Unknown output_field type in Subquery in Django

I have the following models:

class Parent1(models.Model):
    name = models.CharField(max_length=255)

class Parent2(models.Model):
    parent1 = models.ForeignKey(Parent1)
    label = models.CharField(max_length=255)

class Child(models.Model):
    parent1 = models.ForeignKey(Parent1)
    parent2_label = models.CharField(max_length=255)
    # other fields

I am trying to execute the below queries:

parent1 = Parent1.objects.first()
sq = Child.objects.filter(
    parent1=OuterRef("parent1"), parent2_label=OuterRef("label")
)
parents_2 = Parent2.objects.filter(parent1=parent1).annotate(
    children=Subquery(sq)
)

So basically what I am trying to execute is if Child model had a fk to Parent2 then I would have prefetched the related child instances of the Parent2 instances. Now at present there is no fk relation between Child and Parent2. The relation is based on the label field between those two. fk will be added later but before that for backwards compatibility I have to execute this query.

Now when I try to execute this query then I am getting the following error:

FieldError: Cannot resolve expression type, unknown output_field

I am using Postgresql db. Now what will be the output_field for the above query that I have to pass?

So basically what I am trying to achieve through the subquery is this:

# this is not a working example.
parents_2 = Parent2.objects.filter(parent1=parent1)
for parent in parents_2:
    children = Child.objects.filter(parent1=parent1, parent2_label=parent.label)
    parent.annotate(children=children)

I want the children to be a nested object with each parent_2 in the queryset.