use id as idAttribute for user model, and source to user_id to match Rails naming.
comment getters.
Revert "use id as idAttribute for user model, and source to user_id to match Rails naming."

This reverts commit f86304155007199bbcd6b758af4f9ce209fb1b89.
we need to use name as user's idAttribute because otherwise we can't fetch, but we should fetch it from user_name to avoid naming conflict. and we don't need to autofetch.
add user-comment relation.