title: django query latest of multiple dates
body: <p>I need to find the latest date entry among multiple DateTimeFields.</p>
  
  <p>models.py:</p>
  
  <pre><code>class Presentation(models.Model):
      start = models.DateTimeField(blank=True, null=True, verbose_name=_(u'Start'))
      end = models.DateTimeField(blank=True, null=True, verbose_name=_(u'End'))
  </code></pre>
  
  <p>I am aware of <a href="https://docs.djangoproject.com/en/1.9/ref/models/querysets/#latest" rel="nofollow noreferrer">latest()</a> which sadly only supports querying a single field.</p>
  
  <p>EDIT: I am looking for a one-query solution, if it exists.</p>