public static function unassign_roles($unassignments) {
         global $DB;

        // Do basic automatic PARAM checks on incoming data, using params description
        // If any problems are found then exceptions are thrown with helpful error messages
        $params = self::validate_parameters(self::unassign_roles_parameters(), array('unassignments'=>$unassignments));

        $transaction = $DB->start_delegated_transaction();

        foreach ($params['unassignments'] as $unassignment) {
            // Ensure the current user is allowed to run this function in the unassignment context
            $context = self::get_context_from_params($unassignment);
            self::validate_context($context);
            require_capability('moodle/role:assign', $context);

            // throw an exception if user is not able to unassign the role in this context
            $roles = get_assignable_roles($context, ROLENAME_SHORT);
            if (!array_key_exists($unassignment['roleid'], $roles)) {
                throw new invalid_parameter_exception('Can not unassign roleid='.$unassignment['roleid'].' in contextid='.$unassignment['contextid']);
            }

            role_unassign($unassignment['roleid'], $unassignment['userid'], $context->id);
        }

        $transaction->allow_commit();
    }