'filter_queryset should add value from existing instance attribute
if it is not provided in attributes dict'

def test_filter_queryset_do_not_skip_existing_attribute(self):

    class MockQueryset(object, ):
        def filter(self, **kwargs):
            self.called_with = kwargs
    data = {'race_name': 'bar'}
    queryset = MockQueryset()
    validator = UniqueTogetherValidator(queryset, fields=('race_name', 'position'))
    validator.instance = self.instance
    validator.filter_queryset(attrs=data, queryset=queryset)
    assert (queryset.called_with == {'race_name': 'bar', 'position': 1})
