Not every application starts from scratch, sometime you have to deal with a legacy database. Walking the Rails golden path makes life easy, and there's a perception that stepping off that path is incredibly painful.
It's not true. If your database is well designed and but doesn't follow the Rails naming conventions, it's easy to make them play nicely together. However, if your database structure is crap to begin with, then there's only so much Rails can do for you. ActiveRecord is a mapper between the database and objects, but it's not a DBA-in-a-Box.
In a green-field app, ActiveRecord and the Database fit right together: