test.rb

begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  gem 'activerecord', '4.2.4'
  gem 'goldiloader'
  gem 'sqlite3'
end

require 'active_record'
require 'minitest/autorun'
require 'logger'

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :subclasses, force: true do |t|
    t.string :type
  end

  create_table :participants, force: true do |t|
  end 

  create_table :participants_subclasses, force: true do |t|
    t.integer :subclass_id
    t.integer :participant_id
  end
end

class Subclass < ActiveRecord::Base
end

class ParticipantSubclass < Subclass
end

class Participant < ActiveRecord::Base
     has_and_belongs_to_many :participant_subclasses,
                             association_foreign_key: :subclass_id
end

class BugTest < Minitest::Test
  def test_habtm
    participant = Participant.create!
    participant_subclasses = 3.times.map { ParticipantSubclass.create! }
    participant_subclasses.each do |participant_subclass|
      participant.participant_subclasses << participant_subclass
    end

    found_participant_subclasses = Participant.find(participant.id).participant_subclasses.to_a

    assert_equal(participant_subclasses.sort_by(&:id), found_participant_subclasses.sort_by(&:id))
    assert_equal(3, found_participant_subclasses.size)
  end
end