UniqueTogether validation doesn't verify uniqueness between data items when many=True

[zacharypodbela]

I've found a potential issue very similar in nature to #6395, but with UniqueTogetherValidator instead of UniqueValidator.

Seeing as we now support this behavior for UniqueValidator as of 3.15, it feels like subverted expectation that we would not also support the same behavior for UniqueTogetherValidator.

Does this feel like a real issue? Or something out of scope for the framework?

Example

class UniquenessModel(models.Model):
  field1 = models.CharField(max_length=50)
  field2 = models.CharField(max_length=50)

  class Meta:
    unique_together = ('field1', 'field2',)

class UniquenessSerializer(serializers.ModelSerializer):
    class Meta:
        model = UniquenessModel
        fields = '__all__'
        validators = [
            UniqueTogetherValidator(
                queryset=UniquenessModel.objects,
                fields=['field1', 'field2']
            )
        ]

data = [{'field1': 'foo', 'field2': 'bar'}, {'field1': 'foo', 'field2': 'bar'}]
serializer = UniquenessSerializer(data=data, many=True)
serializer.is_valid()   # True
serializer.save()  # django.db.utils.IntegrityError

[zacharypodbela]

I've also found another issue with UniqueTogetherValidator unfortunately, which is that it does not exclude based on pk from the queryset so it will also incorrectly throw a validation error when there is not an error in the following case:

class UniquenessModel(models.Model):
  id = models.AutoField(primary_key=True, unique=True)
  field1 = models.CharField(max_length=50)
  field2 = models.CharField(max_length=50)

  class Meta:
    unique_together = ('field1', 'field2',)

class UniquenessSerializer(serializers.ModelSerializer):
    class Meta:
        model = UniquenessModel
        fields = '__all__'
        validators = [
            UniqueTogetherValidator(
                queryset=UniquenessModel.objects,
                fields=['field1', 'field2']
            )
        ]

p = UniquenessModel.objects.create(field1='foo', field2='bar')
p.id   # 3

data = [{'id': 3, 'field1': 'foo', 'field2': 'bar'}, {'id': 4, 'field1': 'irrelevant', 'field2': 'values'}]
serializer = UniquenessSerializer(data=data, many=True)
serializer.is_valid()   # False. Reason is that foo/bar must be unique combo (it see it that this pair already exists in the database but does not put together that it is the same object based on pk).

[kalekseev]

Probably serializers shouldn't create UniqueTogether validators when many=True is used.
@zacharypodbela in your example with many=True I would expect instance=queryset to be provided to serializer because you expect update of some objects based on pk, the update logic is not implemented by ListSerializer, so it make sense that uniqueness should be implemented by user as well.

The error that I get right now if instance=queryset, many=True passed to serializer:

tests/test_validators.py:586: in test_unique_together_many
    assert serializer.is_valid()
rest_framework/serializers.py:775: in is_valid
    self._validated_data = self.run_validation(self.initial_data)
rest_framework/serializers.py:634: in run_validation
    value = self.to_internal_value(data)
rest_framework/serializers.py:693: in to_internal_value
    validated = self.run_child_validation(item)
rest_framework/serializers.py:653: in run_child_validation
    return self.child.run_validation(data)
rest_framework/serializers.py:442: in run_validation
    value = self.to_internal_value(data)
rest_framework/serializers.py:499: in to_internal_value
    validated_value = field.run_validation(primitive_value)
rest_framework/fields.py:541: in run_validation
    self.run_validators(value)
rest_framework/fields.py:553: in run_validators
    validator(value, self)
rest_framework/validators.py:72: in __call__
    queryset = self.exclude_current_instance(queryset, instance)
rest_framework/validators.py:60: in exclude_current_instance
    return queryset.exclude(pk=instance.pk)
E   AttributeError: 'QuerySet' object has no attribute 'pk'