search for: subtopics_us

Hi, So i have users in the system and i have topics. Topics also have_many subtopics. What is the cleanest way to setup the AR relationships if a user can either have_many topics or subtopics ? Its basically a dual select box dropdown, where the second dropdown is optional. Is the best to have user_topics, and user_subtopics tables and manage that way ? or combine into one table with STI