public static function listGroups(modResource &$resource, array $sort = array('id' => 'ASC'), $limit = 0, $offset = 0) {
        $result = array('collection' => array(), 'total' => 0);
        $c = $resource->xpdo->newQuery('modResourceGroup');
        $c->leftJoin('modResourceGroupResource', 'ResourceGroupResource', array(
            "ResourceGroupResource.document_group = modResourceGroup.id",