How to exclude posts that are already related elsewhere?

Hi,

I have 2 post types: Courses and Lessons. I’m using a Relationship field to relate the Lessons to the Courses. What I would like to do is exclude Lessons that have already been related to another course.

Essentially, each Lesson would only be able to be related to ONE course.

I’m using the acf/fields/relationship/query/ filter, and I assume that there should be some way to accomplish this in conjunction with your plugin… but I’m just not sure. Please let me know if I’m totally off base.

Thanks!