Updates the rows in this queryset

def update(self, **values):
        """ Updates the rows in this queryset """
        if not values:
            return

        nulled_columns = set()
        us = UpdateStatement(self.column_family_name, where=self._where, ttl=self._ttl,
                             timestamp=self._timestamp, transactions=self._transaction)
        for name, val in values.items():
            col_name, col_op = self._parse_filter_arg(name)
            col = self.model._columns.get(col_name)
            # check for nonexistant columns
            if col is None:
                raise ValidationError("{}.{} has no column named: {}".format(self.__module__, self.model.__name__, col_name))
            # check for primary key update attempts
            if col.is_primary_key:
                raise ValidationError("Cannot apply update to primary key '{}' for {}.{}".format(col_name, self.__module__, self.model.__name__))

            # we should not provide default values in this use case.
            val = col.validate(val)
            
            if val is None:
                nulled_columns.add(col_name)
                continue

            # add the update statements
            if isinstance(col, Counter):
                # TODO: implement counter updates
                raise NotImplementedError
            elif isinstance(col, (List, Set, Map)):
                if isinstance(col, List):
                    klass = ListUpdateClause
                elif isinstance(col, Set):
                    klass = SetUpdateClause
                elif isinstance(col, Map):
                    klass = MapUpdateClause
                else:
                    raise RuntimeError
                us.add_assignment_clause(klass(col_name, col.to_database(val), operation=col_op))
            else:
                us.add_assignment_clause(AssignmentClause(
                    col_name, col.to_database(val)))

        if us.assignments:
            self._execute(us)

        if nulled_columns:
            ds = DeleteStatement(self.column_family_name, fields=nulled_columns, where=self._where)
            self._execute(ds)