Add tests for M2M relations with inheritance and a through model

[zorun]

This particular setup worked fine up to DRF 3.2.5, but is broken in DRF
3.3.0. Bisecting indicates that 322bda8
first introduced the regression.

This commit adds a few testcases that pass with DRF 3.2.5 but fail with
DRF 3.3.0 and newer.

[zorun]

Note that the CI checks have failed for this PR. This is perfectly normal, since the new tests exercise a regression :)

[tomchristie]

Thanks!

[tomchristie]

Could you also outline the simplest possible example case, what you would expect the behavior to be, and what the behavior currently is?

[zorun]

The simplest possible example case is unfortunately not so simple, it's the one in the new tests introduced by this PR. Basically:

• start with a model A
• add a model B that inherits from model A
• model B has a M2M relationship with model A, using a "through" model C
• model C has two foreign keys, one to model A, one to model B, as described in https://docs.djangoproject.com/en/1.9/topics/db/models/#extra-fields-on-many-to-many-relationships

The expected behaviour is that DRF is able to serialize the objects (for all three models). The current behaviour (starting with 322bda8) is that DRF throws an exception when serializing:

django.core.exceptions.FieldDoesNotExist: ManyToManyThrough has no field named 'manytomanythroughtarget_ptr'

ManyToManyThrough in the test corresponds to model C in the description above. It seems that the reverse related name between model C and model A is not correctly detected by DRF (but I don't know enough about the internals of DRF to debug this).

See https://travis-ci.org/tomchristie/django-rest-framework/jobs/108339903 for an example of failure.

[tomchristie]

Blimey - that's a pretty obscure use-case corner you've found there.

[zorun]

Bump? I still get the same error with DRF 3.3.3 and Django 1.9.7.

As a reminder, I had bisected the issue to this commit: 322bda8

[tomchristie]

Still not high enough priority just now, given edge-case-ness. Focus is on 3.4.0 first.
Proposed solutions / pull requests most welcome (might help raise the priority as further work on it then has better effort/reward ratio), but we've 179 open issues and pull requests, so there's plenty in the queue.

[carltongibson]

I'm going to de-milestone for now. We can reassess after v3.7

[auvipy]

It would be great to know, if this change is still needed with django 2.0+ versions and latest drf versions? hope you would take the time to consider verifying the validity of proposed change with newer releases.

[tomchristie]

I'm just going to close this off as stale at this point - can reassess if it gets bumped sometime.