Copying events to new instance - backbone.js

Question

I have a collection of Tasks. I have added some filter methods to the collection declaration to return a subset of the collection in a new instance. What I want to do is create a model in the original collection and have the add (and any other) event filter down to any new instances of the Tasks collection so my view partials can update accordingly.
Here is a JSBin demonstrating my problem. Notice that when I add a new model to the original collection on the last line the list doesn't update because, obviously, the filter methods return new instances of that collection (which seems clean to me) therefore any listeners aren't fired - the view is using a different collection to the one that has had an item added to it.
How can I filter a collection semantically and cleanly, but keep any events bound to the set being filtered bound on the subset of models returned?
For example, the add event for notAsManyTasks isn't fired for the below code (the alert() never shows):
// Task model
var Task = Backbone.Model.extend();
// Tasks collection
var TasksCollection = Backbone.Collection.extend({
model: Task,
byProject: function(projectId) {
var matches = this.filter(function(task) {
return task.get('projectId') == projectId;
});
return new TasksCollection(matches);
},
complete: function(state) {
return new TasksCollection(this.where({ complete: state }));
}
});
// Example collection
var lotsOfTasks = new TasksCollection([
{ id: 1, projectId: 1, complete: false },
{ id: 2, projectId: 1, complete: true },
{ id: 3, projectId: 2, complete: false },
{ id: 4, projectId: 2, complete: true }
]);
var notAsManyTasks = lotsOfTasks.byProject(1);
notAsManyTasks.on('add', function() {
alert("Added");
});
// Does not fire `add` event on `notAsManyTasks` which is my problem, however
// it _does_ fire on `lotsOfTasks`, as it should
lotsOfTasks.add({ id: 5, projectId: 1, complete: false });
How can I get round this problem? Ideally I don't want to store the collection's original state, return a subset, then restore all models again.

Answer 1

Without doing some complicated things, you should be able to do something along those lines:
mySubset.listenTo(myCollection, 'all', function() {
this.trigger.apply(this, arguments);
});
You listen to all the events of your collection, and echo them with your subset. Though I guess that's only half a solution because your subset wouldn't be updated and you'd do the filter in the listener for all your subsets.
Another solution to do the job only once would be for your collection to listen to its own add event, filter the new model, and trigger a custom event so that only the subset(s) that actually need(s) to do something do something.