private function getPendingPayments(PAP_Model_User $user = null)
 {
     $payments = array();
     $unpaidCharges = PAP_Model_Charge::getUnpaidCharges($user);
     $periods = array();
     foreach ($unpaidCharges as $charge) {
         $period = new PAP_Model_Period();
         $period->loadByCode($charge->getPeriod());
         $periods[$charge->getId()] = $period;
     }
     if (!isset($user)) {
         $payments = PAP_Model_Payment::getAllPayments($periods);
     } else {
         $payments = PAP_Model_Payment::getPayments($user, $periods);
     }
     return $payments;
 }