nodejs async parallel inside an async each

Question

Firstly, I am new to nodejs. Working on an API where I have to do multiple queries inside an each loop. So, I started using async module with express. The exact code that I am using is

        new_or_updated = {};
        async.each(rows, function(e, callbackParent) {
            new_or_updated.id = e.id;
            async.parallel([
                function(callback){
                    db.query("SELECT sender_string FROM  filter_positive_rules_sender WHERE rule_id = ? AND is_deleted = 0", e.id,function(err, rows, fields){
                        if (err) {
                            return callback(err);
                        }
                        new_or_updated.sender = rows.map(x => x.sender_string);
                        callback(null);
                    });
                },
                function(callback){
                    db.query("SELECT subject_string FROM filter_positive_rules_subject WHERE rule_id =  ? AND is_deleted = 0", e.id ,function(err, rows, fields){
                        if (err) {
                            return callback(err);
                        }
                        new_or_updated.subject = rows.map(x => x.subject_string);
                        callback(null);
                    });  
                },
                function(callback){
                    db.query("SELECT body_string FROM filter_positive_rules_body WHERE rule_id = ?  AND is_deleted = 0", e.id ,function(err, rows, fields){
                        if (err) {
                            return callback(err);
                        }
                        new_or_updated.body = rows.map(x => x.body_string);
                        callback(null);
                    });
                }
            ],
            function(err) {
                if (err) {
                    res.status(500).json(
                        "Internal server error"
                    ).send();
                }
                console.log(new_or_updated):
                callbackParent(null, new_or_updated);
            });

        },
        function(err, result) {
            if (err) {
                res.status(500).json(
                    "Internal server error"
                ).send();
            }
            console.log(result)
        });

As you can see I am trying to populate an object using the async module. When I console new_or_updated in the parallel async, the array is properly built. However, when I send the variable to the each async callback and console it (result variable) I get undefined. Also, when I console new_or_updated in the parentCallback, I only get one element in the array(the last array build using each async module).

What am I doing wrong here? Could someone explain to me how nested async is actually supposed to work in practice.

Use a global variable to store the values. Try initializing the variable outside your function that performs the async operations — ArUn
– ArUn, Commented Jan 12, 2017 at 6:07
@ArUn The new_or_updated variable is initialized outside both async's. — Fahad Sadiq
– Fahad Sadiq, Commented Jan 12, 2017 at 6:27

Rohit Nayal · Accepted Answer · 2017-01-12 10:18:29Z

1

As per code base of async, "async.each" is not designed to take two parameters in the callback.

It would always be invoked with a single "err" only (or without any parameter) callback parameter.

Also while setting a global object's property inside "async.each" operation would overwrite the the property in case of any asynchronous operation. This means the value of "new_or_updated.id" set in the first iteration may be overwritten by any subsequent iteration of "async.each".

So ideally the global object here "new_or_updated" should be an array of objects with some unique property value for each object. so as to hold the value and context of each iteration on "async.each"

answered Jan 12, 2017 at 10:18

Rohit Nayal

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Comments

Fahri · Accepted Answer · 2017-04-28 17:35:58Z

Return to callback, .. try this. I'm using async.map and async.waterfall Sorry if wrong and the dangers of bad English, I'm just a beginner I was just a student.

async.map(rows, function(e, callbackParent) {
    async.waterfall([
        function(callback){
            db.query("SELECT sender_string FROM  filter_positive_rules_sender WHERE rule_id = ? AND is_deleted = 0", e.id,function(err, rows, fields){
                if (err) {
                    return callback(err);
                }
                e.sender = rows.map(x => x.sender_string);
                callback(null, e );
            });
        },
        function(Obj, callback){
            db.query("SELECT subject_string FROM filter_positive_rules_subject WHERE rule_id =  ? AND is_deleted = 0", e.id ,function(err, rows, fields){
                if (err) {
                    return callback(err);
                }
                Obj.subject = rows.map(x => x.subject_string);
                callback(null, Obj);
            });
        },
        function(Obj, callback){
            db.query("SELECT body_string FROM filter_positive_rules_body WHERE rule_id = ?  AND is_deleted = 0", e.id ,function(err, rows, fields){
                if (err) {
                    return callback(err);
                }
                Obj.body = rows.map(x => x.body_string);
                callback(null, Obj);
            });
        }
    ],
    function(err, result) {
        callbackParent(err, result);
    });
},
function(err, result) {
    if (err) {
        res.status(500).json(
            "Internal server error"
        ).send();
    }
    res.send( result )
});

I test like this is work

function test( callback ) {
    callback( [
        {
            'name':'Anu'
        },
        {
            'name':'hem'
        }
    ] );
}
test( function( aaaaaaaaaaa ) {
    async.map( aaaaaaaaaaa, function( e, callbackParent ) {
        async.waterfall( [
            function( callback ) {
                e.number = 123;
                return callback( null, e );
            },
            function( obj, callback ) {
                obj.kelas = '1';
                return callback( null, obj );
            }
        ], function( eerr, r ) {
            callbackParent( eerr, r );
        } );
    }, function( erro, ok ) {
        res.send( ok );
    } );
} );

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