player.rb

class Player < ActiveRecord::Base

  # ...

  def skills
    @skills ||= (Skill.where(:player_id => self.id).first || \
                 Skill.create(:player_id => self.id))
  end

end

skill.rb

class Skill
  include Mongoid::Document
  field :player_id, :type => Integer

  field :accuracy, :type => Integer, :default => 0
  field :craftsmanship, :type => Integer, :default => 0
  field :perception, :type => Integer, :default => 0

  index :player_id, :unique => true

  def player
    Player.find_by_id(player_id)
  end
end