zend framework - Database expression not used in query -

can tell me why expression not used in query below?

select accountreset.* accountreset (reset_id = '34') limit 1

public function findbyresetid($resetid, $model = null) {     $result = null;     if (isset($resetid)) {         $select = $this->getdao()->select(             array('expiration' => new zend_db_expr('unix_timestamp(expiration)'))         );         $select->where('reset_id = ?', $resetid);         $row = $this->getdao()->fetchrow($select);         if (null != $row) {             if (!($model instanceof stage5_model_passwordresetter)) {                 $model = new stage5_model_passwordresetter();             }                            // vul het model object             $model->setresetid($row->reset_id);             $model->setuserid($row->user_id);             $model->setexpiration($row->expiration);             $result = $model;          }     }     return $result;  } 

your zend_db_expr should go from() method instead of select()

$select = $this->getdao()                ->select()                ->from(                     $this->getdao()->info('name'),                     array('expiration' => new zend_db_expr('unix_timestamp(expiration)'))                 );