old_contents: 
  
  from __future__ import unicode_literals
  
  from django.db import models
  from django.utils.translation import ugettext_lazy as _
  
  ORIENTATION_CHOICES = (
      ('vertical', 'Vertical'),
      ('horizon', 'Horizon'),
  )
  
  
  class NavigationWidgetMixin(object):
  
      display_active = models.NullBooleanField(
          verbose_name=_('Display active'), default=True)
  
      display_in_nav = models.NullBooleanField(
          verbose_name=_('Display in navigation'), default=True)
  
      orientation = models.CharField(
          verbose_name=_("Orientation"), max_length=20,
          choices=ORIENTATION_CHOICES, default='horizontal')
  
new_contents: 
  
  from __future__ import unicode_literals
  
  from django.db import models
  from django.utils.translation import ugettext_lazy as _
  
  ORIENTATION_CHOICES = (
      ('vertical', 'Vertical'),
      ('horizon', 'Horizon'),
  )
  
  
  class NavigationWidgetMixin(models.Model):
  
      display_active = models.NullBooleanField(
          verbose_name=_('Display active'), default=True)
  
      display_in_nav = models.NullBooleanField(
          verbose_name=_('Display in navigation'), default=True)
  
      orientation = models.CharField(
          verbose_name=_("Orientation"), max_length=20,
          choices=ORIENTATION_CHOICES, default='horizontal')
  
      class Meta:
          abstract = True
  
subject: fix model inheritence