I got it working! Here’s my code – just in case someone else finds it useful later on…
<?php // args
$args = array(
'numberposts' => -1,
'order' => 'ASC',
'orderby' => 'menu_order name',
'post_type' => 'lesson',
'meta_key' => 'attached_unit',
'meta_value' => get_the_ID()
);
// get results
$the_query = new WP_Query( $args );
// The Loop
?>
<?php if( $the_query->have_posts() ): ?>
<ul>
<?php while ( $the_query->have_posts() ) : $the_query->the_post(); ?>
<li>
<a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
</li>
<?php endwhile; ?>
</ul>
<?php endif; ?>
<?php wp_reset_query(); ?>
Thanks so much for your help, Elliot! (ACF is simply superb!)
Andy