My issue here is that BorrowABook is a verb. This isn't a model class, and in my understanding of OOP, doesn't really deserve a class by itself at all. You could instead put the borrow method in a class called Book that represented the model of your application. Then you call borrow on the book, the book does some logic, and returns a status code to the controller. Or since it's a bit silly to ask a book to borrow itself, have a library model object that handles a bunch of book objects, and put the borrow method in the library class. That's how I do MVC anyways.