update_or_create_reverse_relations does not set parent on sub-serializers

[MattFisher]

I have a use case where I need to customise the create method of a nested serializer, because the model it creates depends on a field in the parent serializer.
If I override create(self, validated_data), self.parent (and self.root) are None within the method body. Normally self.parent and some other fields are set in rest_framework.fields.Field.bind() when the serializer is initially defined, but because update_or_create_reverse_relations creates a new serializer for the nested field, rather than mutating values on the existing one, these fields don't get set.

The simplest fix would be to add serializer.parent = self in one of two places indicated below, but I don't know whether there would be unintended side effects.

Another option would be to change the instance and/or data values on the the existing serializer (field in the code below) rather than creating a new one, but I presume there were side-effects from this that led to the current approach in the first place.

Within BaseNestedModelSerializer.update_or_create_reverse_relations:

for data in related_data:
    obj = instances.get(
        self._get_related_pk(data, field.Meta.model)
    )
    serializer = self._get_serializer_for_field(
        field,
        instance=obj,
        data=data,
    )
    # serializer.parent = self  # <== possibly add this here?
    serializer.is_valid(raise_exception=True)
    related_instance = serializer.save(**save_kwargs)

# elsewhere in BaseNestedModelSerializer
def _get_serializer_for_field(self, field, **kwargs):
    kwargs.update({
        'context': self.context,
        'partial': self.partial,
    })
    serializer = field.__class__(**kwargs)
    # serializer.parent = self  # <== alternatively add this here?
    return serializer

[cjroth]

I think this should fix this: #29