title: With AngularJS : Bind a radio button input to a Boolean with possible null value
body: <p>Using AngularJS v1.5.0, I am trying to bind a radio input field to a Boolean variable that can be : true, false or null. Using the ng-value directive sets a null value to false.</p>
  
  <pre><code>&lt;input type="radio" ng-model="foo.value" ng-value="true"  ng-change="update()"&gt;
  &lt;input type="radio" ng-model="foo.value" ng-value="false" ng-change="update()"&gt;
  &lt;input type="radio" ng-model="foo.value" ng-value="null"  ng-change="update()"&gt; 
  </code></pre>
  
  <p>Would you have any idea about how this could be done ?
  Thank you !</p>