Extending a model in a Rails 3 (pre-3.1) engine

baby_talk.rb

module BabyTalk
  class Engine < Rails::Engine
    config.to_prepare do
      User.class_eval do
        has_many :participations, :foreign_key => "participant_id"
        has_many :rooms, :through => :participations
      end
    end
  end
end