mock a rollback in pylons unit testing

@fudge.patch('samples.model.meta.Session')
def test_add_user_commit_exception(FakeSession):
    FakeSession.provides('query').returns(1).provides("filter").returns_fake().\
                provides("all").returns_fake()
    FakeSession.provides('commit').raises(ValueError("Commit excepion!"))
    FakeSession.expects("rollback").returns_fake()
    try:
        user.add_user("name")
    except Exception, ex:
        pass