Serializer validate function is not called DRF

Question

class ChildSerializer(serializers.ModelSerializer):


    class Meta:
         model = Child
         fields = '__all__'


class ParentSerializer(serializers.ModelSerializer):
    """
    Serializer for task
    """
    def validate_title(self, data):
        if not data.get('title'):
            raise serializers.ValidationError('Please set title')
        return data

Validate Function is not called when Post ,Also how can i give custom errors to ChildSerializer ,

Answer 1

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.)

Answer 2

Your ParentSerializer validation method has some issues. Assumes that there is a title field in your ParentSerializer model. For field level validation, you will get the field instead of whole data. That is validate_title function should have title(title field of the data) as parameter not data. So you dont have to check data.get('title') for the existance of title. Reference

class ParentSerializer(serializers.ModelSerializer):
    """
    Serializer for task
    """
    def validate_title(self, title):
        if not title:
            raise serializers.ValidationError('Please set title')
        return title