In order to get or remove specific Models from a Collection, Backbone needs to know how to index these Models. Backbone does this in two ways:
Using the Model's
idproperty, if any (which, as we discussed in the previous chapter, can either be set directly or indirectly by setting the Model'sidAttributeproperty)Using the Model's
cidproperty (which all Models have)
When you add a Model or attributes to a Collection, Backbone uses both of the preceding forms of identification to register the Model in the Collection's _byId property. _byId is yet another one of Backbone's hidden properties, but it's a private property as well (because its name is prefixed by _). This means that, even more so than with other hidden properties, you should avoid using _byId directly and instead use it indirectly through methods such as get. The get method returns the Model with the provided ID (if any) by using _byId:
var garfield = new Cat({id: 'cat1', name: 'Garfield'});
var cats = new Backbone.Collection...