Alternative for Just One Category plugin for 3.1

Since Just One Category isn’t working on 3.1 I searched for a solution.

Here you’ll find a workaround to exclude child posts from parent categories. I deactivated Just One Category, made the changes to archive.php as mentioned in the link below and upgraded. Now things are working again.

Before your category archive loop

<?php
$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
$current_cat = get_query_var('cat');
$args=array(
	'category__in' => array($current_cat),
	'paged' => $paged
);
query_posts($args);
?>

After category loop

<?php
set_query_var("cat",$current_cat);
?>

Source: https://www.kristarella.com/2010/03/wordpress-exclude-child-categories-from-category-archives/