title: django - filter after slice / filter on queryset where results have been limited
body: <p>having trouble understanding why I can't filter after a slice on a queryset and what is happening.</p>
  
  <pre><code>stuff = stuff.objects.all()
  stuff.count()
  </code></pre>
  
  <p>= 7</p>
  
  <p>If I then go </p>
  
  <pre><code>extra_stuff = stuff.filter(stuff_flag=id)
  extra_stuff.count()
  </code></pre>
  
  <p>= 6. Everything is all good and I have my new queryset in extrastuff no issues</p>
  
  <pre><code>stuff = stuff.objects.all()[:3]
  extra_stuff = stuff.filter(stuff_flag=id)
  </code></pre>
  
  <p>I get the error "Cannot filter a query once a slice has been taken." </p>
  
  <p>How can I filter further on a queryset where I have limited the number of results?</p>