column creator does not exist

[allenwq]

After #12, the creator association is delayed loaded. However, it breaks in the cases when columns is not called before the query is generated.

This can be reproduced in console:

> Course::Announcement.where(creator: current_user)

ActiveRecord::StatementInvalid: PG::UndefinedColumn: ERROR:  column course_announcements.creator does not exist

 SELECT "course_announcements".* FROM "course_announcements" WHERE "course_announcements"."creator" = 15082