MongoError：请求了多数读取问题，但服务器未启动--enableMajorityReadConcern

Question

所以，我尝试使用MongoDB数据库启动我的Express服务器，但是我遇到了下一个错误的问题：

MongoError：请求了多数读取问题，但服务器未启动--enableMajorityReadConcern。

我的server.js：

const express = require('express');  
const app = express();  
const server = require('http').createServer(app);  
const io = require('socket.io')(server);

const MongoClient = require('mongodb').MongoClient;
const mongoURL = "mongodb://myDB";

let dbArr = {};

MongoClient.connect(mongoURL, (err, client) => {
        let db = client.db('test_db'); 
        let changeStream = db.collection('employers').watch();
        // change_strems.on('change', function(change){
        //   console.log(JSON.stringify(change));
        // });
        changeStream.on('change', function(change) {
          expect(change).to.exist;

          // Since changeStream has an implicit seession,
          // we need to close the changeStream for unit testing purposes
          changeStream.close();
          client.close();
          done();
        });
        db.collection('employers').find().toArray((err, results) => {
          dbArr = results;
          // console.log(results)
        });
});

app.get('/users', function(req, res,next) {  
    res.send(dbArr);
});

io.on('connection', function(client) {  
    client.on('form_message', function(data) {
      db.collection('employers').save(data, (err, result) => {
        if (err) return console.log(err)
      });
      // db.collection('employers').find(data, (err, data) => {
      //     if (err) return console.log(err)
      //     if (data) {
      //         client.emit('click', data);
      //     }
      // });
      console.log('New Message', data);
    });
});

server.listen(8000);