Answer for SO question 22569738

what_you_have.rb

gem 'activerecord', '4.0.2'
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 :users do |t|
    t.string :username
  end

  create_table :accounts do |t|
    t.integer :user_id
    t.string :name
  end
end

class User < ActiveRecord::Base
  has_one :account, dependent: :destroy, autosave: true
  validates :username, presence: true, uniqueness: true
end

class Account < ActiveRecord::Base
  belongs_to :user
  validates_presence_of :name
end

class BugTest < Minitest::Test
  def test_basic_assumptions
    refute User.new.valid?
    assert User.new(username: 'bob').valid?

    refute Account.new.valid?
    assert Account.new(name: 'alice inc.').valid?
  end

  def test_both_are_valid
    users_before, accounts_before = User.count, Account.count
    user = User.new(username: 'tom')
    account = user.create_account(name: 'jerry inc.')
    user.save && account.save

    assert_equal users_before+1, User.count
    assert_equal accounts_before+1, Account.count
  end

  def test_invalid_user_valid_account
    users_before, accounts_before = User.count, Account.count
    invalid_user = User.new
    account = invalid_user.create_account(name: 'batman')
    invalid_user.save && account.save

    assert_equal users_before, User.count
    assert_equal accounts_before, Account.count
  end

  def test_valid_user_invalid_account
    users_before, accounts_before = User.count, Account.count
    user = User.new(username: 'robin')
    invalid_account = user.create_account
    user.save && invalid_account.save

    assert_equal users_before, User.count
    assert_equal accounts_before, Account.count
  end
end

what_you_need.rb

# Activate the gem you are reporting the issue against.
gem 'activerecord', '4.0.2'
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 :users do |t|
    t.string :username
  end

  create_table :accounts do |t|
    t.integer :user_id
    t.string :name
  end
end

class User < ActiveRecord::Base
  has_one :account, dependent: :destroy, autosave: true
  validates :username, presence: true, uniqueness: true
end

class Account < ActiveRecord::Base
  belongs_to :user
  validates_presence_of :name
end

class BugTest < Minitest::Test
  def test_basic_assumptions
    refute User.new.valid?
    assert User.new(username: 'bob').valid?

    refute Account.new.valid?
    assert Account.new(name: 'alice inc.').valid?
  end

  def test_both_are_valid
    users_before, accounts_before = User.count, Account.count
    user = User.new(username: 'tom')
    account = user.build_account(name: 'jerry inc.')
    user.save

    assert_equal users_before+1, User.count
    assert_equal accounts_before+1, Account.count
  end

  def test_invalid_user_valid_account
    users_before, accounts_before = User.count, Account.count
    invalid_user = User.new
    account = invalid_user.build_account(name: 'batman')
    invalid_user.save

    assert_equal users_before, User.count
    assert_equal accounts_before, Account.count
  end

  def test_valid_user_invalid_account
    users_before, accounts_before = User.count, Account.count
    user = User.new(username: 'robin')
    invalid_account = user.build_account
    user.save

    assert_equal users_before, User.count
    assert_equal accounts_before, Account.count
  end
end