expressjs中的MongoDB find()查询在第二次运行时返回空结果

const express = require('express');
const app = express();
const MongoClient = require('mongodb').MongoClient;

// 连接MongoDB数据库
const url = 'mongodb://localhost:27017/mydb';
const connectDB = async () => {
  const client = await MongoClient.connect(url, { useNewUrlParser: true });
  return client.db();
};

// 创建Express路由
app.get('/', async (req, res) => {
  try {
    const db = await connectDB();
    const collection = db.collection('mycollection');
  
    // 执行查询操作
    const result = await collection.find({}).toArray();
  
    res.send(result);
  } catch (error) {
    console.error(error);
    res.status(500).send('Internal Server Error');
  }
});

// 启动Express服务器
app.listen(3000, () => {
  console.log('Server started on port 3000');
});