NestJS -如何自引用mongoDB模式@nestjs/mongoose？

npm install --save @nestjs/mongoose

import { Module } from '@nestjs/common';
import { MongooseModule } from '@nestjs/mongoose';

@Module({
  imports: [
    MongooseModule.forRoot('mongodb://localhost:27017/mydatabase'),
  ],
})
export class AppModule {}

import { Schema, Prop, SchemaFactory } from '@nestjs/mongoose';
import { Document } from 'mongoose';

@Schema()
export class User extends Document {
  @Prop()
  name: string;

  @Prop({ type: [SchemaTypes.ObjectId], ref: 'User' })
  friends: User[];
}

export const UserSchema = SchemaFactory.createForClass(User);

import { Injectable } from '@nestjs/common';
import { InjectModel } from '@nestjs/mongoose';
import { Model } from 'mongoose';
import { User } from './user.schema';

@Injectable()
export class UserService {
  constructor(@InjectModel(User.name) private userModel: Model<User>) {}

  async findUserById(id: string): Promise<User> {
    return this.userModel.findById(id).populate('friends');
  }
}