title: fast lookup for the last element in a Django QuerySet?
body: <p>I've a model called Valor. Valor has a Robot. I'm querying like this:</p>
  
  <pre><code>Valor.objects.filter(robot=r).reverse()[0]
  </code></pre>
  
  <p>to get the last Valor the the r robot. Valor.objects.filter(robot=r).count() is about 200000 and getting the last items takes about 4 seconds in my PC.</p>
  
  <p>How can I speed it up? I'm querying the wrong way?</p>