title: How to make a query on related_name field?
body: <p>I have to models connected by a ForeignKey</p>
  
  <pre><code>class User(AbstractUser):
     ...
  </code></pre>
  
  <p>and </p>
  
  <pre><code>class PrivateMessage(models.Model):
      user_from = models.ForeignKey(
          User,
          verbose_name=u'From',
          related_name='sent_messages',
      )
      user_to = models.ForeignKey(
          User,
          verbose_name=u'To',
          related_name='received_messages',
      )
  </code></pre>
  
  <p>Is there any way to get all the addresses for a particular user. For example, if</p>
  
  <pre><code>u = User.objects.get(id=1)
  messages = PrivateMessage.objects.filter(user_from=u)
  for m in messages:
      users.add(m.user_to)
  </code></pre>
  
  <p>How to obtain a list of users that appear in <code>user_to</code> for these messages using only Django ORM methods?</p>