Multi-sort on any number of Backbone Collection attributes. Sort happens from arguments[0] to arguments[n]

MultiSortCollection.js

var MultiSortCollection = Backbone.Collection.extend({
	
	/**
	 * Sort by supplied attributes.  First param is sorted first, and
	 * last param is final subsort
	 * @param {String} sortAttributes
         * @example collection.sortBy("last_name","first_name")
	 */
	sortBy : function(sortAttributes){
		var attributes = arguments;
                if(attributes.length){
                	this.models = this._sortBy(this.models,attributes);
                }	
	},
	
	/**
	 * Recursive sort
	 */
	_sortBy : function(models,attributes){
		var attr,
				that = this;
		//base case
		if(attributes.length === 1){
			attr = attributes[0];
			return _(models).sortBy(function(model){
				return model.get(attr);
			});
		}
		else{
			attr = attributes[0];
			attributes = _.last(attributes,attributes.length-1);
			
			//split up models by sort attribute, 
			//then call _sortBy with remaining attributes
			models = _(models).chain().
				sortBy(function(model){
					return model.get(attr);
				}).
				groupBy(function(model){
					return model.get(attr);
				}).
				toArray().
				value();
			
			_(models).each(function(modelSet,index){
				models[index] = that._sortBy(models[index],attributes);
			});
			return _(models).flatten();	
		}
	}
});

MultiSortCollectionExamples.js

var models = new MultiSortCollection,
model;

models.add([
	{name : "Charlie",number: 5},
	{name : "Billy", number: 7},
	{name : "Albert",number: 1},
	{name : "Charlie",number: 4}
]);

//collection order is [Charlie 5, Billy 7, Albert 1, Charlie 4]

models.sortBy("number","name");  //collection order is now [Albert 1, Charlie 4, Charlie 5, Billy 7]
models.sortBy("name","number");  //collection order is now [Albert 1, Billy 7, Charlie 4, Charlie 5]

model = new Backbone.Model({name : "Charlie",number:4.5});
console.log(models.sortIndex(model));		//returns 3
models.add(model);	//colleciton order is now [Albert 1, Billy 7, Charlie 4, Charlie 4.5, Charlie 5]

I have added this.models.reverse(); at the end