MongoMapper::Randomizer plugin

randomizer.rb

module MongoMapper
  # From https://gist.github.com/901929
  module Randomizer
    extend ActiveSupport::Concern

    included do
      key :randomizer, Integer
      before_create Proc.new { |doc| doc.randomizer = self.class.count }
    end

    module ClassMethods
      # Finds non-sequential random documents.
      # When using the :selector option, ensure you're hitting an index that includes :random as the last listed key.
      # Additionally, do not use range queries in the selector: {http://bit.ly/gHxnLN MongoDB Indexing Advice}.
      #
      # @param [Hash] :limit => the number of documents to find, :selector => randomly select documents matching the criteria
      # @return [Array] the randomly selected documents
      def random(*args)
        opts = { :limit => 1, :selector => { } }.update(args.extract_options!)

        [].tap do |results|
          opts[:limit].times do
            number = rand(self.count)
            result =  where(opts[:selector]).first(:randomizer => { :$gte => number }) ||
                      where(opts[:selector]).first(:randomizer => { :$lte => number })
            results << result if result
          end
        end
      end
    end
  end
end