This is a very good comment. Every complex Rails app I've worked on, we end up having the same debates:

"Our models are too fat, let's refactor."

"Where should the code go?"

"Let's put them in a utility class."

"No, a utility class is a code smell from the point of view of true Object Oriented theory."

"Then let's put them in a service class."

"No, a service class is just a utility class with a different name."

"Then where should the code go?"

A long debate ensues, and then we end up with some crazy solution that includes everything that jeff_vader just suggested as a problematic:

" Where's the code? 'lib'? 'core'? 'app'? 'api'?"