Remove null assignment check for non-nullable foreign key fields

[Tim Graham]

As raised in #25349 and ​discussed on django-developers, the following behavior should be removed:

>>> obj.fk = None
ValueError('Cannot assign None: "Obj.fk" does not allow null values.)

(i.e. no more exception if you assign None to a non-nullable foreign key). It applies to ForwardManyToOneDescriptor and ReverseOneToOneDescriptor. The change should be documented in the "backwards-incompatible changes" section of the release notes.

[Tim Graham]

A partial implementation of this is ​on a pull request for #25349.

After this ticket is completed, we'll rebase that pull request and proceed with that fix.

[Zach Liu]

I have made the changes and created the pull request ticket_26179

I removed both value=None check in ForwardManyToOneDescriptor and ReverseOneToOneDescriptor.

Because of that, several tests failed because they have assertRaise for ValueError. So I removed those tests.

This is my first time working on Django. So please let me know if there's anything I can do better:)

Thanks.

[Tim Graham]

​PR (please add a link like this in the future)

You should also check "Has patch" on the ticket so it appears in the review queue. Since I left some comments for improvement on the pull request, I'll mark "Patch needs improvement." Please uncheck it after you update the pull request. Thanks!

[Zach Liu]

Thanks for the help. I have updated the pull request (tests for None allowed) and also the backwards-incompatible changes in 1.10.txt.

will remember to update the ticket going forward.

[Tim Graham <timograham@…>]

In 04e13c8:

Fixed #26179 -- Removed null assignment check for non-nullable foreign key fields.