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?

score 2 · Accepted Answer · answered Feb 25 '14 at 18:47

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 '14 at 18:47

Bergi

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Great. Could you please complete it using `Q.defer()`? – Afshin Mehrabani Feb 25 '14 at 18:59
@AfshinMehrabani: Why would I need to? One [never uses `Q.defer`](https://github.com/petkaantonov/bluebird/wiki/Promise-anti-patterns#wiki-the-deferred-anti-pattern) manually. `Q.ninvoke` and the other [callback-function method](https://github.com/kriskowal/q/wiki/API-Reference#wiki-promise-for-function-methods) perfectly avoid this. – Bergi Feb 25 '14 at 19:03
Just for educational purposes. – Afshin Mehrabani Feb 25 '14 at 19:12
1

I think that's pretty well covered in [*Using deferreds* in the Q docs](https://github.com/kriskowal/q/#using-deferreds). You can translate the example with `FS.readFile` directly into one for `collection.find` – Bergi Feb 25 '14 at 19:17

score 1 · Answer 2 · answered Feb 25 '14 at 18:43

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 '14 at 18:43

mvw

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Interesting...do you have any examples of this? I'm pretty intrigued – tymeJV Feb 25 '14 at 18:45
I hand coded such, but it must exist in the Node.js ecosystem. gimme a moment. – mvw Feb 25 '14 at 18:49
@mvw: You're looking for [async.js' `parallel` function](https://github.com/caolan/async#parallel)… However, notice that OP wants to use Q. – Bergi Feb 25 '14 at 18:51
@Bergi: yes. The q Solution i don't know. – mvw Feb 25 '14 at 18:55

How to synchronize MongoDB async query in NodeJS

2 Answers2