Best way to connect to MongoDB using Node.js [duplicate]

Question

I've read a few guides on how to use Mongo with Node, and they all seem to connect to databases differently. One particular way that worked well for me was:

MongoClient.connect("mongodb://localhost:27017/exampleDb", function(err, db) {
  if(err) { return console.dir(err); }

  db.createCollection('users', function(err, collection) {});

  //Do all server/database operations in here

});

However, this seems inefficient/odd to me, I would have to reconnect to the database every time there is an app.get(), like for making a new user or retrieving information.

Another way that seems better suited to me is

var mongoose = require("mongoose")
var db = mongoose.connect("localhost:27107/users");

db.createCollection('users', function(err, collection) {});

I've seen several sites do something along these lines, but I personally can't get the above to work. I keep getting the error TypeError: db.createCollection is not a function server-side. So, my question is why the above code doesn't work, if the first code is a good alternative, and if there are any other ways to do this.

Answer 1

You can use a global variable to hold the connection (e.g. db), for example:

var db = null // global variable to hold the connection

MongoClient.connect('mongodb://localhost:27017/', function(err, client) {
    if(err) { console.error(err) }
    db = client.db('test') // once connected, assign the connection to the global variable
})

app.get('/', function(req, res) {
    db.collection('test').find({}).toArray(function(err, docs) {
        if(err) { console.error(err) }
        res.send(JSON.stringify(docs))
    })
})

Or, if you prefer, you can also use the Promise object that is returned by MongoClient if it is called without a callback argument:

var conn = MongoClient.connect('mongodb://localhost:27017/') // returns a Promise

app.get('/', function(req, res) {
    conn.then(client=> client.db('test').collection('test').find({}).toArray(function(err, docs) {
        if(err) { console.error(err) }
        res.send(JSON.stringify(docs))
    }))
})

Please note that I used the ES6 fat arrow function definition in the second example.

You are absolutely correct that you should not call MongoClient every time. Using a global variable or Promises allows the MongoDB node.js driver to create a connection pool, which achieves at least two good things:

• Connections are reused in a pool, so there is no multiple expensive setup/teardown process for the lifetime of your application. You connect once, and let the driver take care of the rest for you.
• You can control the amount of connection your application makes into the database, by limiting the size of the connection pool.

Edit 2018-08-24: The MongoClient.connect() method in node.js driver version 3.0 and newer returns a client object instead of a database object. The examples above were modified to keep it up to date with the latest node.js driver version.