nodejs-async: return mysql query result in for loop

Question

I have three operations to do one after another

1.Fetch some rows from db

2.Another mysql query in forloop for getting some data and store in variable

3.Display the data

For that i am using async waterfall method.

async.waterfall([
            function(callback){
                //first sql and pass the result to second function
                collection.getAllCollections(function (status,error,result) {
                    callback(null,result);
                });
            },
           //running another query in loop with result with previous
            function(result, callback){
                for(var i=0;i<result.length;i++){
                    collection.getImages(result[i].id,function (status,error,user) {
                        //append the query result to old result
                         result[i]['users'] = user;
                    });
                }
                callback(null,result);
            }
        ], function (err, result) {

            console.log("result",result);
        });

But the problem final result does not contains the user results because the second query(query in for loop is asynchronous)

jpschack · Accepted Answer · 2017-05-09 12:50:44Z

1

You realised the problem at hand. Your callback basically has to wait for the for loop to end. For example like this:

async.waterfall([
    function(next){
        //first sql and pass the result to second function
        collection.getAllCollections(function (status,error,result) {
            next(null, result);
        });
    },
    function(result, next){
        var calls = [];

        //putting every call in an array
        result.forEach(function(resultObject){
            calls.push(function(callback) {
                collection.getImages(resultObject.id, function (status, error, user) {
                     resultObject['users'] = user;
                     callback(null, resultObject);
                });
            }
        )});

        //performing calls async parallel to each other
        async.parallel(calls, function(err, results) {
            //executed as soon as all calls are finished
            if (err) {
                next(err, null);
            } else {
                next(null, results);
            }
        });
    }
], function (err, result) {

    console.log("result",result);
});

Documentation: http://caolan.github.io/async/docs.html#parallel

edited May 9, 2017 at 12:50

answered May 9, 2017 at 8:34

jpschack

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5 Comments

Jabaa Over a year ago

But where can put isDone function?? i am using async waterfall method

jpschack Over a year ago

@Jabaa I updated my code to clarify my solution. This should work but I didn't test it.

jpschack Over a year ago

@Jabaa I updated my answer again. There is a better, cleaner and more efficient solution with async.parallel available. That should get you in the right direction.

Jabaa Over a year ago

Thanks let me try it

jpschack Over a year ago

@Jabaa please accept my answer if it works for you :)

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