issue in printing a query from mongodb

I'm trying to learn more aboute node.js(express framework) and mongodb (official driver).

I've this code

  db.open(function(err,db){
    if(!err){
      console.log ('we are connected');

      db.collection('post',function(err,collection){

        collection.find(function(err,posts){
          if(!err){
          res.json(posts);
          }
        });

     });

  }
 });

It should query the db and print the results. But I cannot figure out how to print the query to the browser in json format.

I've already tried the JSON.stringify but without any result :(

Doing:

    collection.find().toArray(function(err,posts){
      if(!err){
        res.json(posts);
      }
    });

(note the toArray function) it works, but I've heard that this is a bad practice because wastes a lot of memory.

Is there any more efficient way of doing it?