correctly implement backbone comparators

Question

I'm getting a bit stuck implemented a backbone comparator, I basically want to select different sorting methods based on the route and use the comparator to sort the collection. Ideally I want to keep the sorting logic encapsulated within the collection but seem to be getting stuck. For example

    Requests = Backbone.Collection.extend({
        model : Request,
        comparator : function(ab) {
            return -ab.id;
        },          
        nooffers : function() {
            return this.sortBy(function(ab) {               
                 return ab.get('offers');
            });
        }
    }); 

So by default it sorts based on the default comparator - but in my routing I wish to be able to resort e.g. do something like

   routes : {
        "" : "index",
        '/ordering/:order' : 'ordering'
    },
    ordering : function(theorder) {
        ordering = theorder;
        if(theorder == 'nooffers') {
            Request.comparator = Request.nooffers();
        }
        Request.sort();
        listView.render();  
        howitworksView.render();
    }

However in that case I get an error ('c.call is not a function') any ideas?

Answer 1

You have a few things wrong here.

This doesn't do what you think it does:

if(theorder == 'nooffers') {
    Request.comparator = Request.nooffers();
}

That executes the nooffers method and assigns its result to Request.comparator. But sortBy returns the sorted list:

nooffers : function() {
    return this.sortBy(function(ab) {               
        return ab.get('offers');
    });
}

and setting that list as the comparator function doesn't do anything useful.

You want to change the assignment to use the function rather that its return value:

if(theorder == 'nooffers') {
    Request.comparator = Request.nooffers;
}

and change the function to be a valid comparator function:

nooffers : function(ab) {
    return ab.get('offers');
}

Demo (run with your console open): http://jsfiddle.net/ambiguous/AAZCa/

But having someone from the outside fiddling with the collection's methods like that smells bad and you shouldn't do it. Instead, you should ask the collection to change its ordering with something like this:

var Requests = Backbone.Collection.extend({
    model: Request,
    comparator: function(ab) {
        if(this._order_by == 'offers')
            return ab.get('offers');
        else if(this._order_by == 'id')
            return -ab.id;
        //...
    },
    order_by_offers: function() {
        this._order_by = 'offers';
        this.sort();
    },
    order_by_default: function() {
        this._order_by = 'id';
        this.sort();
    },
    _order_by: 'id'
});
//...
rs.order_by_offers();

Demo: http://jsfiddle.net/ambiguous/uM9av/

Or you could let the collection swap its own comparator to avoid all the conditional logic inside the comparator:

var Requests = Backbone.Collection.extend({
    model: Request,
    initialize: function() {
        this._order_by_id = this.comparator;
    },
    comparator: function(ab) {
        return -ab.id;
    },
    order_by_offers: function() {
        this.comparator = this._order_by_offers;
        this.sort();
    },
    order_by_default: function() {
        this.comparator = this._order_by_id;
        this.sort();
    },
    _order_by_offers: function(ab) {
        return ab.get('offers');
    }
});

Demo: http://jsfiddle.net/ambiguous/Pjfq2/