MongoDB query within JS function

Question

Im using MongoDB query within JS function. The script looks like this:

var continous = ['b', 'e', 'LBE'];

continous.forEach(e => izracun(e));

function izracun(atr) {
    var query1 = { "$match": { [atr]: {"$ne" : -1} } };
    var query2 = { "$group": { "_id": atr, "avg": { "$avg": "$"+[atr] }, "stdev": { "$stdDevPop": "$"+[atr] }, "nonMissing": { "$sum": 1 }}};

    db.ctg.aggregate([query1, query2]); 
}

When I execute it in mongo shell with load("script.js"), the shell returns "true". I tried using the query with fixed parameter values (instead of passing arguments) inside the function, but I still don't get the results (returns "true" only) that should look like example below:

{ "_id" : "b", "avg" : 878.4397930385701, "stdev" : 893.8744489449962, "nonMissing" : 2126 }

If I console log the query and run it directly in mongo shell, it works fine.

What am I doing wrong?

EDIT: I tried handling the promise with:

db.ctg.aggregate([query1, query2]).then(function (results) { //this block will run synchronsly to the aggregate statement
    console.log(results);
});

Im getting the following error:

uncaught exception: TypeError: db.ctg.aggregate(...).then is not a function

The statement db.ctg.aggregate([query1, query2]) returns a promise. You need to handle the promise via a .then(results => { // Code here }). All-in-all, it looks like: db.ctg.aggregate([query1, query2]).then(results => { // Code here }) — Evan Bechtol
– Evan Bechtol, Commented Jan 28, 2020 at 16:04

newbeski · Accepted Answer · 2020-01-28 16:38:40Z

1

The query returns a promise and then runs asynchronously in the background while the rest of your code carries on - you could change the code to be like the below to get an output:

var continous = ['b', 'e', 'LBE'];

continous.forEach(e => izracun(e));

function izracun(atr) {
   var query1 = { "$match": { [atr]: {"$ne" : -1} } };
   var query2 = { "$group": { "_id": atr, "avg": { "$avg": "$"+[atr] }, "stdev": {"$stdDevPop": "$"+[atr] }, "nonMissing": { "$sum": 1 }}};

    db.ctg.aggregate([query1, query2]).then(function (results) { //this block will run synchronsly to the aggregate statement
        console.log(results);
    }); 
    console.log("after"); //<-- this will apear before the results do despite being after it
}

If you need the function to wait on the query you could look at async/await (https://javascript.info/async-await) or your could return the promise and handle that elsewhere it depends on the greater context of your code.

answered Jan 28, 2020 at 16:38

newbeski

1414 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Marin Leontenko Over a year ago

I'm getting the following error if I try .then() : uncaught exception: TypeError: db.ctg.aggregate(...).then is not a function

newbeski Over a year ago

Hi, Sorry it could be that you are using a different driver can you try db.ctg.aggregate([query1, query2], function(err, result) { console.log(results)}); instead and see if that works?

Collectives™ on Stack Overflow

MongoDB query within JS function

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related