title: Rails model to recursive json
body: <p>I had a Menu model, that has submenus of same type. Something like:</p>
  
  <ul>
  <li>level 1
  <ul>
  <li>level 1.1</li>
  <li>level 1.2</li>
  </ul></li>
  <li>level 2
  <ul>
  <li>level 2.1</li>
  <li>...</li>
  </ul></li>
  </ul>
  
  <p>So, I need a way to include in my json, all levels, in a recursive way.</p>