gistfile1.js

var whatsOn = [];
Collection.findOne({url: 'whats-on'}).populate('children', null, null, {sort: 'position'}).exec(function (error, collection) {
  async.forEachSeries(collection.children, function (child, callback) {
    Collection.findAndPopulate({_id: child._id}, function (error, child) {
      if (child) whatsOn.push(child);
      callback();
    });
  }, function (error) {
    callback(error, whatsOn);
  });
});