service代替controller操作数据库，使controller更加简洁
controller只需要接收前端的数据和返回响应的内容

定义service

在app目录中新建service目录再创建student.js

const Service = require('egg').Service;

class StudentService extends Service {
    async getStudentList(){
        try{
            let studentList = await this.app.model.Student.findAll();
            return studentList;
        }catch(e){
            return null;
        }
        
    }
}

module.exports = StudentService;

controller中student.js
不再关注数据库和业务逻辑，只关心响应的内容

const { Controller } = require('egg');
class studentController extends Controller {
    async index() {
      //调用service的student的getStudentList方法
        let list = await this.ctx.service.student.getStudentList();
        if(list){//如果list有对象，返回true
          this.ctx.body = {
            code:20000,
            data:list,
          }
          }else{
            this.ctx.body = {
              code:50000,
              msg:"服务器异常"
            }
        }
    }
  }
  
  module.exports = studentController;

service能保持业务逻辑的独立性，体现在比如程序中还有些地方需要获取学生列表，直接调用service对应的方法如getStuentList
实现了方法的复用

再写个添加数据的业务
service
student.js

const Service = require('egg').Service;

class StudentService extends Service {
    async getStudentList(){
        try{
            let studentList = await this.app.model.Student.findAll();
            return studentList;
        }catch(e){
            return null;
        }
    }

    async createStudent(name,score,clazz_id){//数据是前端通过controller传过来的
        try{
            await this.app.model.Student.create({
                name:name,
                score:score,
                clazz_id:clazz_id
            });
            return true;//告诉controller已经成功了
        }catch(e){
            return false;
        }
    }
}

module.exports = StudentService;

controller
student.js

const { Controller } = require('egg');
class studentController extends Controller {
    async index() {
      //调用service的student的getStudentList方法
        let list = await this.ctx.service.student.getStudentList();
        if(list){//如果list有对象，返回true
          this.ctx.body = {
            code:20000,
            data:list,
          }
          }else{
            this.ctx.body = {
              code:50000,
              msg:"服务器异常"
            }
        }
    }

    async create(){
      let name = this.ctx.request.body.name;
      let score = this.ctx.request.body.score;
      let clazz_id = this.ctx.request.body.clazz_id;
      let result = await this.ctx.service.student.createStudent(name,score,clazz_id);
      if(result){
        this.ctx.body = {
          code:20000,
          msg:"添加成功"
        }
      }else{
        this.ctx.body = {
          code:50000,
          msg:"添加失败"
        }
      }
    }
  }
  
  module.exports = studentController;