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How do I display related posts based on categories instead of tags?

Open buddy/lib/admin/inc/shortcodes/related-posts.php and find:

$tags = wp_get_post_tags($id);
$tempQuery = $wp_query;

if($tags) {
$tag_ids = array();

foreach($tags as $individual_tag) $tag_ids[] = $individual_tag->term_id;

$args=array(
'post_type' => explode(',', $content),
'post_status' => 'publish',
'cat' => $cats,
'paged' => 1,
'ignore_sticky_posts' => 0,
'orderby' => $orderby,
'order' => $order,
'posts_per_page' => $per_page,
'offset' => $offset,
'tag__in' => $tag_ids,
'post__not_in' => array($id)
);

Replace with:

$tags = get_the_category($id);
$tempQuery = $wp_query;

if($tags) {
$tag_ids = array();

foreach($tags as $individual_tag) $tag_ids[] = $individual_tag->term_id;

$args=array(
'post_type' => explode(',', $content),
'post_status' => 'publish',
'cat' => $cats,
'paged' => 1,
'ignore_sticky_posts' => 0,
'orderby' => $orderby,
'order' => $order,
'posts_per_page' => $per_page,
'offset' => $offset,
'category__in' => $tag_ids,
'post__not_in' => array($id)
);