code: public static function getFieldColumns(TableMapper $databaseTableMapper): array 
      {
          if (!isset(self::$tableMapper)) {
              self::$tableMapper = $databaseTableMapper;
          } elseif (self::$tableMapper !== $databaseTableMapper) {
              throw new \InvalidArgumentException("Table mapper mismatch");
          }
  
          if (!isset(self::$fieldColumns)) {
              self::setFieldColumns();
          }
  
          return self::$fieldColumns;
      }
docs: allow access without constructing