title: Django 1.5 custom User model error. "Manager isn't available; User has been swapped"
body: <p>I extend the django user model as described in the dev doc. I wan't to keep most of the original User model features so I extend the AbstractUser class. I've defined in settings.py:</p>
  
  <pre><code>AUTH_USER_MODEL = 'myapp.CustomUser'
  </code></pre>
  
  <p>My user class: </p>
  
  <pre><code>class CustomUser(AbstractUser):
    custom_field = models.ForeignKey('OtherModel')
    objects = UserManager()
  </code></pre>
  
  <p>Everything seems to work fine but when I try to make it managed by the admin site:</p>
  
  <pre><code>admin.site.register(CustomUser, UserAdmin)
  </code></pre>
  
  <p>I get this error on the admin CustomUser creation page (after validation of the password confirmation form): </p>
  
  <pre><code>AttributeError: Manager isn't available; User has been swapped for 'myapp.CustomUser'
  </code></pre>
  
  <p>The point is that I need this model managed by the admin site in order to have the same creation process as with the original User model (two step process with password validation).</p>