title: What's the meaning of require: 'ngModel'?
body: <p>This is the HTML for my directive:</p>
  <pre><code>&lt;textarea data-modal=&quot;modal&quot; data-mydir ng:model=&quot;abc&quot;&gt;&lt;/textarea&gt;
  </code></pre>
  <p>In my directive I have this:</p>
  <pre><code>return {
    require: 'ngModel',
    replace: true,
    scope: {
      modal: '=modal',
      ngModel: '=',
      pid: '=pid'
    }
  }
  </code></pre>
  <p>Can someone tell me, what's the significance of require: <code>ngModel</code> ?  I see this in many different directives. Could I call this data-modal?</p>
  <p>I am confused because when I change it to <code>data-modal</code> I get a message from Angular saying</p>
  <pre><code>Controller 'ngModel', required by directive 'textarea', can't be found!
  </code></pre>