Sequential execution of nodejs code with mongodb query

Question

I am having situation where I need to fire a mongodb queries from nodejs(expressjs) code and pass the result of the query as a response to the request of my web api, but before my query executes the next lines of code gets execute and the blank response is sent back. The code that I have written is as follows,

router.get('/recent', function (req, res){
    var result = [];
    router.db.collection('Posts').find({},{ _id : 1, uid : 1, imagePath : 1, cntLikes : 1, uploadDate : 1}).limit(5).sort({ uploadDate : -1 }).toArray(function(err, docs) {
                docs.forEach( function (doc){
                        router.db.collection('Users').findOne({ "_id" : mongodb.ObjectId(doc.uid)}, function (err, user){
                            doc.username = user;
                            result.push(doc);
                            });
                });


    res.send(result);     // this line send empty result to caller
    });
});

I have tried callback methods, async.waterfall and programmatically stop the execution but everything failed. I have also tried promises but i dont think promises can help me in this scenario.

Jim Jeffries · Accepted Answer · 2015-04-10 09:55:15Z

1

You are using another asynchronous call within your loop, which is being executed after res.send(result). I think what you're after is async.map, so maybe something along the lines of this within your posts callback

function addUserToPost(item, callback) {
    router.db.collection('Users').findOne({ "_id" : mongodb.ObjectId(doc.uid)}, function (err, user){
        doc.username = user;
        callback(err, doc);
    });
}

async.map(docs, addUserToPost, function(err, posts) {
    res.send(posts);
});

You want to map from you 'Posts' to Posts with user information added.

answered Apr 10, 2015 at 9:55

Jim Jeffries

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1 Comment

Jim Jeffries Over a year ago

Although @subburaj's answer is cleaner IMO

Subburaj · Accepted Answer · 2015-04-10 09:57:21Z

Try for this:

router.get('/recent', function (req, res){
    var result = [];
    router.db.collection('Posts').find({},{ _id : 1, uid : 1, imagePath : 1, cntLikes : 1, uploadDate : 1}).limit(5).sort({ uploadDate : -1 }).toArray(function(err, docs) {
    var counter = 0;
                docs.forEach( function (doc, index){
                        counter++;
                        router.db.collection('Users').findOne({ "_id" : mongodb.ObjectId(doc.uid)}, function (err, user){
                            doc.username = user;
                            result.push(doc);
                            if(counter === index){
                             res.send(result);    
                            }
                            });
                });        
    });
});

Thanks for your solution @Subburaj it worked but I found that if query for retrieving last record executes before any other query or if any other query takes more time to complete than the last query then result doesn't contain that record(more time consuming query's record) so I have to miss that record so can you tell me how to tackle that situation.

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