database.yml

development:
  adapter: postgresql
  encoding: unicode
  database: app_development
  pool: 25
  username: notmyrealusername
  password: notmyrealpassword

review.rb

class Review < UserEvent
  belongs_to :user, counter_cache: true
  belongs_to :item, counter_cache: true
  after_commit :submit_compute_jobs, if: :persisted?
  
  private
  
  def submit_compute_jobs
    SimilarityComputeJobCreator.perform_async(id)
  end
end

seeds.rb

Item.all.each_with_index do |item, index|
  users.sample(20).each do |user|
    review = Review.new(rating: 1 + rand(5))
    review.user = user
    review.item = item
    review.place = item.place
    review.save
  end
end

similarity_compute_job.rb

class SimilarityComputeJob < ActiveRecord::Base
  attr_accessible :item1_id, :item2_id
end

similarity_compute_job_creator.rb

class SimilarityComputeJobCreator
  include Sidekiq::Worker
  
  def perform(review_id)
    review = Review.find review_id
    user_reviews = review.user.reviews
    
    if user_reviews.count > 1
      # create SimilarityComputeJobs for this review and the user's other reviews
      user_reviews.each do |other_review|
        unless review.id == other_review.id
          item1_id, item2_id = [review.item_id, other_review.item_id].sort
          SimilarityComputeJob.create(item1_id: item1_id, item2_id: item2_id)
        end
      end
    end
  end
end