title: Sum fields in sqlAlchemy
body: <p>I recently upgraded to the most recent version of sqlalchemy and some of my code no longer works. I'm having difficulty finding how to fix it and could use a hand.</p>
  
  <p>Previously the query appeared as so.</p>
  
  <pre><code>self.db.query(Drive).filter(Drive.package_id==package.package_id)\
      .filter(Drive.wipe_end!=None).sum(Drive.wipe_end - Drive.wipe_start)
  </code></pre>
  
  <p>this worked before to retrieve the sum of some durations but now I get the following error:</p>
  
  <pre><code>'Query' object has no attribute 'sum'
  </code></pre>
  
  <p>Any googling I do gets me information that is several years old.</p>