https://github.com/rails/rails/issues/9961

association-delete-ar.rb

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

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

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

# 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 :blogs do |t|
    t.string :name
    t.timestamps
  end

  create_table :posts do |t|
    t.string :title
    t.references :blog
    t.timestamps
  end

  create_table :comments do |t|
    t.string :comment
    t.references :post
    t.timestamps
  end
end

class Blog < ActiveRecord::Base
  has_many :posts, dependent: :destroy
  has_many :comments, through: :posts, dependent: :destroy
end

class Post < ActiveRecord::Base
  belongs_to :blog
  has_many :comments, dependent: :destroy
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class BugTest < Minitest::Test
  def test_association_stuff
    blog = Blog.create!
    blog.posts << Post.create!
    blog.posts.first.comments << Comment.create!

    assert_equal 1, blog.posts.first.comments.count
    assert_equal 1, Comment.count
    assert_equal blog.posts.first.id, Comment.first.post.id

    Post.first.destroy

    assert_equal 0, Comment.count

  end
end