/**
  * Detaches an object from the object
  * eg:
  * <code>
  * // detach old posts
  * foreach( $topic->find_attached('Post') as $post )
  * {
  *  if( $post->created < mktime()-5000000 )
  *  {
  *    $topic->detach($post);
  *  }
  * }
  * </code>
  *
  * @param object  $obj  object to be detached
  */
 public function detach(&$obj)
 {
     return MyActiveRecord::UnLink($this, $obj);
 }