title: Update FK model DateField after save
body: <p>I have a person model and a group model, as shown below:</p>
  
  <pre><code>class Person (models.Model):
    name = models.CharField(max_length=75)
    group = models.ForeignKey('Group', blank=True, null=True)
  
  class Group (models.Model):
    name = models.CharField(max_length=100)
    last_join = models.DateField(auto_now = True)
  </code></pre>
  
  <p>What I would like is when I add a new person to a group, the group's last_join is updated with the current time. Can I do this within the models?</p>