How to modify authentication to use against a bespoke database schema

Hi all,

I am a little stuck for documentation. I am attempting to use authlogic in a project that is a mini app added to a much larger, established system. This system has a different way of storing user login information coming from multiple tables to that which is defaulted in authlogic.

It also uses three login fields (company id, user id and password) rather than the authlogic email or login field.

My problem is that I can't find any documentation to help me figure our how to configure authlogic to authenticate against the existing system's schema. I see OpenID addons but surely there must be tons of people using authlogic in code that has to be compatible with an existing database? How many greenfield sites are there these days?

I think that I need to modify the User model and fiddle around with acts_as_authentic. But if anyone can give me any pointers I would greatly appreciate it. My problem is that I am completely ignorant about the authlogic details. I have read through several tutorials and can get a basic authlogic installation up and running, but that's using the authlogic way of authenticating, not adapting authlogic to fit and existing shoe.