Django Rest Framework: validate_username and update methods not called during PUT request

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?

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