[BUG] TypeError: Subscripted generics cannot be used with class and instance checks

[slavashvets]

Describe the bug

When using Python 3.12 with Beanie and a Pydantic (v2) model that has a union-typed field using a discriminated union (via Annotated), Beanie's internal encoder raises a TypeError because it attempts to do an isinstance() check against subscripted generics.

See https://bugs.python.org/issue44529

To Reproduce

from typing import Annotated
from pydantic import BaseModel, Field
from pydantic_ai.messages import ModelRequest, ModelResponse
from beanie import Document

class MyBase(BaseModel):
    # A discriminated union
    data: Annotated[ModelRequest | ModelResponse, "kind"] = Field(...)

class ChatMessage(MyBase, Document):
    class Settings:
        # ...
        pass

Expected behavior
Handle these union-typed fields without errors.

Actual behavior

TypeError: Subscripted generics cannot be used with class and instance checks

Because Beanie's internal _get_encoder does something like:

isinstance(obj, Union[ModelRequest, ModelResponse])

Additional context
I've had to monkey-patch Beanie's _get_encoder to work around this issue:

from typing import get_args, get_origin

import beanie.odm.utils.encoder as encoder_module

_original_get_encoder = encoder_module._get_encoder

def _get_encoder_patched(obj, custom_encoders):

    encoder = custom_encoders.get(type(obj))
    if encoder is not None:
        return encoder

    for cls, encoder in custom_encoders.items():
        origin = get_origin(cls)
        if origin is not None:
            for arg in get_args(cls):
                if isinstance(arg, type) and isinstance(obj, arg):
                    return encoder
        else:
            if isinstance(obj, cls):
                return encoder
    return None

encoder_module._get_encoder = _get_encoder_patched