Clients, services... the original developer didn't see anything wrong with it, since transactions exist and foreign key relationships make sure the data model stays consistent, right? And you can combine some frequently used methods accessing the database in some helper classes, and if some of the clients use those, that's basically your business logic!

It's a pretty impressive house of cards.

Refactoring (or rather re-designing) this sort of thing while still churning out new features is tricky to say the least.