I excluded the OneToOneField because otherwise a validation error was thrown.  I'm not sure of a good way to make these not required in the case of a nested serializer.  It might be that we should just say users should exclude them since it doesn't make sense to try and update them anyway.  Another option would be to pass the field_name that a nested serializer should be updating, and check this during `.field_from_native` to not raise a validation error.  Something like:

```python
class OneToOneTargetSourceSeriailizer(serializers.ModelSerializer):
    source = OneToOneSourceSerializer(fk_field_name='target_source')

    class Meta:
        model = OneToOneTargetSource
````