Fix read_only + default unique_together validation.

[carltongibson]

In order to validate a unique_together relation on a read_only field, you need to specify a default on the field to be used for validation purposes.

e.g.:

    owner = serializers.PrimaryKeyRelatedField(
        read_only=True,
        default=serializers.CreateOnlyDefault(serializers.CurrentUserDefault()),
    )

Where owner is part of a unique_together relation on the model.

#5886 unwittingly broke this behaviour:

• The UniqueTogetherValidator was not added to the serialiser.
• Nor was the default value used in validation.

This PR:

• Adds a test case covering the expected behaviour. (That passes on 3.7.7 but fails on 3.8.0)
• Ensures the UniqueTogetherValidator is correctly generated
• Uses the default value for read_only fields (where specified) only for validation, in line with the changes for Alter read_only+default behaviour #5886.

[carltongibson]

@akx Any chance you could install this branch and see if your problem is resolved? Thanks.

@xordoquy @rpkilby: If you have capacity your eye over this would be helpful. Thanks.

[akx]

@carltongibson Yup, seems to work. 🏆

[tomchristie]

Yup, looks pretty reasonable to me.

[carltongibson]

v3.8.2 is now available on PyPI. Thanks all.

[anveshagarwal]

if we are using a field with field_name different from the field in model that is used as source then this is failing and it should be

defaults[field.source] = default

[rpkilby]

I somehow missed your comment on this, but you're correct. The code above specifically checks for a compatible field source, but uses the serializer field_name instead of the source model field.

[anveshagarwal]

i had reported this and tried to work on it, but couldnt continue
and along with this issue other issue is there defined in the Issue below
#7005
#7003

[rpkilby]

Hi @anveshagarwal. I'm currently following up on that PR. That's what prompted my comment. 😄

[rpkilby]

For posterity's sake, my reasoning above was slightly wrong. The source checks are an indicator that we should be operating on field.source instead of field.field_name, however the way to determine this is by looking at run_validation. Before it calls run_validators (then _read_only_defaults), it calls to_internal_value. At this point, the data has been converted to its source structure. So, it makes sense that the read-only defaults would also map to its source structure, instead of the raw field names.