queryBuilder + getFieldNames

Hi 

This works fine 

PHP Code:

$db  = \Config\Database::connect();
      $r = $db->query("select * from table");
      $fn = $r->getFieldNames();
      var_dump($fn); 


But how to bring out the same thing with queryBuilder ?
This cannot be done

PHP Code:

$users = new \App\Models\UserModel();
      $r =  $users->findAll();
      var_dump($r->getFieldNames());