model with a field that is a root model - a tree

[marekro]

I would like to use Pydantic v2 to manage a tree structure. The basic idea is to have a "node" model, which is a container for the actual item object, and has a field which represents the children. I could use a list for the children, but I would like to use a Pydantic root model to be able to add some helper methods to this model, which list does not have.
See the code below for my draft; the issue I am having is with the constructor, which throws an unexpectedly large number of validation errors on the children field. I tried to debug that, but this seems to be part of the core, which is compiled. I am running this with Python 3.10.14, pydantic 2.7.1.
Is there anything wrong with my code, or is this a bug in Pydantic v2? Any guidance is appreciated!

from typing import Type, TypeVar, Generic
from pydantic import BaseModel, PrivateAttr, RootModel, Field
from typing_extensions import Self


T = TypeVar("T", bound=BaseModel)


class Container(RootModel[list[T]]):
    _item_class: Type[T] = PrivateAttr()
    root: list[T]
    # this is meant to be a more intelligent list


def _empty_container() -> Container[T]:
    return Container[T]([])


class TreeNode(BaseModel, Generic[T]):
    _path: str = PrivateAttr(default="")
    item: T
    children: Container[Self] = Field(default_factory=_empty_container)
    # this is meant to be a base class for a tree model


class MyTestItem(BaseModel):
    my_id: int
    kind: str
    name: str
    # a sample item class


class MyTestNode(TreeNode[MyTestItem]):
    pass


def main() -> None:
    children = Container[MyTestNode]([
        MyTestNode(item=MyTestItem(my_id=101, kind="Item", name="one")),
        MyTestNode(item=MyTestItem(my_id=102, kind="Item", name="two")),
        MyTestNode(item=MyTestItem(my_id=103, kind="Item", name="three"))
    ])
    parent = MyTestNode(item=MyTestItem(my_id=100, kind="Folder", name="Folder"), children=children)
    assert len(parent.children.root) == 3


if __name__ == "__main__":
    main()

Here is the exception:

$ python3 tree_model.py 
Traceback (most recent call last):
  File "./tree_model.py", line 48, in <module>
    main()
  File "./tree_model.py", line 43, in main
    parent = MyTestNode(item=MyTestItem(my_id=100, kind="Folder", name="Folder"), children=children)
  File ".venv/lib/python3.10/site-packages/pydantic/main.py", line 176, in __init__
    self.__pydantic_validator__.validate_python(data, self_instance=self)
pydantic_core._pydantic_core.ValidationError: 28 validation errors for MyTestNode
children.0.0
  Input should be a valid list [type=list_type, input_value='root', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.0.0.0
  Input should be a valid list [type=list_type, input_value='item', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.0.0.1.0.0
  Input should be a valid list [type=list_type, input_value='my_id', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.0.0.1.0.1
  Input should be a valid list [type=list_type, input_value=101, input_type=int]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.0.0.1.1.0
  Input should be a valid list [type=list_type, input_value='kind', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.0.0.1.1.1
  Input should be a valid list [type=list_type, input_value='Item', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.0.0.1.2.0
  Input should be a valid list [type=list_type, input_value='name', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.0.0.1.2.1
  Input should be a valid list [type=list_type, input_value='one', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.0.1.0
  Input should be a valid list [type=list_type, input_value='children', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.0.1.1.0.0
  Input should be a valid list [type=list_type, input_value='root', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.1.0.0
  Input should be a valid list [type=list_type, input_value='item', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.1.0.1.0.0
  Input should be a valid list [type=list_type, input_value='my_id', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.1.0.1.0.1
  Input should be a valid list [type=list_type, input_value=102, input_type=int]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.1.0.1.1.0
  Input should be a valid list [type=list_type, input_value='kind', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.1.0.1.1.1
  Input should be a valid list [type=list_type, input_value='Item', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.1.0.1.2.0
  Input should be a valid list [type=list_type, input_value='name', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.1.0.1.2.1
  Input should be a valid list [type=list_type, input_value='two', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.1.1.0
  Input should be a valid list [type=list_type, input_value='children', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.1.1.1.0.0
  Input should be a valid list [type=list_type, input_value='root', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.2.0.0
  Input should be a valid list [type=list_type, input_value='item', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.2.0.1.0.0
  Input should be a valid list [type=list_type, input_value='my_id', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.2.0.1.0.1
  Input should be a valid list [type=list_type, input_value=103, input_type=int]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.2.0.1.1.0
  Input should be a valid list [type=list_type, input_value='kind', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.2.0.1.1.1
  Input should be a valid list [type=list_type, input_value='Item', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.2.0.1.2.0
  Input should be a valid list [type=list_type, input_value='name', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.2.0.1.2.1
  Input should be a valid list [type=list_type, input_value='three', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.2.1.0
  Input should be a valid list [type=list_type, input_value='children', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type
children.0.1.2.1.1.0.0
  Input should be a valid list [type=list_type, input_value='root', input_type=str]
    For further information visit https://errors.pydantic.dev/2.7/v/list_type