Backbone.js firing Collection change event multiple times - backbone.js

Question

In one of by Backbone.js views I am updating the attribute "read" of the current model (instance of Message) by using this.model.set( { read: true } );. I verified that this command is only executed once (I know about "ghost events"). As you can see below I configured the Collection to fire an update event in which the whole Collection gets saved into a variable.
Unfortunately the saveToVar function gets called 3 times instead of one! Also, the first time saveToVar is called, this correctly consists of all the collection's models, whilst the 2nd and 3rd time this only has one model, namely the one I did the update on.
I tracked everything down piece by piece but I have no clue why this happens.
window.Message = Backbone.Model.extend({
});
window.MessageCollection = Backbone.Collection.extend({
model: Message,
initialize: function()
{
this.on("change", this.saveToVar);
},
saveToVar: function(e)
{
App.Data.Messages = this.toJSON();
return;
}
});

Answer 1

In your jsfiddle, you're doing this:
App.Collections.message = new MessageCollection([ ... ]);
var elements = App.Collections.message.where({ id: 4 });
var item = new MessageCollection(elements);
Your where call will return models that are in the message collection, not copies of those models but exactly the same model objects that are in message. Now you have two references to your id: 4 model:
The original one buried inside App.Collections.message.
The one in elements[0].
Both of those references are pointing at the same object. Then you add elements to another MessageCollection. Now you have something like this:
App.Collections.message.models[3] item.models[0]
| |
+--> [{id: 4}] <--+
Both of those collections will be notified about change events on the id: 4 model since collections listen to all events on their members:
Any event that is triggered on a model in a collection will also be triggered on the collection directly, for convenience.
And your collection listens for "change" events in itself:
initialize: function()
{
this.on("change", this.saveToVar);
}
So when you do this:
this.model.set({ read: true });
in your view, both collections will be notified since that model happens to be in both collections.
If we alter your event handler to look like this:
saveToVar: function() {
console.log(_(this.models).pluck('cid'));
}
then you'll see that the same cid (a unique identifier that Backbone generates) appears in both collections. You can also attach a random number to each collection and see what you get in saveToVar: http://jsfiddle.net/ambiguous/mJvJJ/1/
You probably shouldn't have one model in two collections. You probably shouldn't have two copies of the same model kicking around either so cloning elements[0] before creating item might not be a good idea either. You might need to reconsider your architecture.