__init__.py 文件源码

def _cmp(cls, value1, value2):
        """
        Comparison method that takes into account Django's special rules when
        ordering by a field that is a model:

            1. Try following the default ordering on the related model.
            2. Order by the model's primary key, if there is no Meta.ordering.

        """
        if isinstance(value1, Model) and isinstance(value2, Model):
            field_names = value1._meta.ordering

            # Assert that the ordering is the same between different models.
            if field_names != value2._meta.ordering:
                valid_field_names = (set(cls._get_field_names(value1)) &
                                     set(cls._get_field_names(value2)))
                raise FieldError(
                    "Ordering differs between models. Choices are: %s" %
                    ', '.join(valid_field_names))

            # By default, order by the pk.
            if not field_names:
                field_names = ['pk']

            # TODO Figure out if we don't need to generate this comparator every
            # time.
            return cls._generate_comparator(field_names)(value1, value2)

        return cmp(value1, value2)