title: Using ng-required vs required
body: <p>I have the following multi select box:</p>
  
  <pre><code>&lt;select 
      multiple="multiple" 
      data-ng-model="rightSelected" 
      data-ng-options="slide as slide.SlideBarcode for slide in form.Slides" 
      data-ng-required="form.Slides.length &gt; 0" 
  /&gt;
  </code></pre>
  
  <p>In my controller I do this at initialization:</p>
  
  <pre><code>$scope.form.Slides = [];
  </code></pre>
  
  <p>I want this element of the form to be valid if there are slides in the Slide array.  These are added dynamically- it's kind of a slide bucket users can add slides to.</p>
  
  <p>But I'm not getting how the ngRequired stuff works... if I change it to <code>data-ng-required="true"</code> then my form looks okay, but of course it doesn't do what I want.  If I use the <code>form.Slides.length &gt; 0</code> which is checking for what I really want, not only does this not work, but it messes with my form structure, one of my elements vanishes seemingly at random.  </p>
  
  <p>What is the proper way to use this?  The docs are pretty sparse on this one.</p>