Asynchronous calls inside loop

Possible Duplicate:
Coordinating parallel execution in node.js

First, hands on pseudo-code:

forEach(arrayelements) {
  asyncQueryFunction(function(qres) {
    //work with query results.
  });
}
// finally, AFTER all callbacks did return:
res.render("myview");

How to do that?

If that wasn't clear enough, I would explain:

I need to do a series of "update" queries (in mongodb, via mongoose), looping over a list of document ids. For each id in my array I will call an asynchronous function which will return query results (I don't need to do anything with them, actually).

I know I have to use .forEach() javascript loop, but how can I execute my "final" callback only when ALL of my asynchronous queries did finish?

I'm already using the excellent async library (https://github.com/caolan/async) for achieving this kind of task when I've got a "limited" series of task to execute. But I don't think I can pass it an array of different functions.

CAN I?

very simple pattern is to use 'running tasks' counter:

var numRunningQueries = 0
forEach(arrayelements) {
  ++numRunningQueries;
  asyncQueryFunction(function(qres) {
    //work with query results.
    --numRunningQueries;
    if (numRunningQueries === 0) {
       // finally, AFTER all callbacks did return:
       res.render("myview");
    }
  });
}

or, alternatively, use async helper library such as Async.js

If I understand correctly, asyncQueryFunction is always the same, as in you're applying the same update to each document.

I use a helper method to callback after saving (just swap for update) multiple mongoose documents (converted from CoffeeScript, so it may not be perfect):

function saveAll(docs, callback) {

  // a count for completed operations, and save all errors
  var count = 0
    , errors = [];

  if (docs.length === 0) {
    return callback();
  } else {
    for (var i = 0; i < docs.length; i++) {

      // instead of save, do an update, or asyncQueryFunction
      docs[i].save(function(err) {

        // increase the count in each individual callback
        count++;

        // save any errors
        if (err != null) {
          errors.push(err);
        }

        // once all the individual operations have completed,
        // callback, including any errors
        if (count === docs.length) {
          return callback(errors);
        }
      });
    }
  }
};

saveAll(arrayElements, function(errors) {
  // finally, AFTER all callbacks did return:
  res.render("myview");
}