/**
  * adds a new relation
  * 
  * @param   Tinebase_Model_Relation $_relation 
  * @return  Tinebase_Model_Relation|NULL the new relation
  * @throws  Tinebase_Exception_Record_Validation
  */
 protected function _addRelation(Tinebase_Model_Relation $_relation)
 {
     $_relation->created_by = Tinebase_Core::getUser()->getId();
     $_relation->creation_time = Tinebase_DateTime::now();
     if (!$_relation->isValid()) {
         throw new Tinebase_Exception_Record_Validation('Relation is not valid' . print_r($_relation->getValidationErrors(), true));
     }
     try {
         $result = $this->_backend->addRelation($_relation);
     } catch (Zend_Db_Statement_Exception $zse) {
         Tinebase_Core::getLogger()->warn(__METHOD__ . '::' . __LINE__ . ' Could not add relation: ' . $zse->getMessage());
         $result = NULL;
     }
     return $result;
 }