@n2ygk
Changing renderer always has some risks, but making it better conform to JSON api there is no way around it 😄

However good testing is always a good thing. I have therefore test run this in the projects I am using DJA and it turned out green. If you have any projects to test run it in that would be great too, to lower the barrier that there is a regression in this PR.

@sliverc Should I look at this now or wait until the test failures are resolved?

@sliverc: I'm confused as this was already working per the spec for me. I've compared

git+https://github.com/django-json-api/django-rest-framework-json-api.git@8c075d0 vs
git+https://github.com/sliverc/django-rest-framework-json-api.git@null_relationships

And both return an empty list for a to-many relationships and null for a to-one.

Is this only for HyperlinkedIdentityField? I don't use that in my code.

PS: My sample code is here: https://github.com/columbia-it/django-jsonapi-training

@n2ygk
It's just in corner cases where a relationship is not rendered but skipped. It is basically only when utils.get_relation_instance cannot resolve to a relation instance.
The only known case where utils.get_relation_instance cannot resolve is with a reverse OneToOneField leading to a DoesNotExist exception.

When thinking about it again it is properly better just to address this OneToOneField issue which is known and do not touch the other only hypothetical errors which might actually never happen.

Hence I have simplified this PR and clarified it in comment and changelog. Best have a look again.