related.py 文件源码

def __set__(self, instance, value):
        # The similarity of the code below to the code in
        # ReverseSingleRelatedObjectDescriptor is annoying, but there's a bunch
        # of small differences that would make a common base class convoluted.

        # If null=True, we can assign null here, but otherwise the value needs
        # to be an instance of the related class.
        if value is None and self.related.field.null is False:
            raise ValueError(
                'Cannot assign None: "%s.%s" does not allow null values.' % (
                    instance._meta.object_name,
                    self.related.get_accessor_name(),
                )
            )
        elif value is not None and not isinstance(value, self.related.related_model):
            raise ValueError(
                'Cannot assign "%r": "%s.%s" must be a "%s" instance.' % (
                    value,
                    instance._meta.object_name,
                    self.related.get_accessor_name(),
                    self.related.related_model._meta.object_name,
                )
            )
        elif value is not None:
            if instance._state.db is None:
                instance._state.db = router.db_for_write(instance.__class__, instance=value)
            elif value._state.db is None:
                value._state.db = router.db_for_write(value.__class__, instance=instance)
            elif value._state.db is not None and instance._state.db is not None:
                if not router.allow_relation(value, instance):
                    raise ValueError('Cannot assign "%r": the current database router prevents this relation.' % value)

        related_pk = tuple(getattr(instance, field.attname) for field in self.related.field.foreign_related_fields)
        if not self.related.field.allow_unsaved_instance_assignment and None in related_pk:
            raise ValueError(
                'Cannot assign "%r": "%s" instance isn\'t saved in the database.' %
                (value, instance._meta.object_name)
            )

        # Set the value of the related field to the value of the related object's related field
        for index, field in enumerate(self.related.field.local_related_fields):
            setattr(value, field.attname, related_pk[index])

        # Since we already know what the related object is, seed the related
        # object caches now, too. This avoids another db hit if you get the
        # object you just set.
        setattr(instance, self.cache_name, value)
        setattr(value, self.related.field.get_cache_name(), instance)