UniqueConstraint violation_error_message as error response in drf

[AGM-90]

Example Model
class UniqueTest(BaseModel):
answer = models.ForeignKey(Answer, on_delete=models.CASCADE)
subject = models.ForeignKey(Subject, on_delete=models.CASCADE)
date = models.DateField()

class Meta:
    constraints = [
        models.UniqueConstraint(
            fields=["answer", "subject","date"],
            condition=Q(deleted_at=None),
            name="uniq_if_not_del_pjt_site_sub_mil",
            violation_error_message="test error message"
        )
    ]

Sample error response for violation
{
"non_field_errors": [
"The fields answer, subject, date must make a unique set."
]
}
Expected error response
{
"non_field_errors": [
"test error message"
]
}

[s-aleshin]

Yes, I encountered this issue as well. When the serializer gathers constraints from the model, it doesn’t account for the fact that a constraint might include a custom error message. Additionally, the UniqueTogetherValidator is created with a default message. https://github.com/encode/django-rest-framework/blob/master/rest_framework/serializers.py#L1595-L1621

[igdimer]

Hi! I face to the same issue.

[soldatov-ss]

guys, any news? short workarounds?

[s-aleshin]

I created a custom validator by subclassing UniqueTogetherValidator and overriding its __call__ method. This allows me to catch the default error and raise my own ValidationError with a custom message and error code.

class CustomUniqueTogetherValidator(UniqueTogetherValidator):
    def __call__(self, attrs, serializer):
        self.enforce_required_fields(attrs, serializer)
        try:
            super().__call__(attrs, serializer)
        except serializers.ValidationError as exc:
            [msg] = exc.detail
            raise serializers.ValidationError(
                str(msg), code="custom_unique_constraint"
            ) from exc


class MySerializer(serializers.ModelSerializer):
    class Meta:
        model = MyModel
        fields = "__all__"
        validators = [
            CustomUniqueTogetherValidator(
                queryset=MyModel.objects.filter(...).all(),
                fields=["field1", "field2"],
                message="My custom message."
            ),
        ]

⚠️ When you override the validators attribute in your serializer’s Meta, it replaces all automatically generated validators based on model constraints (https://github.com/encode/django-rest-framework/blob/master/rest_framework/serializers.py#L1561-L1563). DRF will no longer auto-generate them — so you must explicitly define any additional validators you need (e.g., for other UniqueConstraints or CheckConstraints).

If an issue is opened based on this discussion, I’d be happy to submit a PR with a fix.

[efefre]

I have a very similar situation to the one described, with the difference that I have a UniqueConstraint with a single field in fields and a condition. In this case, unfortunately, violation_error_message is ignored.

Unfortunately, in #9766, this case was not taken into account.