Issue in returning data retrieved from DB queries called in the loop

Question

I making multiple mongoDB queries in loop. and want to send the all results as one data array.But when I simple use the return for send the data it simply return undefined and do not wait for results of all DB request. I also tried to use q.moulde but same issue.

Code:

var getPrayerInCat = function(data){
    var result ;
    var finalData = [];
    if(data.length >0){
             data.forEach(function(data2){
                 var id= data2.id;
                 Prayer.find({prayerCat:id},function(err,prayer){
                     var deferred = Q.defer()
                     if (err) { // ...
                         console.log('An error has occurred');
                         // res.send(err);
                         result= finalData = err
                     } else {
                         if(!prayer){
                             // console.log(data2.id+'--0');
                             data2.prayersCount = 0;
                             result = deferred.resolve(finalData.push(data2))
                         } else {
                             // console.log(data2.id+'--'+prayer.length);
                             data2.prayersCount = prayer.length;
                             // console.log(prayer)
                             result =  deferred.resolve(finalData.push(data2))
                         } // else for data forward
                     }
                     deferred.promise;
                 })
                // deferred.resolve(finalData);

             })
             /*if(finalData.length > 0) { return finalData;}*/
        }
}

finalData is returned undefined.

its a promise. I can remove it, i was trying to solve the prob so i tried this thing — Muhammad Mudassir
– Muhammad Mudassir, Commented Sep 9, 2014 at 19:00
Your function getPrayerInCat() doesn't return anything or return anything to a callback. Also it would be helpful if you removed the comments from your code and updated your post to include what your excepted output should be. And should you be return deferred.promise;, instead of just showing it? — Larry Battle
– Larry Battle, Commented Sep 9, 2014 at 19:10
I think finalData is undefined because it's being access before it's populated from your deferred object. — Larry Battle
– Larry Battle, Commented Sep 9, 2014 at 19:12
@MuddassirKhanani: Afair, Q has no moulde method. What do you mean? Where did you use it in your code? — Bergi
– Bergi, Commented Sep 9, 2014 at 19:23

Community · Accepted Answer · 2017-05-23 12:17:32Z

21

Let's start with the general rule for using promises:

Every function that does something asynchronous must return a promise

Which functions are these in your case? It's getPrayerInCat, the forEach callback, and Prayer.find.

Hm, Prayer.find doesn't return a promise, and it's a library function so we cannot modify it. Rule 2 comes into play:

Create an immediate wrapper for every function that doesn't

In our case that's easy with Q's node-interfacing helpers:

var find = Q.nbind(Prayer.find, Prayer);

Now we have only promises around, and do no more need any deferreds. Third rule comes into play:

Everything that does something with an async result goes into a .then callback

…and returns the result. Hell, that result can even be a promise if "something" was asynchronous! With this, we can write the complete callback function:

function getPrayerCount(data2) {
    var id = data2.id;
    return find({prayerCat:id})
//  ^^^^^^ Rule 1
    .then(function(prayer) {
//  ^^^^^ Rule 3
        if (!prayer)
            data2.prayersCount = 0;
        else
            data2.prayersCount = prayer.length;
        return data2;
//      ^^^^^^ Rule 3b
    });
}

Now, we have something a bit more complicated: a loop. Repeatedly calling getPrayerCount() will get us multiple promises, whose asynchronous tasks run in parallel and resolve in unknown order. We want to wait for all of them - i.e. get a promise that resolves with all results when each of the tasks has finished.

For such complicated tasks, don't try to come up with your own solution:

Check the API of your library

And there we find Q.all, which does exactly this. Writing getPrayerInCat is a breeze now:

function getPrayerInCat(data) {
    var promises = data.map(getPrayerCount); // don't use forEach, we get something back
    return Q.all(promises);
//  ^^^^^^ Rule 1
}

If we needed to do anything with the array that Q.all resolves to, just apply Rule 3.

edited May 23, 2017 at 12:17

CommunityBot

11 silver badge

answered Sep 10, 2014 at 2:48

Bergi

671k162 gold badges1k silver badges1.5k bronze badges

Sign up to request clarification or add additional context in comments.

10 Comments

Muhammad Mudassir Over a year ago

I have used your code but same issue it return form find and donot wait for resukt from db . So the final result is undefined.

Muhammad Mudassir Over a year ago

getPrayerCat('').then(function(data){ getPrayerInCat(data) }).then(function(data2){ console.log(data2) }) var getPrayerInCat = function(data){ var result ; var finalData = []; if(data.length >0){ var promises = data.map(getPrayerCount); return Q.all(promises); } }

Muhammad Mudassir Over a year ago

Bergi Over a year ago

You forgot about Rule 1 in your then callback. It needs to be getPrayerCat('').then(function(data){ return getPrayerInCat(data); }).then(…). Without returning anything, the promise resolves to undefined…

Muhammad Mudassir Over a year ago

I again have same issue this time one more loop is added @Bergi

|

Collectives™ on Stack Overflow

Issue in returning data retrieved from DB queries called in the loop

1 Answer 1

10 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

10 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related