DB Connections with hardcoded database.yml username? How do I connect as actual user?

I'm a bit (OK..a lot) confused about the way Rails establishes a DB connection with the login/passwd that is hardcoded in database.yml, even in production mode.

I'm using Rails to develop internal, inward-facing apps for my company, rather than an external, outward-facing public web site.

Once an end-user of my app authenticates w/ his login and passwd, I want him to connect to the database with *his* login, rather than some generic database.yml login that everyone uses. Different users have different DB privileges, and diagnosing back-end DB issues is easier when I am seeing unique logins rather than a generic one. (I'm a DBA).

I can certainly understand that, for a public web site, with potentially hundreds or thousands of users, you don't want each user to have an official OS-level login that they can connect to the DB with. But in my case, each of my several dozen users already has an OS-level login. I use DB2, and its logins are based on those in the OS.

Am I missing something here? Can Rails be influenced to conect to the DB in this way?

Any help appreciated.