Fix contrib.related.ManyToManyFieldDefinition.

[charettes]

The app has multiple failures and do not support symmetric M2M. This might have something to do with deferred SQL.

[audricschiltknecht]

What would be the status of this bug? I noticed there is a m2m branch, but it is 4 years old. Is there a specific area (modules, classes, etc) to look at if people want to provide some help/fixes?

[charettes]

The code changed a lot in the past 4 years and since I've never required support for m2m I never got into fixing everything. People interested into getting this fixed should have a look at the m2m branch and the commented out tests in tests/test_contrib_related/tests.py.