Attempt to address Django 2.0 deprecate warnings related to field.rel

[jplock]

This is related to django/django#4241

rest_framework/utils/model_meta.py:83: RemovedInDjango20Warning: Usage of field.rel has been deprecated. Use field.remote_field instead.
  while pk.rel and pk.rel.parent_link:
rest_framework/utils/model_meta.py:92: RemovedInDjango20Warning: Usage of field.rel has been deprecated. Use field.remote_field instead.
  for field in [field for field in opts.fields if field.serialize and not field.rel]:
rest_framework/utils/model_meta.py:107: RemovedInDjango20Warning: Usage of field.rel has been deprecated. Use field.remote_field instead.
  for field in [field for field in opts.fields if field.serialize and field.rel]:
rest_framework/utils/model_meta.py:147: RemovedInDjango20Warning: Usage of field.rel has been deprecated. Use field.remote_field instead.
  to_many=relation.field.rel.multiple,
rest_framework/utils/model_meta.py:163: RemovedInDjango20Warning: Usage of field.rel has been deprecated. Use field.remote_field instead.
  (getattr(relation.field.rel, 'through', None) is not None) and
rest_framework/utils/model_meta.py:164: RemovedInDjango20Warning: Usage of field.rel has been deprecated. Use field.remote_field instead.
  not relation.field.rel.through._meta.auto_created

I'm not sure how best to handle this for versions prior to Django 1.9

[xordoquy]

Thanks for the PR. Unfortunately this breaks Django < 1.9 compatibility.
This can be solved with a compatibility function (compat.py) to handle the various cases.

[jplock]

@xordoquy thanks for the feedback, I added a compatibility function and the tests are now passing.

[jpadilla]

Could we instead have this set in a var after line 83? remote_field = get_remote_field(pk) and use throughout?

[tomchristie]

Looks like this just needs bringing up to date with master.
Milestoning to keep it on my radar.

[tomchristie]

Thanks for raising this @jplock. Now resolved!

[jplock]

Thanks for fixing this. Once I tried to implement @jpadilla's suggestion, I got stuck and distracted.

[tomchristie]

S'all good. We got there in the end!