How to synchronize MongoDB async query in NodeJS

Question

I have a for-loop statement and an async MongoDB inside loop body. What I want to do is to make a find query from my MongoDB database, and push the result into an Array.

Here is the code:

function() arrResult() {
  var arr = [];
  for(...) {
    collection.find({ foo: i }, function (err, cursor) {
      arr.push(cursor);   
    }    
  }
  return arr;
}

But it's obvious that the return value of the function would be an empty Array.

I want to tackle this problem using Q module. Is there any solutions?

Bergi · Accepted Answer · 2014-02-25 18:47:44Z

2

I want to tackle this problem using Q module. Is there any solutions?

Yes, promises are a very easy abstraction to deal with this. You can execute the queries in parallel, and collect their results with all.

In particular, with Q it would look like this:

function arrResult(…) {
    var promises = [];
    for (…)
        promises.push( Q.ninvoke(collection, "find", {foo: i}) );
    return Q.all(promises);
}

arrResult(…).then(function(arr) {
    …
}, function(err) {
    // first error, if any occured
});

answered Feb 25, 2014 at 18:47

Bergi

671k162 gold badges1k silver badges1.5k bronze badges

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

4 Comments

Afshin Mehrabani Over a year ago

Great. Could you please complete it using Q.defer()?

Bergi Over a year ago

@AfshinMehrabani: Why would I need to? One never uses Q.defer manually. Q.ninvoke and the other callback-function method perfectly avoid this.

Afshin Mehrabani Over a year ago

Just for educational purposes.

Bergi Over a year ago

I think that's pretty well covered in Using deferreds in the Q docs. You can translate the example with FS.readFile directly into one for collection.find

mvw · Accepted Answer · 2014-02-25 18:43:25Z

1

You need a sync mechanism that acts like a process gate. Each returning query has to arrive at the gate, e.g. decrements some counter and deposit its result. When all arrived at the gate, a final callback does return the collected results.

answered Feb 25, 2014 at 18:43

mvw

5,1151 gold badge30 silver badges36 bronze badges

4 Comments

tymeJV Over a year ago

Interesting...do you have any examples of this? I'm pretty intrigued

mvw Over a year ago

I hand coded such, but it must exist in the Node.js ecosystem. gimme a moment.

Bergi Over a year ago

@mvw: You're looking for async.js' parallel function… However, notice that OP wants to use Q.

mvw Over a year ago

@Bergi: yes. The q Solution i don't know.

Collectives™ on Stack Overflow

How to synchronize MongoDB async query in NodeJS

2 Answers 2

4 Comments

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related