code.rb

class Code
  include Mongoid::Document

  field(:name)
  field(:codes, :default => [])

  validates_presence_of(:name)
  validates_uniqueness_of(:name)
  index(:name, :unique => true)

  Cache = Map.new

  class << Code
    def for(name)
      name = name.to_s

      Cache[name] ||= (
        begin
          create!(:name => name)
        rescue
          where(:name => name).first || create!(:name => name)
        end
      )
    end

    alias_method('[]', 'for')
  end

  def next
    42.times do
      code = String.random

      query = {
        '$and' => [
          {:_id => id},
          {'codes' => {'$nin' => [code]}}
        ]
      } 

      update = {
        '$addToSet' => {:codes => code}
      }

      if Code.collection.find_and_modify(:query => query, :update => update)
        reload
        return code
      end
    end

    raise IndexError
  end
end