I ran into a similar problem where my custom validation field was not being called. I was writing it to bypass an incorrect DRF validation (more details shown below, but not necessary for the answer).
Looking into the DRF source code I found my problem: DRF always validates your field using its code before validating with your custom code.
''' rest-framework/serializers.py '''
for field in fields:
validate_method = getattr(self, 'validate_' + field.field_name, None)
primitive_value = field.get_value(data)
try:
# DRF validation always runs first!
# If DRF validator throws, then custom validation is not called
validated_value = field.run_validation(primitive_value)
if validate_method is not None:
# this is your custom validation
validated_value = validate_method(validated_value)
except ValidationError as exc:
errors[field.field_name] = exc.detail
except DjangoValidationError as exc:
errors[field.field_name] = get_error_detail(exc)
Answer: Custom validators cannot be used to bypass DRF's validators, as they will always run first and will raise an exception before you can say it is valid.
(for those interested, the validation error I hit was like this: ModelSerializer used for ModelA, which has a OneToOne relation to ModelB. ModelB has a UUID for its pk. DRF throws the error '53abb068-0286-411e-8729-0174635c5d81' is not a valid UUID.
when validating, which is incorrect, and really infuriating.)
validate_title()
won't invokes unless you callserializer.is_valid()