add_action('mb_relationships_init', function () {
    $post_id = null;
    if ( isset( $_GET['post'] ) ) {
        $post_id = intval( $_GET['post'] );
    } elseif ( isset( $_POST['post_ID'] ) ) {
        $post_id = intval( $_POST['post_ID'] );
    }

    global $wpdb;
    $filteredIds = $wpdb->get_col( $wpdb->prepare( 'SELECT `from` FROM wp_mb_relationships WHERE wp_mb_relationships.to = %s', $post_id ) );

    MB_Relationships_API::register(array(
        'id' => 'p2p',
        'from' => array(
            'type' => 'post',
            'post_type' => 'post',
            'admin_column' => true,
            'meta_box' => [
                'title' => 'Pages'
            ],
            'query_args' => [ 'post__in' => $filteredIds ],
        ),
        'to' => array(
            'type' => 'post',
            'post_type' => 'page',
            'meta_box' => [
                'title' => 'Posts'
            ],
        ),
    ) );
} );