Knockout.js binding radio group does not work

Question

I feel really stupid but can't make it work :)

http://jsfiddle.net/btkmR/

I made this simple Fiddle just to prove that I'm not missing something in my big project.

HTML:

<div>
    Preferred flavor
    <div><input type="radio" name="flavorGroup" data-bind="checked: cherryOn" /> Cherry</div>
    <div><input type="radio" name="flavorGroup" data-bind="checked: almondOn" /> Almond</div>
    <div><input type="radio" name="flavorGroup" data-bind="checked: mgOn" /> Monosodium Glutamate</div>
</div>

JS:

var viewModel = {
    cherryOn: ko.observable(true);
    almondOn: ko.observable(false);
    mgOn: ko.observable(false);
};

ko.applyBindings(viewModel);

I expect to see Cherry selected on start..

Answer 1

From Knockout's documentation (http://knockoutjs.com/documentation/checked-binding.html):

For radio buttons, KO will set the element to be checked if and only if the parameter value equals the radio button node’s value attribute.

Example: http://jsfiddle.net/btkmR/2/

<div>
    Preferred flavor
    <div><input type="radio" name="flavorGroup" value="cherry" data-bind="checked: flavor" /> Cherry</div>
    <div><input type="radio" name="flavorGroup" value="almond"  data-bind="checked: flavor" /> Almond</div>
    <div><input type="radio" name="flavorGroup" value="Monosodium"  data-bind="checked: flavor" /> Monosodium Glutamate</div>
</div>

var viewModel = {
    flavor: ko.observable("cherry")
};

ko.applyBindings(viewModel);

Answer 2

For those who, like me, are struggling to get it working with dynamically bound radio group name and values, pay attention to order of bindings in the data-bind.

Following will NOT work, as value and name are bound after checked:

<input 
    type="radio" 
    data-bind="
        checked: $parent.correctAnswerId, 
        attr: {name: 'correctAnswerFor' + $parent.id, value: id}
    "
/>

correct order is :

        attr: {name: 'correctAnswerFor' + $parent.id, value: id},
        checked: $parent.correctAnswerId