title: django get sum of column based on where clause
body: <p>I have read the documentation related to django aggregation but I am still have trouble figuring out how exactly I can get a sum of points per user. </p>
  
  <p>Here is what I tried: </p>
  
  <pre><code>ModelName.objects.filter(username=rUser.username).aggregate(Sum('points'))
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre><code>Request Method: GET
  Request URL:    http://localhost:8001/mypage/
  Django Version: 1.4.8
  Exception Type: NameError
  Exception Value:    
  global name 'Sum' is not defined
  </code></pre>