title: Add a custom column
body: <p>So I have a query...</p>
  
  <pre><code>qry = Query(models.UserList).filter(...)
  </code></pre>
  
  <p><code>models.UserList</code> contains a mixin method that will return a <code>count</code> of a related table</p>
  
  <p>So what I would like to do is something like ...</p>
  
  <pre><code>qry = qry.order_by(models.UserList.contactCount)
  </code></pre>
  
  <p>However this produces invalid sql of course.</p>
  
  <p>So how can I do something like</p>
  
  <pre><code>qry.add_column((this.contactCount).label("contactCount"))
  </code></pre>
  
  <p><code>this.contactCount</code> is just bad psuedocode, but hopefully you get the point.</p>
  
  <p>I've been looking at how to do this for a while, but sqlalchemy doc isnt the easiest thing to find stuff in.</p>
  
  <p>any help appreciated</p>