Node.js MongoDb ensure collection async workflow

Question

Node.js MongoDb ensure collection async workflow

I am pretty new to Node.js and I am using the MongoDb driver, I understand the workflow of Node.js is eventdriven and everything runs asynchronous but I just don't understand how I can do things like this:

var mongo = require('mongodb');
var db = new mongo.Db('meta', new mongo.Server('localhost', 27017, { auto_reconnect: true }));

db.open(function(error, db){
  if(error){
    throw error;
  }

  db.collection('logs', function(error, collection){
    if(!collection){
      db.createCollection('logs', callback); // How does this callback works? I mean, how can we continue the previous execution of the script?
    }

    // Have fun.
  });
});

So with the script above I want to:

Open the connection
Check for logs collection, if not create, else continue...
Create a user? Do work?

I am still getting used to the Asynchronous model, thanks for help already.

java
node.js
mongodb

Answer 1

What I usually do in this sort of scenario is wrap the continuation in a named callback:

if (!collection) {
    db.createCollection('logs', next);
} else {
    next(null, collection);
}

function next(err, collection) {
    // do something
}

That being said, the createCollection method will also return the collection object and only create it if it does not already exist. You can just write:

db.createCollection('logs', function(err, collection) {
    // do something
});