achievement.rb

class Achievement
  include MongoMapper::Document
  
  key :name, String
  key :description, String
  key :requirements, String
  key :lang, String
end

achievement_system.rb

class AchievementSystem
  
  # Scans to see if the character has completed any Achievements of type
  def self.scan(player, type)
    completed = []
    
    achievements = Achievement.find(:all, :type => type)
    
    for achievement in achievements
      found = false
      player.character_achievements.each do |pach|
        if pach.achievement_id == achievement.id
          found = true
        end
      end
      next if found
      
      case type
        when "FAMILIAR"
          completed << achievement if eval("player.character_familiars.count %s" % achievement.requirements)
        when "QUEST"
          completed_quests = player.character_quests.find(:all, :is_completed => true)
          completed << achievement if eval("completed_quests.size %s" % achievement.requirements)
        when "GOLD"
          completed << achievement if eval("player.total_gold %s" % achievement.requirements)
      end
    end
    
    return completed.collect do |ach|
      CharacterAchievement.create(:character_id => player.id, :achievement_id => ach.id)
    end
  end
  
end

achievements.yml

- name: A Familiar!
  description: Received first familiar.
  requirements: " == 1"
  type: FAMILIAR
  
- name: A Quest!
  description: Completed first quest.
  requirements: " == 1"
  type: QUEST
  
- name: The Rich Get Richer
  description: Received 100 gold.
  requirements: " >= 100"
  type: GOLD

seeds.rb

#######################################
# Achievements
#######################################

puts "Populating Achievements..."
achievements = YAML.load(File.open("#{RAILS_ROOT}/db/load/en/achievements.yml"))
for achievement in achievements
  Achievement.create(
    :name => achievement['name'],
    :description => achievement['description'],
    :requirements => achievement['requirements'],
    :type => achievement['type'],
    :lang => 'en'
  )
end