How to make a for statement wait multiple mongoose queries? - node.js

Question

What I am currently trying to do is to display all the playlists with the songs in it. To do that I first find every playlists, then I do a for to loop through them all (in the same time I initialize globalArr and put the values then it will be sended as json because it's an API) and the problem is when I do another find in the loop(PlaylistSong.find or Song.find) well since it's asynchronous the find will be made when the for will be over, and I will have 0 results because they will take the value of increment when he will be at his maximum. I heard of async, I even googled but I really don't understand how to put through this code because it's a combination of for loops and async queries...
Thanks for your help.
router.get('/', function(req, res, next) {
Playlist.find(function (err, playlists) {
if (err) return next(err);
/* Loop through every playlists */
var globalArr = [];
for (var increment = 0; increment < playlists.length; ++increment)
{
globalArr[increment] = [];
globalArr[increment]["name"] = playlists[increment].name;
/* Loop through every links between Songs and Playlist */
PlaylistSong.find({idPlaylist: playlists[increment]._id}, function (err, songs) {
if (err) return next(err);
for (var songIncrement = 0; songIncrement < songs.length; ++songIncrement) {
{
console.log("increment"+increment);
globalArr[increment][songIncrement] = [];
/* Getting the actual song by his ID */
Song.find({_id: song.idSong}, function (err, song) {
if (err) return next(err);
globalArr[increment][songIncrement]["name"] = songs[songIncrement].name;
globalArr[increment][songIncrement]["artist"] = songs[songIncrement].artist;
globalArr[increment][songIncrement]["picture"] = songs[songIncrement].picture;
globalArr[increment][songIncrement]["price"] = songs[songIncrement].price;
globalArr[increment][songIncrement]["file"] = songs[songIncrement].file;
globalArr[increment][songIncrement]["difficulty"] = songs[songIncrement].difficulty;
globalArr[increment][songIncrement]["downloaded"] = songs[songIncrement].downloaded;
});
}
}});
}
res.contentType('application/json');
res.send(JSON.stringify(globalArr));
});
});

Answer 1

See this question and the accepted answer:
Simplest way to wait some asynchronous tasks complete, in Javascript?
It basically says to use the Async module, push all of your async function calls onto it and then use async.parallel() which gives you a callback when all of the async functions have completed.
I haven't tested it, but something like this seems like it might work:
var async = require('async');
var calls = [];
router.get('/', function(req, res, next) {
Playlist.find(function (err, playlists) {
if (err) return next(err);
/* Loop through every playlists */
var globalArr = [];
for (var increment = 0; increment < playlists.length; ++increment)
{
(function() {
var i = increment;
calls.push(function(callback) {
globalArr[i] = [];
globalArr[i]["name"] = playlists[i].name;
/* Loop through every links between Songs and Playlist */
PlaylistSong.find({idPlaylist: playlists[increment]._id}, function (err, songs) {
if (err) return next(err);
for (var songIncrement = 0; songIncrement < songs.length; ++songIncrement) {
{
console.log("increment"+i);
globalArr[i][songIncrement] = [];
/* Getting the actual song by his ID */
Song.find({_id: song.idSong}, function (err, song) {
if (err) return next(err);
globalArr[i][songIncrement]["name"] = songs[songIncrement].name;
globalArr[i][songIncrement]["artist"] = songs[songIncrement].artist;
globalArr[i][songIncrement]["picture"] = songs[songIncrement].picture;
globalArr[i][songIncrement]["price"] = songs[songIncrement].price;
globalArr[i][songIncrement]["file"] = songs[songIncrement].file;
globalArr[i][songIncrement]["difficulty"] = songs[songIncrement].difficulty;
globalArr[i][songIncrement]["downloaded"] = songs[songIncrement].downloaded;
});
}
callback();
}});
});
})();
}
async.parallel(calls, function(err, result) {
if (err) {
// TODO: Handle error here
}
res.contentType('application/json');
res.send(JSON.stringify(globalArr));
});
});
});
or if you don't want then to execute in parallel, you can use async.series() instead.
See this jsFiddle for a simplified example of your situation... https://jsfiddle.net/bpursley/fj22hf6g/

Answer 2

Yeah, you should use async. I'll explain this in more detail later (son must go to bed...)
PlaylistSong.statics.findSongsByPlaylistId = function(id, done) {
PlaylistSong.find({idPlaylist: id}, function(err, songs) {
if (err) {
done(err)
return
}
var getSongsFns = songs.map(function(song) {
return function(callback) {
Song.find({_id: song.idSong}, callback)
}
})
async.parallel(getSongsFns, done)
})
}
router.get('/', function(req, res, next) {
Playlist.find(function (err, playlists) {
if (err) return next(err);
var getSongsFns = playlists.map(function(playlist) {
return function(callback) {
PlaylistSong.findSongsByPlaylistId(playlist._id, callback)
}
})
async.parallel(getSongsFns, function(err, songs) {
if (err) {
res.status(500).send()
return
}
res.contentType('application/json');
res.send(JSON.stringify(songs));
})
});
});