Fix nested serializer required=False multipart data validation bug

[juannyG]

After reviewing #2796, #2719, and #2919, I thought it might not hurt to propose a fix for the simpler use case where mulitpart data for nested serializers with required=False fails validation when the field is not present.

I agree that great care is needed regarding the design decisions still needed on how the data in a serializer's "validated_data" property should be constructed (and that it should be consistent and predictable), but the fact still remains that currently, you cannot use a View with a serializer that defines a nested serializer with required=False.

I'm hoping this fix can make it into the core, so the format/structure/construction of a serializer's "validated_data" properly can be handled as separate issue.

[xordoquy]

Thanks for the PR.
Why do you define FakeMultiDict ? I mean, Is there a reason Django's QueryDict can't be used here ?

[juannyG]

Hey @xordoquy - thanks for the feedback!

The short answer to your question - nope! The QueryDict satisfies what I'm trying to do in the FakeMultiDict definition. I've updated the PR to use QueryDict.

The longer answer is I was piggy-backing off of #2796 - but simply in order to show that these two PRs are related, but not duplicates - I'm trying to solve for a smaller use case. That said, based on your comment, it seems to me that #2796 could ALSO use QueryDict, but we can tackle that separately :-)

[xordoquy]

@juannyG thanks, comment added in #2796 ;)

[xordoquy]

Ok, looks good to me, thanks for the quick fixes !