Fix UniqueTogetherValidator to handle fields w/ source attr

[joshuadavidthomas]

This pull request introduces a failing test case to demonstrate a KeyError encountered during validation when a ModelSerializer field uses the source attribute and the corresponding model field is part of a UniqueConstraint.

When a ModelSerializer field specifies a source attribute to map to a model field with a different name (e.g., serializer_field = serializers.CharField(source="model_field_name")), and model_field_name is included in a UniqueConstraint, the validation process incorrectly attempts to access the validated data using the serializer field name (serializer_field) instead of the model field name (model_field_name) specified by source.

This leads to a KeyError within the validator, as the serializer field name is not present in the dictionary being checked at that stage. The relevant part of the traceback is:

tests/test_validators.py:702: in test_unique_constraint_source
    assert serializer.is_valid()
rest_framework/serializers.py:225: in is_valid
    self._validated_data = self.run_validation(self.initial_data)
rest_framework/serializers.py:446: in run_validation
    self.run_validators(value)
rest_framework/serializers.py:479: in run_validators
    super().run_validators(to_validate)
rest_framework/fields.py:551: in run_validators
    validator(value, self)
rest_framework/validators.py:191: in __call__
    condition_kwargs = {source: attrs[source] for source in self.condition_fields}
E   KeyError: 'raceName'

Note: I just added the test case at the very end of the class purely so I could quickly get a reproduction of this issue. Open to suggestions regarding its placement or structure if a different organization is preferred!

[joshuadavidthomas]

Alright, I have a fix in for the test. I tried to keep the fix localized and only change what is needed to fix the test, so if I need to restructure or extract some of the logic out for clarity, just let me know!

[Copilot]

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (1)

rest_framework/validators.py:191

• The updated mapping now relies on serializer.fields[field_name].source being present in attrs. Consider ensuring that this key exists or handling the potential absence to avoid another KeyError.

condition_kwargs = {

[browniebroke]

Nitpick: I find this repeated serializer.fields[field_name].source difficult to read, especially the second part attrs[serializer.fields[field_name].source].

What do people think of splitting this statement over 2 lines?

condition_sources = (serializer.fields[field_name].source for field_name in self.condition_fields)
condition_kwargs = {source: attrs[source] for source in condition_sources}