Fix model fields not being omitted on output/serialization

[rpatterson]

Description

The documentation says about a field's required argument:

Setting this to False also allows the object attribute or dictionary key to be omitted from output when serializing the instance.

But this isn't the case for Django ORM model instances because they always return at least the field default so there's never an AttributeError. This means that DRF's logic for omitting fields is never triggered and all model fields are always present in the JSON representation regardless of the field's required argument.

This PR fixes that by conditionally checking against the model field's default when the instance is a Django ORM model instance only when DRF's omit logic would take effect and forcing an AttributeError in those cases. There's full test coverage for all the code this PR touches and there are no tox errors that I didn't also see when runagainst encode/master.

http://www.django-rest-framework.org/api-guide/fields/#required

[lovelydinosaur]

"May be omitted" isn't the same as "Should be omitted".

I'm not convinced we want this behavioral change, as is.
The exclusion of "pk: None" in some of those examples isn't necessarily what our users would expect.

[rpatterson]

Ok, how about if it were triggered (via exc_on_model_default) only if a serializer Meta flag, whose default is set by a setting, were set to True?

[rpkilby]

@rpatterson - have you tested your project against the current master branch? #5375 made changes to the behavior of nested attribute access.

[rpatterson]

Just merged master and tested, passes as before. Just pushed the merge.

…

On Wed, Oct 4, 2017 at 9:00 AM Ryan P Kilby ***@***.***> wrote: @rpatterson <https://github.com/rpatterson> - have you tested your project against the current master branch? #5375 <#5375> made changes to the behavior of nested attribute access. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#5473 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AANmAW2SpETZsnvSi4MwPGRf0JZA39g3ks5so6uRgaJpZM4PrJl-> .

[lovelydinosaur]

only if a serializer Meta flag, whose default is set by a setting, were set to True?

It’s not really complexity I’d like to see us us add, no.