old_contents: class NestedViewSetMixin(object):
      def get_queryset(self):
          """
          Filter the `QuerySet` based on its parents as defined in the
          `serializer_class.parent_lookup_kwargs`.
          """
          queryset = super(NestedViewSetMixin, self).get_queryset()
          if hasattr(self.serializer_class, 'parent_lookup_kwargs'):
              orm_filters = {}
              for query_param, field_name in self.serializer_class.parent_lookup_kwargs.items():
                  orm_filters[field_name] = self.kwargs[query_param]
              return queryset.filter(**orm_filters)
          return queryset
  
new_contents: class NestedViewSetMixin(object):
      def get_queryset(self):
          """
          Filter the `QuerySet` based on its parents as defined in the
          `serializer_class.parent_lookup_kwargs`.
          """
          queryset = super(NestedViewSetMixin, self).get_queryset()
          serializer_class = self.get_serializer_class()
          if hasattr(serializer_class, 'parent_lookup_kwargs'):
              orm_filters = {}
              for query_param, field_name in serializer_class.parent_lookup_kwargs.items():
                  orm_filters[field_name] = self.kwargs[query_param]
              return queryset.filter(**orm_filters)
          return queryset
  
subject: Fix to use get_serializer_class method instead of serializer_class