title: How to rename fields of an annotated query?
body: <p>I have the following query</p>
  
  <pre><code>a = Mainfee.objects.values('collected_by__username').
                         distinct().annotate(Sum('amount'))
  </code></pre>
  
  <p>The result looks like this</p>
  
  <pre><code>[{'collected_by__username': u'maindesk', 'amount__sum': 800}]
  </code></pre>
  
  <p>How can I rename the first key to <code>a</code> and second key to <code>b</code>?</p>
  
  <p>I tried the following</p>
  
  <pre><code>m = Mainfee.objects.extra(select = 
     {'a':'collected_by__username'}).values('a').distinct().
      annotate(Sum('amount'))
  </code></pre>
  
  <p>and received this</p>
  
  <pre><code>DatabaseError: no such column: collected_by__username
  </code></pre>
  
  <p>I also tried</p>
  
  <pre><code>m = Mainfee.objects.extra(select = 
     {'a':'collected_by__username'}).values('collected_by__username').distinct().
      annotate(Sum('amount'))
  </code></pre>
  
  <p>and got </p>
  
  <pre><code>[{'collected_by__username': u'maindesk', 'amount__sum': 800}]
  </code></pre>
  
  <p>PS: I want to rename the second field too</p>