docs: def get_order_key(self, **kwargs):
          """
          function to get the order key to apply it in the filtered queryset
  
          :param kwargs: dict: query dict sent by data tables package
          :return: str: order key, which can be used directly in queryset's
            order_by function
          """
          # get the mapping enumeration class from Meta class
          mapping = self.mapping
          # use the first element in the enumeration as default order column
          order_column = kwargs.get('order[0][column]',
                                    [mapping.keys()[0]])[0]
          order_column = ensure(int, order_column, mapping.keys()[0])
          order = kwargs.get('order[0][dir]', ['asc'])[0]
  
          order_key = mapping.from_key(order_column).label
          # django orm '-' -> desc
          if order == 'desc':
              order_key = '-' + order_key
          return order_key