test nested has many through association with unpersisted parent instance which is then saved

gistfile1.txt

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    gem 'rack', github: 'rack/rack'
    gem 'i18n', github: 'svenfuchs/i18n'
    gem 'sqlite3'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

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 :posts do |t|
    t.integer :author_id
  end

  create_table :authors do |t|
  end

  create_table :books do |t|
    t.integer :author_id
  end

  create_table :subscriptions do |t|
    t.integer :book_id
  end
end

class Post < ActiveRecord::Base
  belongs_to :author
end

class Author < ActiveRecord::Base
  has_one :post
  has_many :books
  has_many :subscriptions, through: :books
end

class Book < ActiveRecord::Base
  belongs_to :author
  has_many :subscriptions
end

class Subscription < ActiveRecord::Base
  belongs_to :book
end

class HasManyThroughAssociationWithUnpersistedParentInstance < Minitest::Test
  def test_nested_has_many_through_association_with_unpersisted_parent_instance_then_saved
    post_with_nested_has_many_through = Class.new(Post) do
      def self.name; 'PostWithNestedHasManyThrough'; end
      has_many :books, through: :author
      has_many :subscriptions, through: :books
    end
    post = post_with_nested_has_many_through.new
    post.author = Author.create!
    book = Book.create!
    post.author.books << book
    subscription = Subscription.create!
    book.subscriptions << subscription
    assert_equal [subscription], post.subscriptions.to_a
    post.save
    assert_equal [subscription], post.subscriptions.to_a
  end
end