related_descriptors.py 文件源码

def __set__(self, instance, value):
        """
        Set the related instance through the reverse relation.

        With the example above, when setting ``place.restaurant = restaurant``:

        - ``self`` is the descriptor managing the ``restaurant`` attribute
        - ``instance`` is the ``place`` instance
        - ``value`` is the ``restaurant`` instance on the right of the equal sign

        Keep in mind that ``Restaurant`` holds the foreign key to ``Place``.
        """
        # The similarity of the code below to the code in
        # ForwardManyToOneDescriptor is annoying, but there's a bunch
        # of small differences that would make a common base class convoluted.

        if value is None:
            # Update the cached related instance (if any) & clear the cache.
            try:
                rel_obj = getattr(instance, self.cache_name)
            except AttributeError:
                pass
            else:
                delattr(instance, self.cache_name)
                setattr(rel_obj, self.related.field.name, None)
        elif not isinstance(value, self.related.related_model):
            # An object must be an instance of the related class.
            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,
                )
            )
        else:
            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)
            # 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])

            # Set the related instance cache used by __get__ to avoid an SQL query
            # when accessing the attribute we just set.
            setattr(instance, self.cache_name, value)

            # Set the forward accessor cache on the related object to the current
            # instance to avoid an extra SQL query if it's accessed later on.
            setattr(value, self.related.field.get_cache_name(), instance)