[Python] Django Rest Framework: validate_username and update methods not called during PUT...

I am working on a Django Rest Framework (DRF) project, where I need to validate a username field and handle custom update logic in a serializer. My model has a username field with unique=True. Here's the relevant setup:

Model:



from django.contrib.auth.models import AbstractUser

class CustomUser(AbstractUser):
pass

Serializer:


class CustomUserSerializer(serializers.ModelSerializer):
class Meta:
model = models.CustomUser
fields = ["id", "username", "first_name", "last_name"]
read_only_field = ["id"]
extra_kwargs = {
"password": {"write_only": True},
}

def validate_username(self, value):
print('here.......')
user_model = get_user_model()
request = self.context.get('request')
if request and request.method in ['PUT', 'PATCH']:

if user_model.objects.filter(username=value).exclude(id=self.instance.id).exists():
raise serializers.ValidationError('User with this username already exists.')
else:
if user_model.objects.filter(username=value).exists():
raise serializers.ValidationError('User with this username already exists.')
return value


class CustomUserProfileSerializer(serializers.ModelSerializer):
user = CustomUserSerializer()
roles = ProfileRolesSerializer(many=True)
department = DepartmentSerializer()
gender = GenderSerializer()

Views:



class CustomUpdateView(APIView):
def put(self, request, *args, **kwargs):
serializer = CustomUserProfileSerializer(data=request.data)
if serializer.is_valid():
serializer.save()
return Response(serializer.data, status=status.HTTP_200_OK)
else:
return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)



When I send a PUT request to update a user profile, the validate_username and update methods in the serializer are not executed. Instead, I get a response error:

{
"user": {
"username": [
"User with this username already exists."
]
}
}


I suspect that the default unique=True constraint on the username field is bypassing the serializer-level validation. I have tried:

1. Adding print statements in validate_username and update methods — these are never executed.
2. Switching to a custom APIView (CustomUpdateView) — no change.
3. Ensuring CustomUserProfileSerializer is correctly used in the view.

What could be causing this issue, and how can I ensure that the validate_username and update methods are called during a PUT request?

Continue reading...