public function attachRole( core_kernel_classes_Resource $user,  core_kernel_classes_Resource $role)
    {
    	try{
	        if (false === $this->userHasRoles($user, $role)){
	        	$rolesProperty = new core_kernel_classes_Property(GenerisRdf::PROPERTY_USER_ROLES);
	        	$user->setPropertyValue($rolesProperty, $role);	
	        }
        }
        catch (common_Exception $e){
        	$roleUri = $role->getUri;
        	$userUri = $user->getUri();
        	$msg = "An error occured while attaching role '${roleUri}' to user '${userUri}': " . $e->getMessage();
        	throw new core_kernel_users_Exception($msg);
        }
    }