function delete($id)
 {
     if (!is_array($document = $this->read_single($id))) {
         return false;
     }
     $this->db->transaction_begin();
     parent::delete($id);
     if ($this->db->transaction_commit()) {
         if (file_exists($document['filename'])) {
             unlink($document['filename']);
         }
         return true;
     }
     return false;
 }