old_contents: from django.db import models
  
  def register(cls, admin_cls):
      cls.add_to_class('_prepared_date', models.TextField('Date of Preparation', blank=True, null=True))
  
      def getter(obj):
          if not obj._prepared_date:
              try:
                  return obj.get_ancestors(ascending=True).filter(_prepared_date__isnull=False)[0]._prepared_date
              except IndexError:
                  return None
          return obj._prepared_date
  
      def setter(obj, value):
          obj._prepared_date = value
  
      cls.prepared_date = property(getter, setter)
  
      if admin_cls and admin_cls.fieldsets:
          admin_cls.fieldsets[2][1]['fields'].append('_prepared_date')
  
  
new_contents: from django.db import models
  
  def register(cls, admin_cls):
      cls.add_to_class('_prepared_date', models.TextField('Date of Preparation', default='', blank=True))
  
      def getter(obj):
          if not obj._prepared_date:
              try:
                  return obj.get_ancestors(ascending=True).filter(_prepared_date__isnull=False)[0]._prepared_date
              except IndexError:
                  return ''
          return obj._prepared_date
  
      def setter(obj, value):
          obj._prepared_date = value
  
      cls.prepared_date = property(getter, setter)
  
      if admin_cls and admin_cls.fieldsets:
          admin_cls.fieldsets[1][1]['fields'] += ('_prepared_date',)
  
  
subject: Fix prepared_date admin extension. Add default=''.