Filtering the Backbone.js Collection by Index

Question

I have a collection of Backbone.js that contains (for example) 30 elements.

I want to go to my collection of filtered filters, consisting of every third element in the source collection.

Does anyone know how to do this elegantly? CoffeeScript code is preferred.

Answer 1

Assuming originalCollection is your existing collection

 var newCollection = new Backbone.Collection(); for (var i = 0, l = originalCollection.length; i < l; i++) { if (i % 3 === 0) { newCollection.add(originalCollection.models[i]); } } 

This code works by cycling through each existing model and adds only a new collection if the index is a multiple of 3.

You can do this a little better by using the underline method each , opened by Underscore.js in Backbone Collections:

 var newCollection = new Backbone.Collection(); originalCollection.each(function (model, index) { if (index % 3 === 0) { newCollection.add(model); } }); 

Converting the above to CoffeeScript results in:

 newCollection = new Backbone.Collection() originalCollection.each (model, index) -> newCollection.add model if index % 3 is 0 

Answer 2

The layout collection contains several useful underlining methods . You can use filter to get an array of models that you can pass to the template:

 filteredModels = collection.filter (model, i) -> i % 3 == 0 

Alternatively, you can use array understanding; although I think this is less readable ...

 filteredModels = (model for model, i in collection.models when i % 3 == 0) 

If you really need the Backbone.Collection template in your template, you can create a new one with these filtered models:

 filteredCollection = new Backbone.Collection filteredModels 

Here is a working jsfiddle example.