Connecting Mongodb databse to node.js server-CodePudding

I have been trying to make a web app using node.js and express and decided to work with mongodb. I am using mongodb node.js driver version: 4.3.1 I have tried all the possible ways to connect the node.js server with my mongodb atlas database.

My database also got connected using the following code in my db.js file:

const app = require('./app');

const MongoClient = require('mongodb').MongoClient

const Url = 'mongodb srv://todoAppUser:<myOriginalPasswordHere>@cluster0.6lvjr.mongodb.net/myDatabase?retryWrites=true&w=majority';

MongoClient.connect(Url, function (err, client) {
  if (err) throw err;

  var db = client.db('myDatabase');

  db.collection('products').findOne({}, function (findErr, result) {
    if (findErr) throw findErr;
    console.log(result.name);
    client.close();
  });
});

The above code works fine and gives the output as well. But I want to use MVC (Model-view-Controller) framework for which I need to export the connection.

I made the following change in the above code:

MongoClient.connect(Url, function (err, client) {
  if (err) throw err;

  var db = client.db('myDatabase');

  db.collection('products').findOne({}, function (findErr, result) {
    if (findErr) throw findErr;
    console.log(result.name);
  module.exports = db
    client.close();
  });
});

After the change when I try to access my connection (const productCollection = require('./db').collection("product");) from any other file of mine, it gives me the following error:

const productCollection = require('./db').collection("product");
                                          ^

TypeError: require(...).collection is not a function
    at Object.<anonymous> (D:\Kush- Complete Data\exp-projects\nodeApp\productController.js:1:43)
    at Module._compile (internal/modules/cjs/loader.js:1072:14)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1101:10)
    at Module.load (internal/modules/cjs/loader.js:937:32)
    at Function.Module._load (internal/modules/cjs/loader.js:778:12)
    at Module.require (internal/modules/cjs/loader.js:961:19)
    at require (internal/modules/cjs/helpers.js:92:18)
    at Object.<anonymous> (D:\Kush- Complete Data\exp-projects\nodeApp\router.js:3:27)
    at Module._compile (internal/modules/cjs/loader.js:1072:14)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1101:10)
[nodemon] app crashed - waiting for file changes before starting...

May anyone please guide me and show a possible way out.

Thanks, Kush

CodePudding user response：

This can't work, because modules are evaluated synchronously. Your callback is called asynchronously but your module has been alread evaluated at this time. Means module.exports has no effect and require('./db').collection() is not defined. Please see Node.js Modules for details.

To solve your problem, handle the connection stored in a module internal variable and export a getter instead of the connection variable itself.

// db.js
let client;
let db;

function connectDB(url, dbName) {
    client = new MongoClient(url);

    return new Promise(function(resolve, reject) {
        client.connect(function(err) {
            if(err) return reject(err);

            db = client.db(dbName);
            return resolve(db);
        });
    });
}

function getCurrentDB() {
    return db;
}

module.exports.connectDB = connectDB;
module.exports.getCurrentDB = getCurrentDB;

Then reuse your opened connection in other files like the following:

// other.js
const db = require("./db.js");

db.getCurrentDB().collection("product");

Of course, getCurrentDB() can only return a database connection if a connection has been established via connectDB() beforehand. So you have to wait for the resolution of the Promise.

CodePudding user response：

[SOLVED] I figured out, in newer versions of mongodb they have essentially changed the way of connecting node server to the database. To establish a reusable connection (So that we can access the connected database from any other file), I created an async function in my db.js file where connection is established and then exported it. In the end of the file, I have called the function. The code is as follows:

 const {MongoClient} = require('mongodb')
    
    
    const client = new MongoClient('mongodb srv://todoAppUser:<password>@cluster0.6lvjr.mongodb.net/myDatabase?retryWrites=true&w=majority')
    
    async function start(){
      await client.connect()
      console.log("Connected")
      module.exports = client.db()
      const app = require('./app')
      app.listen(3000)
    }
    
      start()

and while calling it from another file:

const productCollection = require('./db').collection("product");

This code gives me no error and works perfectly fine. With the help of the above code, one can use this conveniently while following the MVC (Model-View-Controller) framework.