Closing mongodb connection and getting correct results with multiple concurrent asynchronous requests

I am new to Mongo and Node. I am currently using Mongoskin and Bluebird to communicate with db connection and queries (as suggested here: https://stackoverflow.com/a/1193475/ ... ).

I have three collections: users, bindings and maps.

The Binders collection contains information about Maps for each user. Each document in Binders has the following properties:

User Id <--- that refers to the User owning the Card
Card Code <--- that refers to a Card
Count <--- that refers to the number of cards owned by the User

      

        
        
        
      

    

I prefer to have a separate collection of maps so that when the Map changes, it changes for all user bindings.

Now I want to get an array for a given user, for example:

[{card: {card document}, count: 4}, ...]

      

        
        
        
      

    

I have the following problems:

• the db connection must be closed after all asynchronous callbacks have been called
• an array of cards should be returned after the last db.collection ('cards'). find returns results

I know my following code is wrong, but might be a starting point for discussion:

var getAllBinderCards = function(req, res){
    var db = req.db;
    var userId = req.userId;

    var promiseBinders = db.collection('binders').find({userId: userId}).toArrayAsync();

    promiseBinders.then(function(binderCards) {
        if (binderCards) {
            var promiseCards;
            //console.log("------ binderCards: ", binderCards);
            var cards = [];
            for (var i = 0; i < binderCards.length; i++) {
                var binderCard = binderCards[i];

                promiseCards = db.collection('cards').find({Code: binderCard.Code}).toArrayAsync();
                promiseCards.then(function(cardsDB){
                    if(cardsDB){
                        //console.log("Cards found: ",binderCard.Code, cardsDB);
                        for (var i = 0; i < cardsDB.length; i++) {
                            cardsDB[i].count = binderCard.count;
                        };
                        cards.concat(cardsDB);
                    }
                }); 
            }

            promiseCards.then(function(){
                db.close();
                console.log("Binder Cards: " , cards);
                res.json(cards);
            });
        }
    });
}

      

        
        
        
      

    

I am struggling to figure out how to properly handle the aphorized successful call to send the whole array and close the db connection.

I think I should try to build a promise before the for loop and use it to bind the request to the promises cards and finally bind the db.close () and res.json (cards) statements.

[EDIT] Perhaps the simplest solution is to simply use the $ in filter inside a single db.collection ('cards'). find ({Code: {$ in: [bindersCodeArray]}}). toArrayAsync (); and avoid this for the loop:

var getAllBinderCards = function(req, res){
    var db = req.db;
    var userId = req.userId;

    var promiseBinders = db.collection('binders').find({userId: userId}).toArrayAsync();

    promiseBinders.then(function(binderCards) {
        if (binderCards) {
            var binderCodes = binderCards.map(function(element){
                return element.Code;
            });
            var promiseCards = db.collection('cards').find({Code: {$in: binderCodes} }).toArrayAsync();
            promiseCards.then(function(cards){

                var bindersDictionary = {};

                for (var i = 0; i < binderCards.length; i++) {
                    bindersDictionary[binderCards[i].Code] = binderCards[i].count;
                };

                for (var i = 0; i < cards.length; i++) {
                    cards[i].count = bindersDictionary[cards[i].Code];
                };

                db.close();
                console.log("Binder Cards: " , cards);
                res.json(cards);
            });

        }
    });
}

      

        
        
        
      

    

However, I am curious if there is an elegant way to solve this puzzle using promises.