Has related is used by Reevoo to create similar product recommendations.

Example Index Builder:

namespace :apache do
  desc "Parse the last apache logs for similar products"
  task :find_similar_products => :environment do
    log_files = ENV["LOG_FILES"] || "/var/log/httpd/access.*.gz"

    prefs = Hash.new{|h,k| h[k] = Hash.new(0) }

    Dir.glob(log_files) do |filename|
      File.open(filename) do |f|
        Zlib::GzipReader.new(f).each_line do |line|
          product_id, ip = parse_log_line(line)
          if product_id and ip
            prefs[product_id][ip] += 1
          end
        end
      end
    end

    HasRelated.dump_dataset(prefs, "Product")
  end
end

(You will need to define your own parse_log_line method)

Example Usage:

class Product < ActiveRecord::Base
  has_related "related_products"
end

most_related_product = Product.related_products.first

Happy hunting.