Executable Testcase for https://github.com/rails/rails/issues/8020#issuecomment-19958061 Note that this bug is gone in AR 4.0.0

marshalling.rb

gem 'activerecord', '3.2.13'
require 'active_record'
require 'minitest/autorun'
require 'logger'

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts do |t|
  end

  create_table :comments do |t|
    t.integer :post_id
  end
end

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class MarshallingTest < MiniTest::Unit::TestCase

  def setup
    Post.create!
    Post.create!
  end

  def teardown
    Post.destroy_all
  end

  def test_marshalling_with_touching_associations
    posts = Post.all
    posts.each do |post|
      post.comments
    end
    marshalled = Marshal.load(Marshal.dump(posts))
    assert_equal Post.all, marshalled
  end

  def test_marshalling_without_touching_associations
    posts = Post.all
    marshalled = Marshal.load(Marshal.dump(posts))
    assert_equal Post.all, marshalled
  end

  def test_marshalling_with_arrayifying_associations
    posts = Post.all
    posts.each do |post|
      post.comments.to_a
    end
    marshalled = Marshal.load(Marshal.dump(posts))
    assert_equal Post.all, marshalled
  end

  def test_dumping_twice
    posts = Post.all
    posts.each do |post|
      post.comments.to_a
    end
    Marshal.dump(posts) # as noted by a commenter, dumping twice helps...
    marshalled = Marshal.load(Marshal.dump(posts))
    assert_equal Post.all, marshalled
  end


end