Been doing Rails development for 6+ years. The most maintainable codebases I've worked on had some kind of service layer between the controller and the model. We used the "interactor" gem to create individual units of business logic that we could reuse and piece together into larger "flows". Business logic stayed in the interactors, persistence logic in the models. This lead to skinny controllers, skinny models and many, many many reusable skinny services. One fortunate side effect is that all these pieces became extremely easy to test in isolation, as well as integration tested.