pre_get_posts to get all posts connected to single object

function custom_query_vars_filter($vars) {
    $vars[] .= 'relationships';
    return $vars;
}
add_filter( 'query_vars', 'custom_query_vars_filter' );

function prefix_get_related_posts( $query ) {
    
    $post_id = get_query_var('relationships');
    
    if ( empty( $post_id ) || is_admin() ) {
        return $query;
    }

    if( is_post_type_archive( 'poa_person' ) && $query->is_main_query() ) {
        $query->set( 'relationship', 
            array( 
                'id' => 'poa_practice_to_poa_person',
                'from' => $post_id,
            )
        );
    }

    return $query;
}
add_filter( 'parse_query', 'prefix_get_related_posts', 10, 1);

parse_query works great. But now my styling is all jacked up. At least the query is working now. Thanks for the help.

I am following the video EXACTLY and am still seeing errors. OMG this is so frustrating. Your video is perfect and is not throwing errors. I setup the freshest install and just installed metabox and mb relationships with custom post types built all in the same file. Made the relationships and ran the query on the default theme like your video. Still seeing the post type being stripped for certain functions like get_post_type_object. I actually think get_post_type_object is returning null since the post type is being sent as a string not an array or object. Is_scalar is exiting early for get_post_type_object.

See the screenshots. I am getting the exact same query you producing with post type being sent as a string.

Running WP 4.9.8 on Flywheel
MySQL 5.6.34
PHP 7.2 and 7.0.3
Nginx
Twenty Seventeen theme

Could that have anything to do with it?

For some reason my query is setting post type to any. For some reason in parse_query it will change the post type any when using a tax_query. I don't have a tax_query nor do I have any taxonomies on these two post types.

https://developer.wordpress.org/reference/classes/wp_query/parse_query/

Search for post_type

add_action( 'parse_query', 'rel_filter_query_by_relationship', 10, 1 );
/**
 * Filter the main query by using URL parameters
 * 
 * @param object $query WP_Query.
 * @return object
 * 
 * @since 3.6.8
 */
function rel_filter_query_by_relationship( $query ) {
    
    $id     = $query->get('relation_id');
    $from   = $query->get('relation_from');
    $to     = $query->get('relation_to');
    
    if ( ! empty( $id ) && ! is_admin() && $query->is_main_query() ) {

        if ( ! empty( $from ) ) {
            
            $query->set( 'relationship', 
                array( 
                    'id'  => esc_attr( $id ),
                    'from'    => intval( $from ),
                )
            );

        } elseif ( ! empty( $to ) ) {

            $query->set( 'relationship', 
                array( 
                    'id'  => esc_attr( $id ),
                    'to'  => intval( $to )
                )
            );
        }
        
        
    }

}

add_filter( 'query_vars', 'rel_add_query_vars' );
/**
 * Add query vars to build query with URL parameters
 *
 * @param array $vars WP_Query variables.
 * @return array
 * @since 3.6.8
 */
function rel_add_query_vars( array $vars ) {
    $vars[] = 'relation_id';
    $vars[] = 'relation_from';
    $vars[] = 'relation_to';
    return $vars;
}

add_action( 'mb_relationships_init', function() {
    MB_Relationships_API::register( array(
        'id'   => 'practices_to_persons',
        'from' => array(
            'object_type' => 'post',
            'post_type' => 'practice',
            'admin_column' => 'after title'
        ),
        'to'   => array(
            'object_type' => 'post',
            'post_type' => 'person',
            'admin_column' => 'after title',
        ),
    ) );
} );