我从 Mongoose 5跳到6，甚至到7。
但是我不能再Map我的user_id属性了，因为这个TypeError: Function.prototype.toString requires that 'this' be a Function

import type { DocumentType } from '@typegoose/typegoose'
import { getModelForClass, prop } from '@typegoose/typegoose'

export class ExperimentUser {
  @prop({})
  public user_id?: mongoose.Types.ObjectId
}

export type ExperimentUserDocument = DocumentType<ExperimentUserSchema>
export const ExperimentUserModel = getModelForClass(ExperimentUser)

import { HydratedDocument } from 'mongoose'

private map(document: HydratedDocument<ExperimentUser>): ExperimentUserDTO {
    return {
      id: document._id.toString(),
      user_id: document.user_id?.toString()
//                              ^
//TypeError: Function.prototype.toString requires that 'this' be a Function

    }
}

import mongoose from 'mongoose'

async create(user_id?: string): Promise<ExperimentUserDTO> {
    const created_user = await ExperimentUserModel.create({
      user_id: user_id ? new mongoose.Types.ObjectId(user_id) : undefined
    })
    return map(created_user)
  }

任何帮助将不胜感激
干杯