TypeError：无法读取未定义的属性“集合”这里有啥错误？

Posted 2023-03-11

【中文标题】TypeError：无法读取未定义的属性“集合”这里有啥错误？

【英文标题】：TypeError: Cannot read property 'collection' of undefined what is the mistake here?TypeError：无法读取未定义的属性“集合”这里有什么错误？

【发布时间】：2016-05-17 22:45:49

【问题描述】：

var mongodb = require('mongodb');
var MongoClient = mongodb.MongoClient;
var url = 'mongodb://localhost:27017/projectone';
var db1=MongoClient.connect(url, function (err, db) 
  if (err) 
    console.log('Unable to connect to the mongoDB server. Error:', err);
   if(!err) 

    console.log('Connection established to', url);


);
exports.findAll = function(req, res) 

var collection = db1.collection('student');
   collection.find().toArray(function (err, result) 
      res.send(result);


    );

这是我正在使用的示例代码，它在执行下面的代码时抛出错误。谁能解释为什么会这样？

我得到的错误：

TypeError：无法读取未定义的属性“集合” export.findAll (/home/android/student/pro1/route/connect1.js:16:6) 在回调 (/home/android/student/pro1/node_modules/express/lib/router/index.js:164:37) 在参数 (/home/android/student/pro1/node_modules/express/lib/router/index.js:138:11) 通过时 (/home/android/student/pro1/node_modules/express/lib/router/index.js:145:5) 在 Router._dispatch (/home/android/student/pro1/node_modules/express/lib/router/index.js:173:5) 在 Object.router (/home/android/student/pro1/node_modules/express/lib/router/index.js:33:10) 接下来 (/home/android/student/pro1/node_modules/express/node_modules/connect/lib/proto.js:174:15) 在 Object.expressInit [作为句柄] (/home/android/student/pro1/node_modules/express/lib/middleware.js:30:5) 接下来 (/home/android/student/pro1/node_modules/express/node_modules/connect/lib/proto.js:174:15) 在 Object.query [作为句柄] (/home/android/student/pro1/node_modules/express/node_modules/connect/lib/middleware/query.js:43:5)

var express=require('express'),
connect=require('./route/connect3');

var app=express();

app.get('/con',connect.findAll);
app.listen(8081);
console.log('listening to port 8081');

【参考方案1】：

试试这个

// connect3.js
var mongodb = require('mongodb');
var MongoClient = mongodb.MongoClient;
var url = 'mongodb://localhost:27017/projectone';

module.exports = MongoClient.connect(url, function (err, db) 
  if (err) 
        console.log('Unable to connect to the mongoDB server. Error:', err);
   if(!err) 
        console.log('Connection established to', url);
        return db;
    
);

exports.findAll = function(req, res) 
    var collection = req.db.collection('student');
        collection.find().toArray(function (err, result) 
            res.send(result);
        );
    

还有你的主文件

var express=require('express'),
const db = require('./route/connect3');
var app = express();
app.use(function(req, res, next) 
    req.db = db;
    next();
)



app.get('/con',connect.findAll);
app.listen(8081);
console.log('listening to port 8081');

【讨论】：

我认为在通用中间件中传递数据库是一种不好的做法，你不觉得吗？