title: Django was deleted parent relation when i delete child
body: <p>I have two models:</p>
  
  <pre><code>class One(models.Model)
      name = models.CharField()
  
  class Two(models.Model):
      one_first = models.OneToOneField(One, related_name='first', null=True, blank=True)
      one_second = models.OneToOneField(One, related_name='second', null=True, blank=True)
  </code></pre>
  
  <p>When I do <code>two_instance.one_second.delete()</code>, Django deletes the one instance (which I want), but it also deletes the two instance. Why does it delete <code>two_instance</code>?</p>