Socialize Models with Redis and Rails

event.rb

# app/models/event.rb
# Event model has a set (event_users_ids) tracking all the users that has an inscription to it.
# The set update is made in the inscription model which models the relation between the event and its participants.
# The operation to get the friends participating in one event is done via redis set operations (&)
class Event < ActiveRecord::Base
  include Redis::Objects
  has_many :inscriptions
  
  set :event_users_ids
  
  def friends_for(user)
    return [] unless user.present?
    
    self.event_users_ids & user.followed_user_ids # operation performed in Redis
  end
end

inscription.rb

# app/models/inscription.rb
# The event set is updated every time a new Inscription instance is created.
class Inscription < ActiveRecord::Base
  include Socialize::Socializer
  belongs_to :event
  belongs_to :user
  
  socialize_for :event, :event_users_ids
end

socialize.rb

# app/models/concern/socialize.rb
# This concern enables the socialization of one model via `socialize_for` class method.
# Every time a new instance is created it updates a redis set in the association with its belonging user.
require 'active_support/concern'

module Socialize
  module Socializer
    extend ActiveSupport::Concern

    included do
      after_create :__update_socialized_models
    end

    def __update_socialized_models
      self.class.model_updates.each do |association, set_name|
        self.send(association).send(set_name) << self.user.id
      end
    end

    module ClassMethods
      def model_updates
        @model_updates ||= []
      end

      def socialize_for(association, set_name)
        model_updates << [ association, set_name ]
      end
    end
  end
end

user.rb

# app/models/user.rb
# user has a set with all the other users following her/him
class User < ActiveRecord::Base
  include Redis::Objects
  
  set :followed_user_ids
end