I'm using doctrine2

I try to get the number of "Events" ordered by number of tickets sold

$manager = $this->getDoctrine()->getManager();
$builder = $manager->createQueryBuilder();

return $builder
    ->select('e')
    ->from('AppBundle:Event', 'e')
    ->leftJoin('e.tickets', 't')
    ->orderBy('COUNT(t)')
    ->groupBy('e.id')
    ->setMaxResults(10)
    ->getQuery()
    ->getResult()
;

This produce the error

[Syntax Error] line 0, col 80: Error: Expected known function, got 'COUNT'

With the following DQL produce

QueryException: SELECT e FROM AppBundle:Event e LEFT JOIN e.tickets t GROUP BY e.id ORDER BY COUNT(t.id) ASC

However if I put the COUNT in the SELECT part, it does work correctly (but then I have to do additional treatment to remove the unwanted field of the SELECT)

return $builder
    ->select('e')
    ->from('AppBundle:Event', 'e')
    ->addSelect('COUNT(t) AS nbrTicketsSold')
    ->leftJoin('e.t', 't')
    ->orderBy('nbrTicketsSold')
    ->groupBy('e.id')
    ->setMaxResults(10)
    ->getQuery()
    ->getResult()
;

Is using COUNT in an ORDER BY clause a known limitation of DQL ?