title: Django queryset for latest of each unique property value
body: <pre><code>class SwallowMigration(models.Model):
      swallow = models.ForeignKey(Swallow)
      date = models.DateTimeField(auto_now_add=True)
      coconuts_carried = models.IntegerField()
  </code></pre>
  
  <p>How can I get the latest migration for each swallow?</p>
  
  <p>Of course .latest() only gives me the very most recent entry.  Is there a way (maybe using Aggregation with Max?) to get the latest for each instance of some value in another field?</p>