grade must be smaller than max grade
done with enforcing upgraded upload. The key point is to add a user_id column to homework_document, pass the current_user.id to this user_id column in the controller. This way, we can see whether the edit/update is being done by a grader or a submitter. Only a grader is required to upload graded file
Check whether graded_file being uploaded via graded_file_file_name.nil? Wonder if there's a better way. Cannot check graded_file.nil?, exists?, present? all doesn't work somehow