Change in `get_all_related_objects` behavior.

[Tom Christie]

In Django 1.8 alpha, the behavior of the internal get_all_related_objects() changes.

If I have a model with a reverse foreign key relationship then...

• In 1.7 get_all_related_objects returns the relationship as django.db.models.related.RelatedObject with the model attribute set to the source of the relationship.
• In 1.8-alpha get_all_related_objects returns the relationship as django.db.models.fields.related.ManyToOneRel with the model attribute set to the target of the relationship.

Effect is that for REST framework ModelSerializers any automatic reverse relationships will be incorrectly generated.

This is private API, but it's also currently API that third party apps do need to rely on in order to be able to function and correctly introspect the model relationships.

I'd also be okay with a resolution where this *isn't* fixed per-se, but that it is at least demonstrated that there's now an alternative route that I can use to correctly determine the reverse relationships.

[Anssi Kääriäinen]

The likely reason for this regression is that field.rel.model was defined differently from field.related.model. After removal of RelatedObject field.related is just a property that returns field.rel. Also, get_all_related_objects() used to return field.related, but now it likely returns field.rel instances.

I see a couple of approaches to solve this issue:

1. Create a proxy object for field.related (or do this only for the return value of get_all_related_objects()). This object should then return the correct model as RelatedObject used to do.
2. Document a way to conditionally access a different property from the field.rel ManyToOneRel instance which returns the correct model instance.

I think approach 1. is preferable. It should be easy to do so, and the field.related API is deprecated, so we get to remove the proxy object in a couple of releases.

[Tom Christie]

Workaround - .related_model will return the source model, so...

    for relation in opts.get_all_related_objects():
        to_model = getattr(relation, 'related_model', relation.model)

Appears to work consistently across Django versions.

[Collin Anderson]

Yes, this was a backwards-incompatible decision we made, but it needs to be documented.

I think we should document the getattr() approach. I've ran into this issue with contribute_to_related_class and used the getattr() approach.

[Tim Graham]

​PR

[Tim Graham <timograham@…>]

In 888054bff7c5878e026bac2e712d5480f7e295c7:

Fixed #24208 -- Documented changes in private model relations.

[Tim Graham <timograham@…>]

In 28bb0ad19947514cd275e5692a5424e45919b5be:

[1.8.x] Fixed #24208 -- Documented changes in private model relations.

Backport of 888054bff7c5878e026bac2e712d5480f7e295c7 from master