How to query a 'join' collection in MongoDb using Node.js?

Question

I'm using Node.js and mongoose to query a mongodb database. It is very simple scenario but I cannot figure out how to do it properly.

I have a collection to query and then given the result, need to fetch object from another collection to populate the result. code sample as below:

q.exec( function (e, docs){
    if (e || !docs) {
        res.status(400);
        res.send("Error while processing request.\n"+ JSON.stringify(e, null, 4));
    }else{
         //do the JOIN here.
         var result = _(docs).map(
            function(doc){
                markModel.findOne(
                   {question_id: doc._id, user_id: user_id},
                   function (err, markDoc){
                     if (markDoc)
                        doc.mark = markDoc;
                   });
              //HOW and what to return here??
            });
         res.send(result);
    }
});

In the code above, docs is the result of the first query; and then I need to find the mark from another collection for each of the doc in docs , and attach it to the doc object. Because the second query is also asyn called, I don't know what should be returned in the map() function, without using defer or promise, etc.

Answer 1

MongoDB is a non-relational database and doesn't support joins.
For this needs please check data models documentation .
Also look at practice of using MapReduce : http://cookbook.mongodb.org/patterns/pivot/

Answer 2

Node.js behaviors is asynchronous in which callbacks will be executed after getting result set . In your code you are doing multiple call to mongodb , which can be avoided by using $in construct . Doing multiple mongo call in loop is bad design where you can't ensure when to return or beak loop , because you don't know when call back will be called . To avoid this just collect all doc _id and do one mongo call using $in construct . This will avoid the confusion of when to send response .Something like :

q.exec(function (e, docs) {
        if (e || !docs) {
            res.status(400);
            res.send("Error while processing request.\n" + JSON.stringify(e, 
                                                                 null, 4));
        } else {
            //do the JOIN here.     
            var docId = _(docs).map(
                function (doc) {
                    return parseInt(doc._id);
                });
            markModel.findOne({
                    question_id: {
                        $in: docId
                    },
                    user_id: user_id
                },
                function (err, markDoc) {
                    if (markDoc)
                        doc.mark = markDoc;
                    res.send(result); // send result
                });


        }
    });