title: Django anonymous user in model
body: <p>I have a model defined as below:</p>
  
  <pre><code>class Example(models.Model):
      user = models.ForeignKey(User, null=True)
      other = models.CharField(max_length=100)
  </code></pre>
  
  <p>The problem is Django refuses to assign django.contrib.auth.models.AnonymousUser directly to Example.user as null field so everytime I have to check if request.user.is_authenticated() ans assign Example.user = None manually.</p>
  
  <p>Is there a default value for AnonymousUser to use in a model field?</p>