public static function database($db_name = 'db')
 {
     $db_var = $db_name;
     $obj = base::getInstance();
     if (isset($obj->{$db_var}) and is_object($obj->{$db_var})) {
         return;
     }
     require SYS . 'database' . DS . 'db_factory' . PHP_EXT;
     $obj->{$db_var} = db_factory::callDB($db_name);
     base::$databases[$db_var] = $db_var;
     if (count(base::$models) >= 0) {
         foreach (base::$models as $model_name) {
             $obj->{$model_name}->{$db_var} =& $obj->{$db_var};
         }
     }
 }