gistfile1.rb

$:.unshift("/Users/miloops/Workspace/github/miloops/rails/activerecord/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')
150.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

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

gistfile2.txt

-- create_table(:accounts)
   -> 0.0597s
-- create_table(:posts)
   -> 0.0009s
-- create_table(:comments)
   -> 0.0006s
==================================================
ActiveRecord::IdentityMap.enabled: false
Objects:
Warmup: 1662829 allocations | 60073516 bytes
Actual: 1662797 allocations | 60072341 bytes
==================================================
ActiveRecord::IdentityMap.enabled: true
Objects:
Warmup: 53212 allocations | 694650 bytes
Actual: 53217 allocations | 694731 bytes
==================================================
ActiveRecord::IdentityMap.enabled: false
Objects:
Warmup: 293610 allocations | 9184219 bytes
Actual: 292247 allocations | 9055118 bytes