gistfile1.rb

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

ActiveRecord::IdentityMap.enabled = true

instances do
  Account.first.posts.map(&:comments).flatten.each do |comment|
    comment.post.account.email
  end
end

ActiveRecord::IdentityMap.enabled = false

instances do
  Account.first.posts.map(&:comments).flatten.each do |comment|
    comment.post.account.email
  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
  Account.first.posts.map(&:comments).flatten.each do |comment|
    comment.post.account.email
  end
end



==================================================
ActiveRecord::IdentityMap.enabled: true
Objects:
Warmup: 117070 allocations | 3643893 bytes
Actual: 15819 allocations | 220567 bytes
==================================================
ActiveRecord::IdentityMap.enabled: false
Objects:
Warmup: 572747 allocations | 20617244 bytes
Actual: 562228 allocations | 20265932 bytes
==================================================
ActiveRecord::IdentityMap.enabled: false
Objects:
Warmup: 96817 allocations | 3031917 bytes
Actual: 95978 allocations | 2956359 bytes