NestJS中Mongoose的populate失效及关联查询无结果问题排查

问题现象 #

在NestJS项目中使用Mongoose时遇到两个问题：

• populate关联查询无法正常工作
• 通过Night文档中teams数组的ObjectId查询Team集合时返回空数组

服务方法代码 #

async getTeamsByNightId(id: string) {
    const night = await this.nightModel.findById({_id: id});
    console.log('night: ', night);
    //@TODO Why is populate not working?
    //const night = (await this.nightModel.findById({_id: id})).populate('teams');

    if (night) {
      const teamIds = night.teams.map((teamId) => teamId.toString());
      console.log('teamIds: ', teamIds);
      try {
        const teams = await this.teamModel.find({ _id: { $in: night.teams } }).exec();
        console.log('teams: ', teams);
      } catch (error) {
        console.log(error);
      }
      
    } else {
      throw new NotFoundException('No matching Night found');
    }
  }

控制台输出 #

Mongoose: nights.findOne({ _id: ObjectId("6552ad936a93bab2b686496d") }, {})
night:  {
  _id: new ObjectId("6552ad936a93bab2b686496d"),
  name: 'a',
  askedQuestions: [],
  teams: [ new ObjectId("6552ada76a93bab2b6864972") ],
  password: '487254',
  __v: 1
}
teamIds:  [ '6552ada76a93bab2b6864972' ]
Mongoose: teams.find({ _id: { '$in': [ ObjectId("6552ada76a93bab2b6864972") ] } }, {})
teams:  []

Schema定义 #

Night Schema #

import { Prop, Schema, SchemaFactory } from "@nestjs/mongoose";
import { HydratedDocument, Types } from "mongoose";

export type NightDocument = HydratedDocument<Night>;

@Schema()
export class Night {
    @Prop()
    name: string;

    @Prop()
    password: string;

    @Prop([{ type: Types.ObjectId, ref: 'Question' }])
    askedQuestions: Types.ObjectId[];

    @Prop([{ type: Types.ObjectId, ref: 'Team' }])
    teams: Types.ObjectId[];;
}

export const NightSchema = SchemaFactory.createForClass(Night);

Team Schema #

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

export type TeamDocument = HydratedDocument<Team>;

@Schema()
export class Team {

  @Prop()
  username: string;

  @Prop()
  password: string;
}

export const TeamSchema = SchemaFactory.createForClass(Team);

添加Team到Night的方法 #

async addTeamToNight(teamId: string, addTeamToNightDTO: AddTeamToNightDTO) {
    const name = addTeamToNightDTO.name;
    const password = addTeamToNightDTO.password;
    const night = await this.nightModel.findOne({name, password}).exec();

    if (night) {
      night.teams.push(new Types.ObjectId(teamId));
      return await night.save();
    } else {
      throw new NotFoundException('Name or Password incorrect');
    }
  }

疑问 #

请问我哪里操作出错了？

问题原因及解决方案 #

1. 查询Team返回空数组的原因 #

你存入Night的teamId对应的文档很可能不存在于Team集合中。可以直接在MongoDB客户端执行db.teams.find({_id: ObjectId("6552ada76a93bab2b6864972")})，验证该ObjectId是否真的对应一个Team文档。另外，检查传入addTeamToNight的teamId是否正确，有没有拼写或格式错误。

2. populate失效的原因 #

你写的(await this.nightModel.findById({_id: id})).populate('teams')写法错误，Mongoose的populate需要在查询链中调用，或者对已查询的文档执行execPopulate()。正确写法有两种：

• 链式调用（推荐）：

  const night = await this.nightModel.findById(id).populate('teams').exec();
  

• 对已获取的文档执行：

  const night = await this.nightModel.findById(id);
  await night.populate('teams');
  

3. 额外修复点 #

• Night Schema中teams字段多了一个分号，修正为：teams: Types.ObjectId[];
• 在addTeamToNight方法中，建议先验证teamId对应的Team是否存在，再添加到Night的teams数组，避免存入无效ObjectId：

  async addTeamToNight(teamId: string, addTeamToNightDTO: AddTeamToNightDTO) {
    const name = addTeamToNightDTO.name;
    const password = addTeamToNightDTO.password;
    
    // 先验证Team存在
    const teamExists = await this.teamModel.exists({_id: teamId});
    if (!teamExists) {
      throw new NotFoundException('Team not found');
    }
    
    const night = await this.nightModel.findOne({name, password}).exec();
    if (night) {
      night.teams.push(new Types.ObjectId(teamId));
      return await night.save();
    } else {
      throw new NotFoundException('Name or Password incorrect');
    }
  }
  

内容的提问来源于stack exchange，提问作者user3481441