Here's some snippets from my Podcast to Episodes relationship. As far as I can tell, this part continues to work fine in the last few updates of Meta Box AIO. Something is different about the way related posts are queried, however.
// register relationship
add_action('mb_relationships_init', array($this,'podcasts_to_episodes_relationship'));
function podcasts_to_episodes_relationship () {
MB_Relationships_API::register( array(
'id' => 'podcasts_to_episodes',
'reciprocal' => true,
'from' => array(
'object_type' => 'post',
'post_type' => 'episode',
'admin_column' => true,
'meta_box' => [
'title' => 'Podcast',
],
),
'to' => array(
'object_type' => 'post',
'post_type' => 'podcast',
'admin_column' => false,
),
));
}
...
// link episode id to podcast id
MB_Relationships_API::add( $episode_id, $podcast_id, 'podcasts_to_episodes');