$qr = Doctrine_Query::create()
            ->select('r.data_invio,privati.nome')
            ->from('richiestepreventivo as r')
            ->innerJoin('richiestepreventivo.idPrivato WHERE richiestepreventivo.idPrivato=privati.id');

SELECT `r`.`id` AS `r__id`, `r`.`data_invio` AS `r__data_invio` FROM `RichiestePreventivo` `r`, `RichiestePreventivo` `r2` INNER JOIN `Privati` `p` ON `r2`.`idprivato` = `p`.`id`