code: def sort(self, *columns, **options):
          sorts = self.meta.setdefault('sort', [])
          for column in columns:
              if isinstance(column, Column):
                  identifier = column.id
              elif isinstance(column, utils.basestring):
                  descending = column.startswith('-') or options.get('descending', False)
                  identifier = self.api.columns[column.lstrip('-')].id
              else:
                  raise ValueError("Can only sort on columns or column strings. Received: {}".format(column))
              if descending:
                  sign = '-'
              else:
                  sign = ''
              sorts.append(sign + identifier)
          self.raw['sort'] = ",".join(sorts)
          return self