protected function get_fieldlist() : array {
        global $DB;

        if (null === $this->fields) {
            $fields = [];
            foreach (array_keys($DB->get_columns($this->tablename)) as $fieldname) {
                $fields["{$this->fieldprefix}{$fieldname}"] = $fieldname;
            }

            $this->fields = $fields;
        }

        return $this->fields;
    }