identity_map.rb

##
# Include this in your app as config/initializers/identity_map.rb
# But don't really, this is untested and not used in any production code.
#
# It is a partial implementation of Martin Fowler's IdentityMap.
# http://martinfowler.com/eaaCatalog/identityMap.html
# 
# Combined with QueryCache it is a complete implementation ( almost ).
#
module IdentityMap

  def self.included(base)

    base.extend ClassMethods

    base.alias_method_chain :create, :identity_map

    class << base
      alias_method_chain :instantiate, :identity_map
    end

  end

  def identity_map_key
    self.class.identity_map_key(self)
  end

  def create_with_identity_map
    result = create_without_identity_map
    if self.class.identity_map_enabled? && result
      complete = self.class.column_names.sort == attribute_names
      self.class.identity_map[identity_map_key] = self if complete
    end
    result
  end

  # TODO 2009-10-22
  # reload will do nothing
  # define a without identity_map reload

  module ClassMethods

    def instantiate_with_identity_map(record)
      if identity_map_enabled? && record[primary_key]
        key = identity_map_key(record)
        if mapped = identity_map[key]
          # puts "HIT [#{key}]"
          mapped
        else
          # puts "MISS [#{key}]"
          unmapped = instantiate_without_identity_map(record)

          complete = column_names.sort == record.keys.sort

          identity_map[key] = unmapped if complete

          unmapped
        end
      else
        instantiate_without_identity_map(record)
      end
    end

    def identity_map_enabled?
      if defined?(@identity_map_enabled)
        @identity_map_enabled == true
      elsif superclass.respond_to?(:identity_map_enabled?)
        superclass.identity_map_enabled?
      end
    end

    def enable_identity_map!
      @identity_map = {} unless identity_map
      @identity_map_enabled = true
    end

    def disable_identity_map!
      remove_instance_variable(:@identity_map) if instance_variable_defined?(:@identity_map)
      @identity_map_enabled = false
    end

    def identity_map
      if defined?(@identity_map)
        @identity_map
      elsif superclass.respond_to?(:identity_map)
        superclass.identity_map
      end
    end

    def identity_map_key(record)
      [table_name, record[primary_key.to_s]].join('-')
    end

    def with_identity_map
      old, @identity_map_enabled = @identity_map_enabled, true
      @identity_map ||= {}
      yield
    ensure
      clear_identity_map
      if old.nil?
        remove_instance_variable :@identity_map_enabled
      else
        @identity_map_enabled = old
      end
    end

    ##
    # allow clearing if you need to instantiate the same records into separate instances
    # also used for testing
    #
    def clear_identity_map
      @identity_map.clear if @identity_map
    end

    def without_identity_map
      old, @identity_map_enabled = @identity_map_enabled, false
      yield
    ensure
      if old.nil?
        remove_instance_variable :@identity_map_enabled
      else
        @identity_map_enabled = old
      end
    end

  end # ClassMethods
end # IdentityMap

ActiveRecord::Base.class_eval do
  include IdentityMap
end

module ActionController
  module IdentitiyMap
    def self.included(base)
      base.alias_method_chain :perform_action, :identity_map
    end

    protected
      def perform_action_with_identity_map
        ActiveRecord::Base.with_identity_map do
          perform_action_without_identity_map
        end
      end
  end
end

ActionController::Base.class_eval do
  include ActionController::IdentitiyMap
end