Validating radio button list - validation

Question

I have a directive that sets a CSS class on a form element based on its $valid value. I want to use it with a radio button list. There a multiple radio buttons in the set but I only need to add the directive to one of them because a) they all control the same model and b) the CSS ultimately will reveal a single image tag which I only want once per radio button list.
Here's some HTML:
<div ng-app="myApp">
<input required type="radio" ng-model="type" name="typeGroup" value="type1">Type 1
<input add-validation-class required type="radio" ng-model="type" name="typeGroup" value="type2">Type 2
</div>
Script:
angular.module('myApp', []).directive('addValidationClass', function() {
return {
require: '^ngModel',
link: function($scope, $element, $attrs, $ngModel) {
if (!$ngModel) return;
var update = function () {
alert($ngModel.$dirty);
$element
.removeClass()
.addClass($ngModel.$dirty ? ($ngModel.$valid ? 'valid' : '') : '');
};
$scope.$watch($attrs.ngModel, function (newValue, oldValue) {
update();
});
}
};
});
Fiddle: http://jsfiddle.net/JqBgs/
You can see from the fiddle that the watch fires update regardless of the radio button clicked on. This is what I expected because both radio buttons control the same model. However you can also see that the left radio button isn't dirty. So the CSS class isn't applied.
Why isn't it dirty?

Answer 1

I put this in the HTML:
<div ng-app="myApp">
<input ng-change="myForm.validateRadio.$dirty = true" required type="radio" ng-model="type" name="typeGroup" value="type1">Type 1
<input name="validateRadio" add-validation-class required type="radio" ng-model="type" name="typeGroup" value="type2">Type 2
</div>
What a ridiculous bug in Angular.