We'll create fresh WordPress site with Post Relation Widget installed. You have 20 minutes to test the plugin after that site we'll be deleted.
With this widget its easy to have relations between posts, depending on their category.
You can create category-relationships on back-end and have an easy widget on front-end. See or remove existing relations between the current post or add a new.
Easy undone deleting changes
Example:
Create a relationship between the categories movies and actors. On front-end add actors playing in a movie.