Fixes unexpected deletion of related objects.

[gibsonbailey]

When updating an instance with foreign key relations, the objects linking to the instance should not be deleted. Instead, the foreign key should simply be removed from the objects to the instance. This commit checks if the related field is a foreign key, and, if so, it unlinks the object. If the field cannot be null, the object still gets deleted, as it does in the current state of the master branch.

[codecov-io]

Codecov Report

Merging #97 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
+ Coverage   98.59%   98.61%   +0.01%     
==========================================
  Files           3        3              
  Lines         213      216       +3     
==========================================
+ Hits          210      213       +3     
  Misses          3        3

Impacted Files	Coverage Δ
drf_writable_nested/mixins.py	98.52% <100%> (+0.02%)	⬆️

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[ruscoder]

@gibsonbailey Thanks for the contribution. Could you please add a failing test which is fixed with your changes?

[gibsonbailey]

Yes, thanks for the consideration.

[gibsonbailey]

@ruscoder Test has been added. Thanks Vadim!

[ruscoder]

@gibsonbailey Thanks for the tests.
I'm not sure that this logic should work in this way.
Let's consider how Django works with null=True and on_delete=CASCADE.
https://docs.djangoproject.com/en/2.2/ref/models/fields/#django.db.models.ForeignKey.on_delete

With CASCADE, Django deletes the instance if the related instance is deleted. To avoid this you should pass SET_NULL/SET_DEFAULT.

So, I think the logic should be changed to use on_delete param instead of null

[ruscoder]

My comment is still actual. So I don't close it and don't accept for now.