[Django]-Django REST Framework: Unique_together validation on Serializers

The ModelSerializer class has this functionality build-in, at least in djangorestframework>=3.0.0, however if you are using a serializer which doesn’t include all of the fields which are affected by your unique_togetherconstrain, then you’ll get an IntegrityError when saving an instance which violates it. For example, using the following model:

class Foo(models.Model):
    class Meta:
        unique_together = ('foo_a', 'foo_b')

    a = models.TextField(blank=True)
    b = models.TextField(blank=True)
    foo_a = models.IntegerField()
    foo_b = models.IntegerField(default=2)

and the following serializer and ViewSet:

class FooSerializer(serializers.ModelSerializer):
    class Meta:
        model = models.Foo
        fields = ('a', 'b', 'foo_a')

class FooViewSet(viewsets.ModelViewSet):
    queryset = models.Foo.objects.all()
    serializer_class = FooSerializer


routes = routers.DefaultRouter()
routes.register(r'foo', FooViewSet)

if you try to save two instances with the same foo_a and foo_b set, you’ll get an IntegrityError. However, if we modify the serializer like this:

class FooSerializer(serializers.ModelSerializer):
    class Meta:
        model = models.Foo
        fields = ('a', 'b', 'foo_a', 'foo_b')

you’ll then get a proper HTTP 400 BAD REQUEST status code, and the corresponding JSON descriptive message in the response body:

HTTP 400 BAD REQUEST
Content-Type: application/json
Vary: Accept
Allow: GET, POST, HEAD, OPTIONS

{
    "non_field_errors": [
        "The fields foo_a, foo_b must make a unique set."
    ]
}

I hope this to result helpful for you, even when this is a somewhat old-posted question 😉

I Needed this to override default message. Solved it by this.

from django.utils.translation import ugettext_lazy as _
from rest_framework import serializers, validators


class SomeSerializer(serializers.ModelSerializer):
  """
  Demostrating How to Override DRF UniqueTogetherValidator Message
  """

    class Meta:
        model = Some
        validators = [
            validators.UniqueTogetherValidator(
                queryset=model.objects.all(),
                fields=('field1', 'field2'),
                message=_("Some custom message.")
            )
        ]

Similarly, you can specify fields.

Yes, you can do it in the .validate() method of the serializer.

def validate(self, attrs):
    try:
        Model.objects.get(field1=attrs['field1'], field2=attrs['field2'])
    except Model.DoesNotExist:
        pass
    else:
        raise serializers.ValidationError('field1 with field2 already exists')

    return attrs

The unique constraint that you set in your model is for creating database constraints, not for validating.

Had the same problem and from this answer https://stackoverflow.com/a/26027788/6473175 I was able to get it to work but had to use self.instance instead of self.object.

def validate(self, data):
    field1 = data.get('field1',None)
    field2 = data.get('field2',None)

    try:
        obj = self.Meta.model.objects.get(field1=field1, field2=field2)
    except self.Meta.model.DoesNotExist:
        return data
    if self.instance and obj.id == self.instance.id:
        return data
    else:
        raise serializers.ValidationError('custom error message')

Well this is kinda stupid and it’s very unlikely that anyone but me would make this mistake, but I had put the same model in two serializers class, as a consequence I had this problem

Hope my mistake help someone!