How to get a callback on MongoDB collection.find()

Question

How to get a callback on MongoDB collection.find()

When I run collection.find() in MongoDB/Node/Express, I'd like to get a callback when it's finished. What's the correct syntax for this?

 function (id,callback) {

    var o_id = new BSON.ObjectID(id);

    db.open(function(err,db){
      db.collection('users',function(err,collection){
        collection.find({'_id':o_id},function(err,results){  //What's the correct callback synatax here?
          db.close();
          callback(results);
        }) //find
      }) //collection
    }); //open
  }

node.js
mongodb
express

Answer 1

That's the correct callback syntax, but what find provides to the callback is a Cursor, not an array of documents. So if you want your callback to provide results as an array of documents, change the find to:

collection.find({'_id':o_id}, function(err, cursor){
    cursor.toArray(callback);
    db.close();
});

Note that your function's callback still needs to provide an err parameter so that the caller knows whether the query worked or not.

Answer 2

Based on JohnnyHK answer I simply wrapped my calls inside db.open() method and it worked. Thanks @JohnnyHK.

app.get('/answers', function (req, res){
     db.open(function(err,db){ // <------everything wrapped inside this function
         db.collection('answer', function(err, collection) {
             collection.find().toArray(function(err, items) {
                 console.log(items);
                 res.send(items);
             });
         });
     });
});

Hope it is helpful as an example.