I am designing a cinema app, so I made a parent table topics (polymorphic table) which have topicable_id and 'topicable_type' fields to get the related model, I think it makes the search faster meanwhile the all titles in the same table, is it a good approach to go-ahead? if I got one million records in movies and series table I'm going to end up with two million records in topics table, is it a bad design?