Fix UniqueTogetherValidator with field sources

[rpkilby]

This is an updated version of #7005 that I was helping @anveshagarwal put together. In short, there are two related bugs:

• As pointed out by @anveshagarwal in Fix read_only + default unique_together validation. #5922 (comment), _read_only_defaults does not correctly account for the field source. The initial check does account for compatible sources, but when assigning the read only default, the field_name is used instead of source.
• Generally, UniqueTogetherValidator doesn't handle field sources. Handling this is a little annoying, because the objective is to validate the serializer fields/data. However, the given attrs use the source/model field names, which are used in the actual validation check.

As to potential breaking changes, we may want to call out that validation against read-only fields that have defaults now obey their sources? ¯\_(ツ)_/¯

Fixes #7003

[carltongibson]

Hey @rpkilby. Thanks for this. Tricky stuff. 🌶 Let me give it a look over. (But first glance it seems clean.)

[carltongibson]

TBH, this looks great. I was worried it was going to be much dirtier than it is. That source just is the field name in the usual case is nice.

As to potential breaking changes, we may want to call out that validation against read-only fields that have defaults now obey their sources? ¯_(ツ)_/¯

Meh, this is clearly a bug. You can't be depending on the broken behaviour here. 😝

That the existing tests here pass is 👍 for me — We did exercise the difficult cases here (when we changed the old behaviour in 3.8(?)

[rpkilby]

I was worried it was going to be much dirtier than it is.

The initial implementation definitely was. At first, I didn't fully understand that the attrs was a map of {field source: value}, so there were a lot of messy code trying to work around handling field name vs source.

[rpkilby]

Expanded the writable field test to include validation for a missing value. This shouldn't extend to read-only fields, since their defaults are always provided.

[ihorm5]

I have encountered this issue when I was searching for a solution for exact same problem. It looks that this solution only works if you provide your own UniqueTogetherValidation in the serializer's meta validators field. While this is OK, without setting

        validators = [
            UniqueTogetherValidator(
                queryset=Model1.objects.all(),
                fields=['f1', 'f2']
            )
        ]

The code will raise KeyError. Is this the desired behavior?

[rpkilby]

Hi @fatalispm. I think you're running into #7143, which hasn't been released yet.