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  • Meanwhile, I’ve converted the Relationship field to a Post Object field since each Artwork only has one Artist associated with it. That has this post object query filter:

    function artist_post_object_query( $args, $field, $post_id ) {
    $args['meta_query'] = array(
        'relation' => 'AND',
        'last_name' => array(
            'key' => 'artist_last_name'
         'first_name' => array(
    	'key' => 'artist_first_name'
          $args['orderby']	= array(
    	'last_name' => 'ASC',
    	'first_name' => 'ASC'
        // return
        return $args;
    add_filter('acf/fields/post_object/query/name=artists_name', 'artist_post_object_query', 10, 3);

    So Artist correctly alphabetized in the Artwork edit screen. (Although that filter doesn’t extend to the List screen sort order, for which I’m using Admin Columns Pro.)

    Essentially I want that filter to work any time the Artworks field ‘artist_name’ is being used as a meta_key to sort so that ‘meta_value’ can be replaced with ‘artist_last_name’ + ‘artist_first_name’ instead of the post ID. It seems like a posts_join and then posts_orderby filter would work but I can’t get this join to work:

    add_filter('posts_join', 'artist_join', 10, 2 );
    function artist_join($joins)
        if(is_post_type_archive('artworks') || (is_admin() && $_GET['post_type'] == 'artworks')) {
            global $wpdb;        
            $joins .= "  INNER JOIN {$wpdb->postmeta} pm_last_name ON pm_last_name.post_id={$wpdb->posts}.ID
           WHERE pm_last_name.meta_key='artist_last_name'" ;
            $joins .= "  INNER JOIN {$wpdb->postmeta} pm_first_name ON pm_first_name.post_id={$wpdb->posts}.ID
           WHERE pm_first_name.meta_key='artist_first_name'" ;
        return $joins;

    Any help would be much appreciated.