/**
  * Returns the ID of the archived subperiods.
  * 
  * @return array Array of the idArchive of the subperiods
  */
 protected function loadSubperiodsArchive()
 {
     $periods = array();
     // we first compute every subperiod of the archive
     foreach ($this->period->getSubperiods() as $period) {
         $archivePeriod = new Piwik_Archive_Single();
         $archivePeriod->setSite($this->site);
         $archivePeriod->setPeriod($period);
         $archivePeriod->prepareArchive();
         $periods[] = $archivePeriod;
     }
     return $periods;
 }