100times

-- create_table(:accounts)
   -> 0.0264s
-- create_table(:posts)
   -> 0.0008s
-- create_table(:comments)
   -> 0.0010s
==================================================
ActiveRecord::IdentityMap.enabled: false
Objects:
Warmup: 1110266 allocations | 40078000 bytes
Actual: 1108696 allocations | 40061024 bytes
==================================================
ActiveRecord::IdentityMap.enabled: true
Objects:
Warmup: 272520 allocations | 8646368 bytes
Actual: 224883 allocations | 6988789 bytes
==================================================
ActiveRecord::IdentityMap.enabled: true
Objects:
Warmup: 226782 allocations | 7061606 bytes
Actual: 39599 allocations | 498807 bytes
==================================================
ActiveRecord::IdentityMap.enabled: false
Objects:
Warmup: 196359 allocations | 6179002 bytes
Actual: 199547 allocations | 6086341 bytes

50times

-- create_table(:accounts)
   -> 0.0262s
-- create_table(:posts)
   -> 0.0008s
-- create_table(:comments)
   -> 0.0010s
==================================================
ActiveRecord::IdentityMap.enabled: false
Objects:
Warmup: 555077 allocations | 20047967 bytes
Actual: 554746 allocations | 20045261 bytes
==================================================
ActiveRecord::IdentityMap.enabled: true
Objects:
Warmup: 118825 allocations | 3707707 bytes
Actual: 17924 allocations | 238183 bytes
==================================================
ActiveRecord::IdentityMap.enabled: true
Objects:
Warmup: 17919 allocations | 238100 bytes
Actual: 17924 allocations | 238181 bytes
==================================================
ActiveRecord::IdentityMap.enabled: false
Objects:
Warmup: 110312 allocations | 3473307 bytes
Actual: 97896 allocations | 3039749 bytes

gistfile1.rb

$:.unshift("/Users/miloops/Workspace/github/miloops/rails/activerecord/lib")
$:.unshift("/Users/miloops/Workspace/github/swistak/weakling/lib")
$:.unshift("/Users/miloops/Workspace/github/railsarel/lib")
require 'active_record'

ActiveRecord::Base.establish_connection(
  :adapter => "sqlite3",
  :database  => ":memory:"
)

ActiveRecord::Schema.define do
  create_table :accounts do |t|
    t.string :email

    t.timestamps
  end

  create_table :posts do |t|
    t.string :title
    t.text :body
    t.integer :account_id

    t.timestamps
  end

  create_table :comments do |t|
    t.string :body
    t.integer :post_id

    t.timestamps
  end
end

class Post < ActiveRecord::Base
  belongs_to :account
  has_many :comments
end

class Account < ActiveRecord::Base
  has_many :posts
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

def instances(&block)
  GC.enable_stats
  GC.clear_stats
  block.call
  warmup_objs = GC.num_allocations
  warmup_bytes = GC.allocated_size
  GC.clear_stats
  block.call
  puts "=" * 50
  puts "ActiveRecord::IdentityMap.enabled: #{ActiveRecord::IdentityMap.enabled}"
  puts "Objects:"
  puts "Warmup: #{warmup_objs} allocations | #{warmup_bytes} bytes"
  puts "Actual: #{GC.num_allocations} allocations | #{GC.allocated_size} bytes"
end


Account.destroy_all

a = Account.create(:email => 'miloops@example.com')
50.times do |i|
  c = a.posts.create!(:title => "test", :body => "Loldel")
  10.times do
    c.comments.create!(:body => ("lol! " * 10))
  end
end

