present) the line evals to false..

am i using this right? in AWDwR 2nd Ed thevery first example makes
heavy use of this style of test..

thanks in advance..
stuart