union + instance_of doesn't work in Django 3

[l0b0]

I've been using several polymorphic model filters to get only the instances of specific types. For example:

class MyModelFilter(FilterSet):
    def polymorphic_type_filter(self, queryset: PolymorphicQuerySet, name: str, type_names: Iterable[str]) -> QuerySet:
        type_models = []
        for model_class in MyModelPolymorphicSerializer.model_serializer_mapping.keys():
            if model_class.__name__ in type_names:
                type_models.append(model_class)

        return queryset.instance_of(*type_models)

    resourcetype__in = CharInFilter(method=polymorphic_type_filter.__name__, field_name="resourcetype")

Django 3 no longer allows filtering a union query set, so after upgrading this filter results in the following error:

django.db.utils.NotSupportedError: Calling QuerySet.filter() after union() is not supported.

I'm looking for a workaround for this, but I'm not sure how easy this is to do. Is there a known fix?

The weird thing is, this ticket says this never used to work, but I've got tests which say otherwise.

[l0b0]

Looks like this might've been a red herring. Sorry for the noise.