related.py 文件源码

def __init__(self, to, to_field=None, rel_class=ManyToOneRel,
                 db_constraint=True, **kwargs):
        try:
            to._meta.model_name
        except AttributeError:  # to._meta doesn't exist, so it must be RECURSIVE_RELATIONSHIP_CONSTANT
            assert isinstance(to, six.string_types), (
                "%s(%r) is invalid. First parameter to ForeignKey must be "
                "either a model, a model name, or the string %r" % (
                    self.__class__.__name__, to,
                    RECURSIVE_RELATIONSHIP_CONSTANT,
                )
            )
        else:
            # For backwards compatibility purposes, we need to *try* and set
            # the to_field during FK construction. It won't be guaranteed to
            # be correct until contribute_to_class is called. Refs #12190.
            to_field = to_field or (to._meta.pk and to._meta.pk.name)

        if 'db_index' not in kwargs:
            kwargs['db_index'] = True

        self.db_constraint = db_constraint

        kwargs['rel'] = rel_class(
            self, to, to_field,
            related_name=kwargs.pop('related_name', None),
            related_query_name=kwargs.pop('related_query_name', None),
            limit_choices_to=kwargs.pop('limit_choices_to', None),
            parent_link=kwargs.pop('parent_link', False),
            on_delete=kwargs.pop('on_delete', CASCADE),
        )
        super(ForeignKey, self).__init__(to, ['self'], [to_field], **kwargs)