public static function assign_roles($assignments) {
        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
        $params = self::validate_parameters(self::assign_roles_parameters(), array('assignments'=>$assignments));

        $transaction = $DB->start_delegated_transaction();

        foreach ($params['assignments'] as $assignment) {
            // Ensure correct context level with a instance id or contextid is passed.
            $context = self::get_context_from_params($assignment);

            // Ensure the current user is allowed to run this function in the enrolment context.
            self::validate_context($context);
            require_capability('moodle/role:assign', $context);

            // throw an exception if user is not able to assign the role in this context
            $roles = get_assignable_roles($context, ROLENAME_SHORT);

            if (!array_key_exists($assignment['roleid'], $roles)) {
                throw new invalid_parameter_exception('Can not assign roleid='.$assignment['roleid'].' in contextid='.$assignment['contextid']);
            }

            role_assign($assignment['roleid'], $assignment['userid'], $context->id);
        }

        $transaction->allow_commit();
    }