title: How to conditionally render a css class with knockoutjs
body: <p>I have some html like the following:</p>
  
  <pre><code>&lt;div class="control-group"&gt;
      &lt;input type="text" data-bind="value: $data.DealCode" name="DealCode" class="input-mini" /&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>However, <code>ifnot: $data.DealCodeIsValid</code>, I need to render the following:</p>
  
  <pre><code>&lt;div class="control-group error"&gt;
      &lt;input type="text" data-bind="value: $data.DealCode" name="DealCode" class="input-mini" /&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>Note the additional class "error" in the div. Is there a way to do that with knockoutjs?</p>