gistfile1.rb

class TopicAssociation < ActiveRecord::Base
  
  belongs_to :user
  belongs_to :topic

  after_create :calculate_popularity
  validates_uniqueness_of :user_id, :scope => :topic_id

  def calculate_popularity
     self.popularity = TopicAssociation.count(:all, :conditions=>{:topic_id => self.topic_id})
     TopicAssociation.update_all( "popularity = #{self.popularity}","topic_id=#{self.topic_id}" )
  end
  
end

class Topic < ActiveRecord::Base
  
  has_many :comments
  has_many :topic_associations
  has_many :users, :through => :topic_associations
  
end

class User < ActiveRecord::Base
  
  has_many :topic_associations
  has_many :topics, :through => :topic_associations
  has_many :topic_user_owns, :foreign_key => :user_id
  has_many :comments
  
  def self.create_with_omniauth(auth)
    create! do |user|
      user.provider = auth["provider"]
      user.uid = auth["uid"]
      user.name = auth["user_info"]["nickname"]
    end
  end
  
end

class TopicsController < ApplicationController

  def index
    @topics = Topic.all(:include => :topic_associations, :order => 'popularity DESC')
  end

end