habtm_before_add_bug.rb

# gem 'activerecord', '=4.0.2'
gem 'activerecord', '=4.1.0.rc1'
require 'active_record'
require 'minitest/autorun'

ActiveRecord::Base.logger = Logger.new(STDOUT)
ActiveRecord::Base.establish_connection(adapter: "postgresql", host: "localhost", user: "cristi", database: "test")

class Message < ActiveRecord::Base
  connection.create_table table_name, force: true do |t|
  end
  connection.create_table :message_threads_messages, force: true, id: false do |t|
    t.references :message
    t.references :message_thread
  end

  has_and_belongs_to_many :message_threads, before_add: :set_as_latest_message

  def set_as_latest_message(message_thread)
    message_thread.latest_message = self
  end
end

class MessageThread < ActiveRecord::Base
  connection.create_table table_name, force: true do |t|
    t.references :latest_message, null: false
  end

  has_and_belongs_to_many :messages
  belongs_to :latest_message, class_name: 'Message'
end

class BugTest < MiniTest::Unit::TestCase
  def test_before_add
    m = Message.create!
    m.message_threads << MessageThread.new
    assert MessageThread.count == 1
  end
end