请教mongoclient连接数据库的问题

Question

我在用mongdb的原生方式链接数据库，请看代码

conn.js文件

'use strict'
const { MongoClient } = require('mongodb');
const url = 'mongodb://localhost:27017/test';
const dbname = 'test';
const dbConnect = () => {
    return new Promise((resolve, reject) => {
        MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {
            if (err) {
                reject(err);
            } else {
                resolve(client.db(dbname));
            }
        });
    });
}
module.exports = {
    dbConnect
}

在其他地方引用，比如在 user.js 文件中引用

const conn = require('../conn');
router
   .post('/', async (ctx, next) => {
      try {
          let user = await conn.dbConnect.collection('user').findOne({email: email});
          console(user);
      } catch (e) {
            ctx.body = {error:true, msg: e instanceof Error ? e.message : e};
      }
})
module.exports = router;

但总是提示错误，db.collection is not a function，请问这个问题怎么解决？谢谢