'Ensure that db_field doesn\'t accept invalid values.'

def test_db_field_validation(self):

    with self.assertRaises(ValueError):
        class User(Document, ):
            name = StringField(db_field='user.name')
    with self.assertRaises(ValueError):
        class User(Document, ):
            name = StringField(db_field='$name')
    with self.assertRaises(ValueError):
        class User(Document, ):
            name = StringField(db_field='name\x00')
