When someone wants to add their favorite, check the database to see if it exists. What's the problem?
-Dave
The problem is that once you dive in, all of the “cool” rails scoping (user.books.create…) fails and you start managing the relationships yourself (user.books << book.find_or_create). It feels “wrong”, but as I’m diving in deeper it seems like the best solution is to create an intermediate class to handle the association management and keep the logic out of the controllers.
What's wrong with "@user.favorite_books << Book.find_or_create_by_title(@title)"? That's still some pretty "cool" Rails scoping. Compared to what you'd have to do in raw SQL, it saves you tons of work, and is still very clear. Sure it's not as simple as "@user.favorite_books.create @title", but hey, if you've got more needs, such as non-duplication, often you need to write more code, and in this case the difference is still small.
Seems to me that "BookListManager.add_to_favorites_for @user, @title" isn't any better than that. It just adds one more layer of indirection, one more place to have to go look to find out what's happening. Of course the "enterprisey" lovers of huge ER diagrams would advocate for that approach (complete with a separate interface class for it to implement), but they can just go pound Java.
-Dave