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

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.