Elgg-Lorea-Hydra/mod/questions/activate.php

<?php
/**
 * This file is executed when the plugin is enabled
 */

// register our own classes
if (!update_subtype('object', 'question', 'ElggQuestion')) {
	add_subtype('object', 'question', 'ElggQuestion');
}

if (!update_subtype('object', 'answer', 'ElggAnswer')) {
	add_subtype('object', 'answer', 'ElggAnswer');
}