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.