'Assure Revision.clean() changes a bad based_on value to None when
there is no current_revision of the English document.'

def test_correct_based_on_to_none(self):

    r1 = RevisionFactory()
    r2 = RevisionFactory.build(based_on=r1)
    self.assertRaises(ValidationError, r2.clean)
    eq_(None, r2.based_on)
