How to implement an achievement system in RoR

I agree with your idea to use an Achievement model.

You should probably not implement the triggers in your controllers, though. Imagine that you have two ways to post a comment; you will inevitably get code duplication. This sort of behaviour belongs in a model.

Suppose you want to track the number of comments that a user makes, and award an achievement for 100 comments. You could have the following models:

class User < ActiveRecord::Base
  has_many :comments
  has_many :achievements

  def award(achievement)
    achievements << achievement.new
  end

  def awarded?(achievement)
    achievements.count(:conditions => { :type => achievement }) > 0
  end
end

class Achievement < ActiveRecord::Base
  belongs_to :user
end

class Comment < ActiveRecord::Base
  belongs_to :user
end

class CommentAchievement < Achievement
  def self.check_conditions_for(user)
    # Check if achievement is already awarded before doing possibly expensive
    # operations to see if the achievement conditions are met.
    if !user.awarded?(self) and user.comments.size > 100
      user.award(self)
    end
  end
end

The different achievements are all subclasses of Achievement model, and use single table inheritance so that they are stored in just one table. The subclasses can contain all logic required for each individual achievement. You can also store additional information in this model, such as the date on which the achievement was awarded. To make sure that the database rejects duplicate achievements, you could create a UNIQUE index on the type and user_id columns.

CommentAchievement.check_conditions_for(user) can be called whenever you wish to. You may create a background job that runs every now and then, or you could create an observer:

# app/models/comment_achievement_observer.rb
class CommentAchievementObserver < ActiveRecord::Observer
  observe :comment

  def after_create(comment)
    CommentAchievement.check_conditions_for(comment.user)
  end
end

# config/environment.rb
config.active_record.observers = :comment_achievement_observer

The above is just one idea of how to do it, of course there may be others. The code is just an example, I haven’t actually tested it. Hopefully it’s of some help to you.