/**
  * Adds an existing channel to the folder. If the given channel is already
  * part of another folder, it will be automatically removed from the old folder.
  *
  * @param MessageChannel $channel The channel which should be added to the folder.
  *
  * @return AbstractPostboxFolder Returns itself (fluent-interface)
  */
 public function addChannel(MessageChannel $channel)
 {
     // first remove the channel
     /* @var $oldFolder AbstractPostboxFolder */
     $oldFolder = $channel->loadRelatedObject('PostboxFolder2MessageChannel');
     if ($oldFolder !== null) {
         $oldFolder->removeChannel($channel);
     }
     $this->createAssociation('PostboxFolder2MessageChannel', $channel);
     return $this;
 }