After working a while with Codeigniter, I am totally in love with the ActiveRecord way of writing mySQL queries. Now, when working on another non-codeigniter project I've been recommended to use an ORM to handle the DB mapping which sounds great, but after looking into Propel & Doctrine, I have become quite afraid of the additional configuration files containing the database structure, more or less.
Why does these ORM:s define the database structure and what advantage has it over an non-defined ORM like the one that's bundled with codeIgniter?