KnockoutJS - Adding computed values to an observable array

Question

I'm binding data to a page using KnockoutJS, the ViewModel is being populated by an JSON response from an AJAX call using the mapping plugin, like this:

$(function () {
    $.getJSON("@Url.Action("Get")", 
        function(allData) {
            viewModel = ko.mapping.fromJS(allData);

            viewModel.Brokers.Url = ko.computed(function()
            {
                return 'BASEURLHERE/' + this.BrokerNum();
            });

            ko.applyBindings(viewModel);
    });
});

The middle part there doesn't work (it works fine without that computed property). "Brokers" is an observable array, and I want to add a computed value to every element in the array called URL. I'm binding that Brokers array to a foreach, and I'd like to use that URL as the href attribute of an anchor. Any ideas?

Answer 1

I've been working through very similar issues and I've found that you can intercept the creation of the Broker objects and insert your own fields using the mapping options parameter:

var data = { "Brokers":[{"BrokerNum": "2"},{"BrokerNum": "10"}] };

var mappingOptions = {
    'Brokers': {
        create: function(options) {
            return (new (function() {
                this.Url = ko.computed(function() {
                    return 'http://BASEURLHERE/' + this.BrokerNum();
                }, this);

                ko.mapping.fromJS(options.data, {}, this); // continue the std mapping
            })());
        }
    }
};

viewModel = ko.mapping.fromJS(data, mappingOptions);

ko.applyBindings(viewModel);

Here is a fiddle to demonstrate this: http://jsfiddle.net/pwiles/ZP2pg/