Id Parse to Username

[eluser]nagata[/eluser]
I am making the blog as a study of php, I have met a problem, I have made an authentication system (All except login/session), And Initially I coded my app so that each time news are added, they set 3 values: Title, Text and Author,
Author is set as Id of user who posts, Its just I plan to have name change feature in future,
And I met a problem exactly here, I need to display user name, and not user id,
Sadly this 2 things are in separate models and controllers, I need an easy fast way to convert user id's on a page to a user name, counting that author name will be displayed on news list page too, meaning going though all news db has (Which will be alot...)
I know, this might be hard to get, But I really need this, I have been trying to make it work like for 2 hours now... no result...