Session error

Got this error when I change git branch from rails2 to rails3. When I am doing some upgrading task.

ActionDispatch::Session::SessionRestoreError
Session contains objects whose class definition isn't available.Remember to require the classes for all objects kept in the session.(Original exception: uninitialized constant ActionController::Flash::FlashHash [NameError])

Just delete the browser's cookie, fixed this issue. I guess this two version have different way to handle the session or cookie. Then I google again. Got this Link , Look like someone already figure it out.