title: I get "parent_id may not be NULL" when creating my Django model
body: <p>I'm creating my own <code>Group</code> model; I'm not referring to the builtin <code>Group</code> model. I want each hroup to be a member of another group (it's parent), but there is the one "top" group that doesn't have a parent group.</p>
  
  <p>The admin interface won't let me create a group without entering a parent. I get the error <code>personnel_group.parent_id may not be NULL</code>. My <code>Group</code> model looks like this:</p>
  
  <pre><code>class Group(models.Model):
      name = models.CharField(max_length=50)
      parent = models.ForeignKey('self', blank=True, null=True)
      order = models.IntegerField()
      icon = models.ImageField(upload_to='groups', blank=True, null=True)
      description = models.TextField(blank=True, null=True)
  </code></pre>
  
  <p>How can I accomplish this?</p>
  
  <p>Thanks.</p>