Self-Contained Active Record http://guides.rubyonrails.org/contributing_to_ruby_on_rails.html#create-a-self-contained-gist-for-active-record-issues ActiveRecord v3.2.12: https://gist.github.com/neerajdotname/5187092

rails4.rb

gem 'activerecord', '4.0.0'
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 BugTest < MiniTest::Unit::TestCase
  def test_association_stuff
    post = Post.create!
    post.comments << Comment.create!

    assert_equal 1, post.comments.count
    assert_equal 1, Comment.count
    assert_equal post.id, Comment.first.post.id
  end
end