NestJS模块依赖注入异常：调用getUsers时出现undefined错误

我用命令nest g module UserModule在NestJS中创建了新模块，相关文件代码如下：

src/user-module/user.resolver.ts代码 #

import { Query, Resolver } from '@nestjs/graphql';
import { UserService } from './user.service';

export class UserResolver {
  
  constructor (private readonly user: UserService) {
   this.user = user; 
  }

  @Query('users')
  async users() {
    return this.user.getUsers();
  }
}

src/user-module/user-module.module.ts代码 #

import { Module } from '@nestjs/common';
import { UserResolver } from './user.resolver';
import { UserService } from './user.service';
import { PrismaService } from './prisma.services';

@Module({
  providers: [UserResolver, UserService, PrismaService]
})

export class UserModuleModule {}

src/user-module/user.service.ts代码 #

import { Injectable } from "@nestjs/common";
import { PrismaService } from "./prisma.services";
import { User } from "src/graphql";

@Injectable()
export class UserService {
  constructor(private readonly prisma: PrismaService) {

  }

  async getUsers(): Promise<User[]> {
    return this.prisma.prismaUsers.findMany({})
  }
}

src/user-module/prisma.services.ts代码 #

import { Injectable, OnModuleInit } from '@nestjs/common';
import { PrismaClient } from '@prisma/client';

@Injectable()
export class PrismaService extends PrismaClient implements OnModuleInit {
  async onModuleInit() {
    await this.$connect();
  }
}

执行npm start启动服务后，运行以下GraphQL查询：

query {
   users {
     id
     name
     email
   }
}

出现如下错误：

[Nest] 24682 - 09/09/2023, 10:59:34 AM ERROR [ExceptionsHandler] Cannot read properties of undefined (reading'getUsers')
TypeError: Cannot read properties of undefined (reading 'getUsers')
at UserResolver.users (/home/raj/Coding/ts/prisma-gql-express-demo/src/user-module/user.resolver.ts:12:22)
at /home/raj/Coding/ts/prisma-gql-express-demo/node_modules/@nestjs/core/helpers/external-context-creator.js:69:29)
at InterceptorsConsumer.intercept (/home/raj/Coding/ts/prisma-gql-express-demo/node_modules/@nestjs/core/interceptors/interceptors-consumer.js:12:20)
at target (/home/raj/Coding/ts/prisma-gql-express-demo/node_modules/@nestjs/core/helpers/external-context-creator.js:74:60)
at Object.users (/home/raj/Coding/ts/prisma-gql-express-demo/node_modules/@nestjs/core/helpers/external-proxy.js:9:30)
at field.resolve (/home/raj/Coding/ts/prisma-gql-express-demo/node_modules/@apollo/server/src/utils/schemaInstrumentation.ts:82:22)
at executeField (/home/raj/Coding/ts/prisma-gql-express-demo/node_modules/graphql/execution/execute.js:492:20)
at executeFields (/home/raj/Coding/ts/prisma-gql-express-demo/node_modules/graphql/execution/execute.js:414:22)
at executeOperation (/home/raj/Coding/ts/prisma-gql-express-demo/node_modules/graphql/execution/execute.js:344:14)
at execute (/home/raj/Coding/ts/prisma-gql-express-demo/node_modules/graphql/execution/execute.js:136:20)

请问我的依赖注入哪里出问题导致了这个错误？

错误核心原因是**UserResolver类缺少@Resolver()装饰器**：NestJS需要通过这个装饰器识别该类为GraphQL解析器，才能将其纳入依赖注入容器管理。没有这个装饰器的话，Nest无法正确注入UserService，导致this.user为undefined，调用getUsers()时抛出错误。

另外，构造函数里的this.user = user;属于多余代码——当你在构造函数参数中使用private readonly修饰符时，TypeScript会自动将参数赋值为类的属性，无需手动重复赋值。

修复后的user.resolver.ts代码如下：

import { Query, Resolver } from '@nestjs/graphql';
import { UserService } from './user.service';

// 添加@Resolver装饰器
@Resolver()
export class UserResolver {
  
  // 移除多余的手动赋值代码
  constructor (private readonly user: UserService) {}

  @Query('users')
  async users() {
    return this.user.getUsers();
  }
}

额外提醒：检查UserService中的prismaUsers是否符合Prisma生成的模型命名规范——通常Prisma会将模型名转为小写驼峰式（比如模型User对应user，查询方法为this.prisma.user.findMany({})），避免后续出现新的查询错误。

内容的提问来源于stack exchange，提问作者user51