title: Django 2.1 TypeError: __init__() got an unexpected keyword argument 'on_delete'
body: <p>My code:</p>
  
  <pre><code>class UserProfile(models.Model):
      user      = models.OneToOneField(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, related_name='profile')
      following = models.ManyToManyField(settings.AUTH_USER_MODEL, on_delete=models.CASCADE, related_name='followed_by')
  
      def __str__(self):
          return str(self.following.all().count())
  </code></pre>
  
  <p>My error:</p>
  
  <pre><code>TypeError: __init__() got an unexpected keyword argument 'on_delete'
  </code></pre>
  
  <p>Any ideas?</p>