UUIDasID.rb

require 'rubygems'
require 'uuidtools'

module UUIDasID
  
  #
  # When included, e.g. <code>include UUIDasID</code>, 
  # this method will be called, passing the class in
  #
  def self.included mod
    
    # this will set up generate_uuid as a before_create callback
    mod.instance_eval( %q{before_create :generate_uuid} )
    
    # here we define the macro method, has_uuid, and a query method, has_uuid?
    mod.class_eval do

      def self.has_uuid column_name, options={}
        @has_uuid = true
        @uuid_options = options.merge(:id => column_name)
      end

      def self.has_uuid?
        @has_uuid || false
      end
    end
  end
  
  #
  # This method is called whenever an ActiveRecord is created
  # but should fail early if has_uuid wasn't called as a macro
  # e.g. <code>has_uuid :guid, :unique_with => [:attr1, attr2]</code>
  #
  def generate_uuid
    # fail early and quietly if this AR doesn't have a UUID
    return true unless self.class.has_uuid?
    
    options = self.class.instance_variable_get :@uuid_options

    # this could help with testing
    return true unless send("#{options[:id]}").nil?
    
    # get each attribute that makes this record unique and make an array of string values
    unique_attributes = options[:unique_with].inject([Time.new.to_s]) do |atts, current_att_name|
      atts << send("#{current_att_name}").to_s
    end rescue ['nuffin'] # or just make a dummy string array
    # set the id to a SHA1 of the timestamp plus unique values (minus the dashes)
    uuid = UUID.sha1_create(UUID_DNS_NAMESPACE, unique_attributes.join('')).to_s.gsub(/-/, '')
    send("#{options[:id]}=", uuid)
    # if the callback method uuid_generated exists, call it with the new uuid
    uuid_generated(send("#{options[:id]}")) if respond_to?(:uuid_generated)
    true
  end
  
end

#
# Go on and put this feller on all ActiveRecords, at least once it's required
# maybe it will be a plugin someday
#
class ActiveRecord::Base
  include UUIDasID
end