This actually represents an extremely common misconception about MVC that's common among Rails developers as well as most of the other web frameworks that have chosen some form of the ActiveRecord pattern for their ORM. Model != Persistence layer.

A Model, in the classic MVC, is a model of your problem domain with classes that represent entities and services and whatever it takes to represent your business logic in code, not just a bunch entities that get saved and retrieved from the database. Your persistence layer generally lives below the model and knows how to persist your model's entities however you so choose. The ActiveRecord pattern really muddles this distinction by putting all of your persistence code right in with your business logic. It's a great pattern when you're building something simple with minimal actual business logic, but quickly becomes unwieldily when you get into more complex situations.