After changes to the Core Data entities, you must recreate the managed object subclass files via "Editor -> Create NSManagedObject Subclass …" from the Xcode menu. Manually editing these files only bears the danger of introducing errors.
Alternatively, use tools like "mogenerator" to create and update the files automatically. See (for example) Mogenerator and Xcode 4 advantages/disadvantages for a comparison between "mogenerator" and the built-in Xcode file generation.