Rating System

I'm trying to create a Rating System for my application.

My Model is called "game" and has a "rating" field. Each logged in User
shall give his rating (preferably via ajax), while the GAme Objects
saved all Ratings given and displays the total amount of them.

Is there a nice gem for this feature? I tried ajaxful_rating, but it
didn't really work. It only caused my application not to accept any more
rails commands. If theres no nice documented gem, what would be the
generel way to program this from scratch?

I guess I'll be needing a Rating Controller,but do I need a Rating
Model, or is the rating field in my game object already enough?

I'd appreciate any kind of advice!