This pull request introduces a failing test case to demonstrate aKeyError encountered during validation when aModelSerializer field uses thesource attribute and the corresponding model field is part of aUniqueConstraint.

When aModelSerializer field specifies asource attribute to map to a model field with a different name (e.g.,serializer_field = serializers.CharField(source="model_field_name")), andmodel_field_name is included in aUniqueConstraint, 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 bysource.

This leads to aKeyError 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:intest_unique_constraint_sourceassertserializer.is_valid()rest_framework/serializers.py:225:inis_validself._validated_data=self.run_validation(self.initial_data)rest_framework/serializers.py:446:inrun_validationself.run_validators(value)rest_framework/serializers.py:479:inrun_validatorssuper().run_validators(to_validate)rest_framework/fields.py:551:inrun_validatorsvalidator(value,self)rest_framework/validators.py:191:in__call__condition_kwargs= {source:attrs[source]forsourceinself.condition_fields}EKeyError:'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!