Hi @andymacleod
When on the unit
single page, you can load the lessons with a get_posts
function. You can learn more about this here:
http://www.advancedcustomfields.com/resources/how-to/how-to-query-posts-filtered-by-custom-field-values/
Notes:
1. You do not need to ‘modify’ the loop
2. Use get_posts to load the lessons and loop over them
3. Your meta_query args should find posts where 'post_object_field_name' => get_the_ID()
Thanks
E