One approach is to store one document per user, with a ratings field that is a hash of item ids to users, for example
class UserRating
include MongoMapper::Document
key :ratings
key :user_id
end
UserRating.create(:user_id => 1, :ratings => {"1" => 4, "2" => 3})
You have to use string keys for the hash. This approach doesn't make it easy to retrieve all the ratings for a given document - if you do that a lot it might be easier to store a document per item instead. It's also probably not very efficient if you only ever need a small proportion of a user's ratings at a time.
Obviously you can combine this with other approaches to increasing write throughput, such as batching your inserts or sharding your database.