Django ManyToMany relationship on unioned queryset from different models - django

Question

I'm unable to join ManyToMany relationship records using a serializer to a unioned queryset from different DB modals.
I have two similar DB models, A and B both with a ManyToMany relationship with another model, Tag.
from django.db import models
class Tag(models.Model):
name = models.CharField(max_length=100)
class A(models.Model):
name = models.CharField(max_length=100)
tags = models.ManyToManyField(Tag)
class B(models.Model):
name = models.CharField(max_length=100)
tags = models.ManyToManyField(Tag)
I want to union A and B, and join the data from Tag via a Serializer. Here's my serializer:
from rest_framework import serializers
from union.models import A, B, Tag
class TagSerializer(serializers.ModelSerializer):
class Meta:
model = Tag
fields = '__all__'
class ABUnionSerializer(serializers.Serializer):
name = serializers.CharField()
tags = TagSerializer(many=True)
When I create a queryset that unions A and B, then pass that to ABUnionSerializer, the serializer does not join the Tag data from B correctly. Example:
from django.test import TestCase
from union.models import A, B, Tag
from union.serializers import ABUnionSerializer
class UnionTestCase(TestCase):
def test_union(self):
ta = Tag.objects.create(name='t-a')
tb = Tag.objects.create(name='t-b')
a = A.objects.create(name='aaa')
a.tags.add(ta)
b = B.objects.create(name='bbb')
b.tags.add(tb)
u = A.objects.all().union(B.objects.all())
s = ABUnionSerializer(u, many=True)
print(s.data)
The Serializer attempts to use relationship table from A instead of B. In this example, this causes the serialized record "bbb" to have tag "t-a" instead of "t-b".
This is likely due to this documented behavior of unioned querysets:
union(), intersection(), and difference() return model instances of the type of the first QuerySet even if the arguments are {{QuerySet}}s of other models.
What's the best way to join Tags to this unioned queryset?