Best way to subscribe to individual changes in collection?

[thinkloop]

trafficstars

In Firebase I am storing a bunch of keys/ids, but no real data. For example I store user IDs, but not user names. So when it comes time to display the user, I make another request to another service to populate the rest of the item. To get this to work, I need to know which individual item has changed in the collection. My first attempt was something like this:

// init knockoutfire
self.suggestions2({ collection: KnockoutFire.observable(recommenderFirebase.child('ratings/suggestions').child(user.id), {
        '.limit': 20,
       "$suggestion" : {
            accountID: true,
            itemID: true,
            relevance: true
        }
    })
});

// ******* TRYING TO SUBSCRIBE TO INDIVIDUAL ELEMENT CHANGES ********
self.suggestions2().collection.subscribe(function(newValue) {
    log(newValue.length);
    log(newValue[0]().itemID());
});

But this listens to the whole collection, returning all 20 items, instead of the individual item that was changed.

A horrible but functional way to solve this would be if I were willing to modify KnockoutFire itself, and populate the rest of the object on the native Firebase changes. So for example, KnockoutFire listens to "child_added" - if in that function I populated the object it would work (see bottom) :

firebaseRef.on("child_added", function(childSnap, prevChildName) {
    //var child = KnockoutFire.observable(childSnap.ref(), map[childVariable]);
    var childMap = map[childVariable];
    var child = KnockoutFire.mapObservable(childMap);
    if (childSnap.getPriority()) {
        child()[".priority"] = childSnap.getPriority();
    }
    child.extend({firebaseObject: {firebaseRef: childSnap.ref(), map: childMap}});
    //console.log(childSnap.ref().path.toString() + ":" + child().firebase);
    self.insert(child, prevChildName, map[".reverse"]);
    self().last(child());

    // ******* POPULATE OBJECT ********
    outsideService.populate(child());
});

Of course I don't want to do that. So whats's the best way to listen to changes to individual elements in the collection, and not the whole collection itself?

[thinkloop]

I'm not sure why I didn't think of this originally, but I can simply subscribe directly to the underlying Firebase independent of KnockoutFire:

recommenderFirebase.child('ratings/suggestions').child(user.id).limit(20).on('child_added', function(data) {
    log(data.val());
});

[hiroshi]

I think ".extend" map option may work.

ko.extenders.suggestion = function(target) {
  outsideService.populate(target());
};

then specify the extender.

"$suggestion" : {
  ".extend": {"suggestion": null},
  ...
}