[Python] How to specify actual class in Pydantic member

I'd like to serialize and deserialize an object holding a polymorphic member.

If the class was polymorphic, I would use a correct concrete type with TypeAdapter. The question is, how to do the same when a member is polymorphic. Here's a minimal example of what I mean:

from abc import ABC, abstractmethod
from pydantic import BaseModel, TypeAdapter


class I_Widget(ABC, BaseModel):
@abstractmethod
def do_stuff(self): ...


class WidgetHolder(BaseModel):
widget: I_Widget


class ActualWidget(I_Widget):
value: int

def do_stuff(self):
print('Doing stuff')


if __name__ == '__main__':
w = ActualWidget(value=42)
wh = WidgetHolder(widget=w)

# Serializes:
json = wh.json()

# Deserializes:
wh2 = TypeAdapter(WidgetHolder).validate_json(json) # Throws an error


When I run it, I get:

File "/home/adam/.cache/pypoetry/virtualenvs/poc-py3.12/lib/python3.12/site-packages/pydantic/type_adapter.py", line 446, in validate_json
return self.validator.validate_json(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: Can't instantiate abstract class I_Widget without an implementation for abstract method 'do_stuff'

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