@Override
 protected Collection<AsteriskServer> retrieveJDOObjects(ProgressMonitor monitor) {
   Collection<AsteriskServer> result =
       PhoneSystemDAO.sharedInstance()
           .getPhoneSystems(
               AsteriskServer.class,
               true,
               FETCH_GROUPS_ASTERISK_SERVER,
               NLJDOHelper.MAX_FETCH_DEPTH_NO_LIMIT,
               monitor);
   return result;
 }
 /*
  * (non-Javadoc)
  * @see org.nightlabs.jfire.base.ui.entity.tree.ActiveJDOEntityTreeCategory#retrieveJDOObjects(java.util.Set, org.nightlabs.progress.ProgressMonitor)
  */
 @Override
 protected Collection<AsteriskServer> retrieveJDOObjects(
     Set<PhoneSystemID> objectIDs, ProgressMonitor monitor) {
   List<PhoneSystem> phoneSystems =
       PhoneSystemDAO.sharedInstance()
           .getPhoneSystems(
               objectIDs,
               FETCH_GROUPS_ASTERISK_SERVER,
               NLJDOHelper.MAX_FETCH_DEPTH_NO_LIMIT,
               monitor);
   Collection<AsteriskServer> result = new ArrayList<AsteriskServer>(phoneSystems.size());
   for (PhoneSystem phoneSystem : phoneSystems) {
     if (phoneSystem instanceof AsteriskServer) result.add((AsteriskServer) phoneSystem);
   }
   return result;
 }