Django Rest Framework unique together validation with field absent from request - django

Question

I'm implementing some voting functionality in an application, where a logged-in user specifies a post that they would like to vote for using a payload like this:
{
"post": 1,
"value": 1
}
As you can tell, the a user field is absent - this is because it gets set in my viewset's perform_create method. I've done this to ensure the vote's user gets set server side. This is what the viewset looks like:
class CreateVoteView(generics.CreateAPIView):
permission_classes = (permissions.IsAuthenticated,)
serializer_class = VoteSerializer
def perform_create(self, serializer):
serializer.save(user=self.request.user)
Here is what the model looks like:
class Vote(models.Model):
post = models.ForeignKey(Post, on_delete=models.CASCADE, related_name='votes', null=False)
user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, related_name='votes', null=False)
class Values(models.IntegerChoices):
UP = 1, _('Up')
DOWN = -1, _('Down')
value = models.IntegerField(choices=Values.choices, null=False)
class Meta:
unique_together = ('post', 'user')
and finally, the serializer:
class VoteSerializer(serializers.ModelSerializer):
class Meta:
model = Vote
fields = ['post', 'value']
From what I understand, in order for DRF to enforce a unique together validation, both fields (in my case, user and post) must be included in the serializer's fields. As I've mentioned, I'd like to avoid this. Is there any other way of implementing this type of validation logic?
EDIT:
To clarify: the records do not save - I receive this error:
django.db.utils.IntegrityError: (1062, "Duplicate entry '1-3' for key 'api_vote.api_vote_post_id_user_id_73614533_uniq'")
However, my goal is to return a Bad Request instead of an Internal Server Error much like I would when traditionally using a DRF serializer and excluding required fields from a payload.

Answer 1

To output a custom error message due to the IntegrityError, you can override the create method in your serializer:
from django.db import IntegrityError
class VoteSerializer(serializers.ModelSerializer):
class Meta:
model = Vote
fields = ['post', 'value']
def create(self, validated_data):
try:
validated_data['user'] = self.context['request'].user
return super().create(validated_data)
except IntegrityError:
error_msg = {'error': 'IntegrityError message'}
raise serializers.ValidationError(error_msg)

Answer 2

You can try this on your views
try:
MoviesWatchList.objects.create(user=request.user, content=movie)
return response.Response({'message': f'{movie} added in watchlist.'}, status=status.HTTP_201_CREATED)
except:
return response.Response({'message': f'{movie} already added to watchlist.'}, status=status.HTTP_304_NOT_MODIFIED)