title: django - get all objects in one table which do not have a foreignkey in another table
body: <p>I have 2 models:</p>
  
  <pre><code>class Friend(models.Model):
      person1 = models.ForeignKey("Person", related_name="friend1")
      person2 = models.ForeignKey("Person", related_name="friend2")
      created = models.DateTimeField(auto_now=True, auto_now_add=True, editable=False)
  
  class Person(models.Model):
      firstname = models.CharField(max_length=100)
      surname = models.CharField(max_length=100)
      created = models.DateTimeField(auto_now=True, auto_now_add=True, editable=False)
  </code></pre>
  
  <p>I want a list of <code>Person</code> objects which do not have any entries in the <code>Friend</code> object. e.g.</p>
  
  <pre><code>no_friends = Person.objects.filter(????)
  </code></pre>
  
  <p>but I'm not sure what the filter should be.</p>