Node.js - mysql query inside foreach

Question

I have this structure of code:

connection.query(query1,function(err,rows) {
    var response = [];
    //doing something with rows
    rows.forEach(function(item) {
        connection.query(queryItem,function(err,rows) {
            //doing something
            result = rows[0].field;
            //and want to push it to an array
            response.push(result);
        });
    });
    console.log(response); //empty
});

I know that forEach is blocking but query is non-blocking. I tried to use promises:

connection.query(query1,function(err,rows) {
    var response = [];
    //doing something with rows
    rows.forEach(function(item) {
        var promise = new Promise(function(resolve,reject) {
            connection.query(queryItem,function(err,rows) {
                //doing something
                result = rows[0].field;
                //and want to push it to an array
                resolve(result);
            });
        });
        promise.then(function(result) {
            console.log(result); //ok
            response.push(result) //not ok, result is empty
        });
    });
    console.log(response); //empty
});

But it's not helped. How can I push value into array from non-blocking function and use it after?

I'm a newbie in node. require('promise'); May be native, i don't know =) — Pavel Trefilov
– Pavel Trefilov, Commented Oct 3, 2015 at 9:46

user400654 · Accepted Answer · 2015-10-02 22:06:03Z

4

Creating promises is a step in the right direction, but you need to take it a step further by aggregating the promises into an array and passing to Promise.all, and then waiting for them all to finish before trying to access response.

connection.query(query1,function(err,rows) {
    var response = [];
    //doing something with rows
    Promise.all(rows.map(function(item) {
        var promise = new Promise(function(resolve,reject) {
            connection.query(queryItem,function(err,rows) {
                //doing something
                result = rows[0].field;
                //and want to push it to an array
                resolve(result);
            });
        });
        return promise.then(function(result) {
            console.log(result); //ok
            response.push(result) //ok
        });
    }).then(function () {
        console.log(response); //not empty
    });
});

answered Oct 2, 2015 at 22:06

user400654

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3 Comments

user400654 Over a year ago

Note that this results in sending n queries to your database a the same time, it would likely be better to combine them all into a single transaction, which would also remove the need for promises in this case. Disregard if this is a SELECT statement, in which case you might be able to just make a single query that gets the data and then group by item instead. Sending queries in a loop like this is usually a bad idea performance-wise.

Pavel Trefilov Over a year ago

It's work! But if I do "console.log" outside Promise.all - this is empty. Is it mean that I must put my next code into promise.then? Or are there any way?

Pavel Trefilov Over a year ago

And thank you for advice about complex SELECT-query. I'll try it.

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