new_contents: from django.db import models
  
  class QuerySetManager(models.Manager):
      def __getattr__(self, name, *args):
          if name.startswith('_'):
              raise AttributeError
          return getattr(self.get_query_set(), name, *args)
  
      def get_query_set(self):
          return self.model.QuerySet(self.model)
  
subject: Add QuerySet manager, for implementing chainable filters.