Using Async Parallel for multiple MongoDB aggregate functions

Question

Hello I need to do multiple aggregate queries, and render the results to a view. My problem right now is figuring out how to make sure all the queries finish before rendering them and I have come upon async.parallel. I have been trying to mess with it but it is not working. For example, this is what I have right now.

    var async = require('async');
    async.parallel({
    one: function(){
        Country.aggregate([
            {
                $match: {
                name: { $in: ["USA", "China", "India", "France", "Japan"]},
                }
            },
            {
                $sort: {
                    year: 1
                }
            },
            {
                $group: {
                    _id: '$name',
                    nominalGDP: {$push: "$nominalGDP"}
                }
            }, 
            {
                $project: {
                _id: 0,
                name: "$_id",
                nominalGDP: 1
                }
            }
            ], function(err, recs){
            if(err){
                console.log(err);
            } else {
                console.log(recs);
            }
        });
    },

    two: function(){
        Country.aggregate([
            {           
                $match: {
                    $and: [
                        {name: { $in: ["USA", "China", "India", "France", "Japan"]}},
                        {year: 2017}
                    ]
                }
            },
            {
                $sort: {
                    "nominalGDP": -1
                }
            },
            {
                $project: {
                _id: 0,
                name: 1,
                'y' : '$nominalGDP'
                }
            }       
            ], function(err, recs){
            if(err){
                console.log(err);
            } else {
                console.log(recs);
            }
        });
    }
}, function(err, results){
    res.render('random', { title: 'Test', data: results.one, pie: results.two });
});

So I have two aggregate functions that each produce their results. The two functions are printing their results properly, but no data is being rendered to my view (random).

what do you expect results.one and results.two will be? Neither of the functions returns anything either, so, how do you expect the results to be available in function(err, results) ? — Jaromanda X
– Jaromanda X, Commented Feb 28, 2018 at 2:54
change one: function(){ to one: function(callback){ ... and instead of function(err, recs){ .... } replace that with callback ... repeat for two — Jaromanda X
– Jaromanda X, Commented Feb 28, 2018 at 3:01
ohhh i see. well just adding 'callback' like how h1b9b and Jaromanda X said did the trick, thanks a lot — Jose Varela
– Jose Varela, Commented Feb 28, 2018 at 3:04

h1b9b · Accepted Answer · 2018-02-28 03:08:29Z

1

The tasks have the callback function as argument
So the return data from a task, it should look like

task: function(callback) {
   ...
   callback(null, result);
}

In this case, you can use the callback as the second argument to Country.aggregate

edited Feb 28, 2018 at 3:08

answered Feb 28, 2018 at 3:00

h1b9b

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Jaromanda X Over a year ago

or, in the case of the code in the question, instead of using function(err, recs) { .... } just use callback

RITESH ARORA · Accepted Answer · 2018-02-28 03:51:43Z

Parllel require a callback function which make sure that function is fully executed and now its time to return.

Even callback take 2 parameters , 1. Err (which will null if success) 2. Data(which need to send in success)

var async = require('async');
    async.parallel({
    one: function(cb){
        Country.aggregate([
            {
                $match: {
                name: { $in: ["USA", "China", "India", "France", "Japan"]},
                }
            },
            {
                $sort: {
                    year: 1
                }
            },
            {
                $group: {
                    _id: '$name',
                    nominalGDP: {$push: "$nominalGDP"}
                }
            }, 
            {
                $project: {
                _id: 0,
                name: "$_id",
                nominalGDP: 1
                }
            }
            ], function(err, recs){
            if(err){
               cb(err);
            } else {
               cb (null , recs);
            }
        });
    },

    two: function(cb){
        Country.aggregate([
            {           
                $match: {
                    $and: [
                        {name: { $in: ["USA", "China", "India", "France", "Japan"]}},
                        {year: 2017}
                    ]
                }
            },
            {
                $sort: {
                    "nominalGDP": -1
                }
            },
            {
                $project: {
                _id: 0,
                name: 1,
                'y' : '$nominalGDP'
                }
            }       
            ], function(err, recs){
            if(err){

                cb(err);
            } else {
               cb(null, recs);
            }
        });
    }
}, function(err, results){
    res.render('random', { title: 'Test', data: results.one, pie: results.two });
});

You can read further details on https://github.com/caolan/async/blob/v1.5.2/README.md#parallel

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