Question

When you look into the source code of Backbone.js, version 0.9.2, there is

“_onModelEvent”:

This method is responsible of forwarding events from the model to the collection (bubbling up).

So when you change something in a model which is part of a collection (with “set”), there is a change event, and because (in the add method) the _onModelEvent method was bound to the model (“all”), this method gets called.

I also see, that there is a destroy method, which triggers a “destroy” event.

But why does the author (in the first line of the _onModelEvent) check, if the event name is “add” or “remove”. Normally there is no “add” event on the model?

I do not understand this.

Can somebody explain it to me?

Thanks in advance

_onModelEvent: function(event, model, collection, options) {
  if ((event == 'add' || event == 'remove') && collection != this) return;

Answer 1

With Backbone you can create nested model structures and collections. So you can create a collection where the model is a collection. That is way the check in place:

So from the annotated source:

_onModelEvent:

Internal method called every time a model in the set fires an event.
Sets need to update their indexes when models change ids. All other
events simply proxy through. “add” and “remove” events that originate
in other collections are ignored.

So if you have a collection as a model. It won’t delegate the add or remove event if items were added/removed to/from the child collections because in that case only the child collection is changed and not the parent.