protected function get_column_type($columnname) {
        if (! class_exists($columnname)) {
            if (class_exists('core_question\\bank\\' . $columnname)) {
                $columnname = 'core_question\\bank\\' . $columnname;
            } else {
                throw new \coding_exception("No such class exists: $columnname");
            }
        }
        if (empty($this->requiredcolumns[$columnname])) {
            $this->requiredcolumns[$columnname] = new $columnname($this);
        }
        return $this->requiredcolumns[$columnname];
    }