has_many_through nested_attributes setter vs collection setter

has_many_through.rb

# This is not a bug report
#
# The purpose of this file is to illustrate
# differences in how Rails treats assignment
# to a nested_attributes setter vs. assignment
# to a collection directly.
#
# In the former, existing records are _not_ deleted if they are not present in the assigned data.
# In the latter, existing records _are_ deleted if they are not present in the assigned data.

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'
  # Activate the gem you are reporting the issue against.
  gem 'activerecord', '4.2.2'
  gem 'sqlite3'
  gem 'database_cleaner'
end

require 'active_record'
require 'database_cleaner'
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 :authors, force: true do |t|
    t.string :name
  end

  create_table :books, force: true do |t|
    t.string :name
  end

  create_table :authorships, force: true do |t|
    t.integer :book_id
    t.integer :author_id
    t.integer :num_chapters_contributed
  end
end

class Book < ActiveRecord::Base
  has_many :authorships
  has_many :authors, through: :authorships
end

class Author < ActiveRecord::Base
  has_many :authorships, dependent: :destroy
  has_many :books, through: :authorships

  accepts_nested_attributes_for :authorships
end

class Authorship < ActiveRecord::Base
  belongs_to :book
  belongs_to :author
end

class BugTest < Minitest::Test

  def setup
    DatabaseCleaner.start
    @author = Author.create(name: 'Ed')
    @book1 = Book.create(name: 'Elephants')
    @book2 = Book.create(name: 'Frogs')
    @book3 = Book.create(name: 'Gazelles')
  end

  def teardown
    DatabaseCleaner.clean
  end

  def test_assigning_through_nested_attributes
    @author.authorships_attributes = [ { book_id: @book1.id, num_chapters_contributed: 5 },
                                      { book_id: @book2.id, num_chapters_contributed: 4} ]
    @author.save

    # We want this to delete the first two authorships, but it doesn't
    @author.authorships_attributes = [ { book_id: @book3.id, num_chapters_contributed: 3 } ]
    @author.save

    assert_equal 1, @author.reload.books.count
  end

  def test_assigning_to_join_collection_directly
    @author.authorships = [Authorship.create(book_id: @book1.id, num_chapters_contributed: 5),
                          Authorship.create(book_id: @book2.id, num_chapters_contributed: 4)]
    @author.save

    assert_equal 2, @author.reload.books.count

    assert_equal 2, Authorship.count
    # Ensure author_id is assigned automatically
    Authorship.all.each do |authorship|
      assert_equal @author.id, authorship.author_id
    end

    # We want this to delete this assignment to delete the first two authorships
    @author.authorships = [Authorship.new(book_id: @book3.id, num_chapters_contributed: 3)]
    @author.save

    assert_equal 1, @author.reload.books.count
  end
end