How Pydantic can handle `id` as string and `_id` as ObjectId for PyMongo

ObjectId and Pydantic.py

from typing import List, Optional
from bson import ObjectId
from pydantic import BaseModel, Field


class PyObjectId(ObjectId):
    @classmethod
    def __get_validators__(cls):
        yield cls.validate

    @classmethod
    def validate(cls, v):
        if not ObjectId.is_valid(v):
            raise ValueError("Invalid objectid")
        return ObjectId(v)

    @classmethod
    def __modify_schema__(cls, field_schema):
        field_schema.update(type="string")


class Entity(BaseModel):
    id: Optional[PyObjectId] = Field(alias="_id")
    ref: Optional[PyObjectId] = Field()
    refs: Optional[List[PyObjectId]] = Field()

    class Config:
        allow_population_by_field_name = True # Pydantic should use the alias when populating the model from a dictionary (which allows you to pass in a dictionary with an _id key rather than an id key)
        json_encoders = {ObjectId: str} # ObjectId should be encoded to a string when converting the model to JSON


test = Entity(
    id="64760b13500e13df454ff03d",
    ref="647e0fa0b4ee216624761493",
    refs=["64760b13500e13df454ff03d", "647e0fa0b4ee216624761493"],
)

print(test)
# id=ObjectId('64760b13500e13df454ff03d') ref=ObjectId('647e0fa0b4ee216624761493') refs=[ObjectId('64760b13500e13df454ff03d'), ObjectId('647e0fa0b4ee216624761493')]

print(test.dict(by_alias=True)) # ensure that the dictionary uses the field aliases (so it has an _id key rather than an id key)
# {'_id': ObjectId('64760b13500e13df454ff03d'), 'ref': ObjectId('647e0fa0b4ee216624761493'), 'refs': [ObjectId('64760b13500e13df454ff03d'), ObjectId('647e0fa0b4ee216624761493')]}

print(test.json())
# {"id": "64760b13500e13df454ff03d", "ref": "647e0fa0b4ee216624761493", "refs": ["64760b13500e13df454ff03d", "647e0fa0b4ee216624761493"]}

print(test.id)
# 64760b13500e13df454ff03d