No, it's not. But since Rails 3 neither is rails really. Rails is more about the ActiveModel pattern, which has it's own API. Active Record inherits from ActiveModel. You can pick and choose what you want in your persistence classes, or even better, make models/classes that have no persistence layer tie.

For instance, if you want to use a class like it was a MySQL table, you could include a few ActiveModel classes which will make things for ActiveSupport helpers work with them.

http://api.rubyonrails.org/classes/ActiveModel.html - pick and choose what you want in your classes. This was one of the major changes in Rails 3

EDIT: just to clear it up, by ActiveModel I mean Model. Rails is convention over configuration and adheres to the MVC pattern and philosophy