Account.first.posts.each do |post|
  post.account.email
  post.comments.each do |comment|
    comment.post.account.email
  end
end

instances do
  ActiveRecord::IdentityMap.enabled = false
  Account.first.posts.each do |post|
    post.account.email
    post.comments.each do |comment|
      comment.post.account.email
    end
  end
end

instances do
  ActiveRecord::IdentityMap.enabled = true
  Account.first.posts.each do |post|
    post.account.email
    post.comments.each do |comment|
      comment.post.account.email
    end
  end
end

module ActiveRecord::IdentityMap
  class << self
    def current
      repositories[current_repository_name] ||= WeakHash.new
    end
  end
end

instances do
  ActiveRecord::IdentityMap.enabled = true
  Account.first.posts.each do |post|
    post.account.email
    post.comments.each do |comment|
      comment.post.account.email
    end
  end
end

Post.belongs_to :account, :inverse_of => :posts
Post.has_many :comments, :inverse_of => :post
Comment.belongs_to :post, :inverse_of => :comments
Account.has_many :posts, :inverse_of => :account

instances do
  ActiveRecord::IdentityMap.enabled = false
  Account.first.posts.each do |post|
    post.account.email
    post.comments.each do |comment|
      comment.post.account.email
    end
  end
end

identity_map.rb

class WeakHash
  def initialize( cache = Hash.new )
    @cache = cache
    @key_map = {}
    @reclaim_key = lambda do |key_id|
      delete(@key_map[key_id]) unless @cache.empty?
    end
  end

   def []( key )
     value_id = @cache[key]
     value_id ? ObjectSpace._id2ref(value_id) : nil
     rescue RangeError
       nil
   end

   def []=( key, value )
     case key
     when Fixnum, Symbol, true, false
       key2 = key
     else
       key2 = key.dup
     end
     @cache[key2] = value.object_id
     @key_map[key.object_id] = key2

     ObjectSpace.define_finalizer(key, @reclaim_key)
   end

   def clear
     @key_map.clear
     @cache.clear
   end

   def delete(key)
     @cache.delete(key)
   end

end


module ActiveRecord
  # = Active Record Identity Map
  #
  # Ensures that each object gets loaded only once by keeping every loaded
  # object in a map. Looks up objects using the map when referring to them.
  #
  # More information on Identity Map pattern:
  #   http://www.martinfowler.com/eaaCatalog/identityMap.html
  #
  # == Configuration
  #
  # In order to disable IdentityMap, set <tt>config.active_record.identity_map = false</tt>
  # in your <tt>config/application.rb</tt> file.
  #
  # IdentityMap is enabled by default.
  #
  module IdentityMap
    extend ActiveSupport::Concern

    class << self
      attr_accessor :repositories
      attr_accessor :current_repository_name
      attr_accessor :enabled

      def current
        repositories[current_repository_name] ||= Weakling::WeakHash.new
      end

      def with_repository(name = :default)
        old_repository = self.current_repository_name
        self.current_repository_name = name

        yield if block_given?
      ensure
        self.current_repository_name = old_repository
      end

      def without
        old, self.enabled = self.enabled, false

        yield if block_given?
      ensure
        self.enabled = old
      end

      def get(class_name, primary_key)
        current[[class_name, primary_key.to_s]]
      end

      def add(record)
        current[[record.class.name, record.id.to_s]] = record
      end

      def remove(record)
        current.delete([record.class.name, record.id.to_s])
      end

      def clear
        current.clear
      end

      alias enabled? enabled
      alias identity_map= enabled=
    end

    self.repositories ||= Hash.new
    self.current_repository_name ||= :default
    self.enabled = true

    module InstanceMethods
      # Reinitialize an Identity Map model object from +coder+.
      # +coder+ must contain the attributes necessary for initializing an empty
      # model object.
      def reinit_with(coder)
        @attributes_cache = {}
        dirty = @changed_attributes.keys
        @attributes.update(coder['attributes'].except(*dirty))
        @changed_attributes.update(coder['attributes'].slice(*dirty))
        @changed_attributes.delete_if{|k,v| v.eql? @attributes[k]}

        _run_find_callbacks

        self
      end
    end

    module ClassMethods
      def identity_map
        ActiveRecord::IdentityMap
      end
    end
  end
end