I came across Jay Fields' blog entry, "Rails: How we test".
I'm glad that I happened to find this article because it contains the ways to use mock object and to make rails unit test not to access database. I like the way of writing unit tests without accessing database by mocking database access. But I didn't know how to do that with Rails. And I wanted to follow the Rails convention because that's one of the strength of Rails. But now I can do whole a lot of better way of writing tests.