NestJS中使用Mongoose的.find()方法如何判断无查询结果？

调用this.userService.findOne(createUserDto.username)时，无法正确判断查询结果是否为空，无论输入什么，判断逻辑总是返回true或false，不符合预期。

控制器代码： #

import { Get, Controller, Body, Post, Param } from "@nestjs/common";
import { RegisterService } from "./register.service";
import * as bcrypt from "bcrypt";
import { CreateUserDto } from "./create-user.dto";

@Controller("register")
export class RegisterController {
  constructor(private userService: RegisterService) {}

  @Post()
  create(@Body() createUserDto: CreateUserDto) {
    let chkUname: any = this.userService.findOne(createUserDto.username);

    let response = {
      usernError: "",
      regOk: true,
    };

    if (!chkUname) {
      // this.userService.createUser(createUserDto);
      return chkUname;
    } else {
      // response.usernError = "Sorry, this username is used!";
      // response.regOk = false;
      return true;
    }
  }
  // create(@Param("uname") uname: any) {
  //   return this.userService.findOne(uname);
  // }
}

服务代码： #

import { Injectable, NotFoundException } from "@nestjs/common";
import { InjectModel } from "@nestjs/mongoose";
import { Model } from "mongoose";
import { CreateUserDto } from "./create-user.dto";
import { Users, UsersDocument } from "./schema/user.schema";

@Injectable()
export class RegisterService {
  constructor(
    @InjectModel(Users.name) private readonly usersModel: Model<UsersDocument>
  ) {}

  async createUser(createUserDto: CreateUserDto): Promise<UsersDocument> {
    const user = new this.usersModel(createUserDto);
    return user.save();
  }

  async findOne(uname: string) {
    return this.usersModel.find({uname}).exec();
  }
}

问题根源及解决方法 #

• 异步逻辑未处理
  服务中的findOne是异步函数（标记了async），直接赋值let chkUname: any = this.userService.findOne(...)得到的是Promise对象而非实际查询结果。Promise作为对象在布尔判断中永远为真，导致!chkUname的判断完全失效。必须用await等待异步操作完成，获取实际结果。

• 查询方法使用错误
  当前服务中用find({uname})查询，该方法返回数组，即使无匹配数据也会返回空数组[]。空数组在布尔判断中为真，因此!chkUname的逻辑完全错误。应改用findOne方法，无匹配结果时返回null，才能通过!chkUname判断是否存在用户。

修正后的代码 #

服务代码

import { Injectable, NotFoundException } from "@nestjs/common";
import { InjectModel } from "@nestjs/mongoose";
import { Model } from "mongoose";
import { CreateUserDto } from "./create-user.dto";
import { Users, UsersDocument } from "./schema/user.schema";

@Injectable()
export class RegisterService {
  constructor(
    @InjectModel(Users.name) private readonly usersModel: Model<UsersDocument>
  ) {}

  async createUser(createUserDto: CreateUserDto): Promise<UsersDocument> {
    const user = new this.usersModel(createUserDto);
    return user.save();
  }

  async findOne(uname: string): Promise<UsersDocument | null> {
    // 改用findOne，无匹配时返回null
    return this.usersModel.findOne({uname}).exec();
  }
}

控制器代码

import { Get, Controller, Body, Post, Param } from "@nestjs/common";
import { RegisterService } from "./register.service";
import * as bcrypt from "bcrypt";
import { CreateUserDto } from "./create-user.dto";

@Controller("register")
export class RegisterController {
  constructor(private userService: RegisterService) {}

  // 标记为async，处理异步逻辑
  @Post()
  async create(@Body() createUserDto: CreateUserDto) {
    // 用await获取实际查询结果
    const chkUname = await this.userService.findOne(createUserDto.username);

    const response = {
      usernError: "",
      regOk: true,
    };

    if (!chkUname) {
      // 无匹配用户，执行注册逻辑
      await this.userService.createUser(createUserDto);
      return response;
    } else {
      // 用户名已存在
      response.usernError = "Sorry, this username is used!";
      response.regOk = false;
      return response;
    }
  }
}

内容的提问来源于stack exchange，提问作者Alms