title: Haml: How to add classes dynamically to an element?
body: <p>I have a <code>&lt;tr&gt;</code> element in my view, and I want to add classes dynamically on this element depending on an association between two models (many to many between company and packaging).</p>
  
  <p>The result should looks like </p>
  
  <pre><code>&lt;tr class="pck1 pck3 pck5"&gt;
  </code></pre>
  
  <p>where <code>pck1</code>, <code>pck3</code> and <code>pck5</code> are packagings associated with a company.</p>