What is ActiveRecord's STI vs polymorphic associations — when to use each?

STI shares one table for multiple subclasses — best when subclasses have the same attributes and differ primarily in behavior. Example: Animal table with Dog and Cat subclasses. Polymorphic associations allow one model to belong to multiple unrelated parent types — best when unrelated models share an associated behavior. Example: both Post and Video can have Comments. Key decision: if subclasses share data structure → STI; if a child model needs to reference multiple unrelated parent types → polymorphic. Neither is ideal for complex hierarchies — consider Multiple Table Inheritance (MTI) via gems or a separate mapping table instead.