Ruby on Rails Fixtures

When I use test fixtures to populate test data into mysql database I am running into an unexpected behavior. Only columns explicitly defined in the fixture is populated into the database. However, created_on, created_by, updated_on, updated_by fields along with all fields normally populated by observers are NULL. For example, I have a couple fields that have their values calculated via observers. I want to test that these calculations are performed correctly by the observer, so I don't want to hard code their values in the fixture.

Has anyone else run into this issue? If so, how did you perform testing of these fields? I realize that I could perform these tests without fixtures by creating and saving the object. Behind the scenes, Active Record populates all the fields in the expected manner. Then I could find the saved record and assert_equal the expected calculations. However this approach seems brittle and I rather use fixtures. It doesn't seem (from the results) that I am getting that the fixtures functionality uses Active Record to serialize data to mysql.

Thanks! Asa