Habtm and foreign key Bug

habtm_bug.rb

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

    # Bundle edge Rails instead: gem 'rails', github: 'rails/rails'
    gem 'rails', '4.1.2'

    #For Heroku
    gem 'pg'

    # Use SCSS for stylesheets
    gem 'sass-rails', '~> 4.0.0'

    # Use Uglifier as compressor for JavaScript assets
    gem 'uglifier', '>= 1.3.0'

    # Use CoffeeScript for .js.coffee assets and views
    gem 'coffee-rails', '~> 4.0.0'

    # See https://github.com/sstephenson/execjs#readme for more supported runtimes
    # gem 'therubyracer', platforms: :ruby

    # Use jquery as the JavaScript library
    gem 'jquery-rails'

    # Build JSON APIs with ease. Read more: https://github.com/rails/jbuilder
    gem 'jbuilder', '~> 1.2'

    group :doc do
      # bundle exec rake doc:rails generates the API under doc/api.
      gem 'sdoc', require: false
    end

    # Use ActiveModel has_secure_password
    # gem 'bcrypt-ruby', '~> 3.1.2'

    # Use unicorn as the app server
    # gem 'unicorn'

    # Use Capistrano for deployment
    # gem 'capistrano', group: :development

    gem 'sqlite3'
    group :development, :test do
      # Use sqlite3 as the database for Active Record
      gem 'debugger'

      gem 'rails-erd'

      #add rack-mini-profiler
      gem 'rack-mini-profiler'
    end

    group :test do
      gem 'cucumber-rails', :require => false
      gem 'selenium-webdriver'
      # database_cleaner is not required, but highly recommended
      gem 'database_cleaner'
      gem 'rspec-rails', '~> 3.0.0.beta'
      gem 'factory_girl_rails'
      gem 'faker'
    end

    gem 'pry-rails', :group => :development

    #for authentication
    gem 'devise'
    gem 'devise-i18n'
    gem 'devise-i18n-views'

    #use haml instead of html
    gem 'haml'
    gem 'haml-rails'

    #twitter bootstrap gem
    gem 'bootstrap-sass', '~> 3.1.0'

    #font-awesome gem
    gem "font-awesome-rails"

    #kaminari gem
    gem 'kaminari'

    #angularjs gem
    gem 'angularjs-rails'
    gem 'angular-rails-templates'
    #the_role gem
    gem 'the_role', '~> 2.3'

    #Auditing and Versioning gem
    gem 'paper_trail', '~> 3.0.0'

    #pdf generating gem
    gem 'prawn'
    gem 'wicked_pdf'

    #excel generating gem
    gem 'axlsx', '~> 2.0'
    gem 'axlsx_rails'
    gem 'acts_as_xlsx'
    gem 'zip-zip'

    #puma web server
    gem 'puma'

    #thread gem
    gem 'sidekiq'

    #calculation gem
    gem 'dentaku'
  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 :posts do |t|
  end

  create_table :comments do |t|
  end

  create_table :comments_posts, id: false do |t|
    t.references :post, null: false
    t.references :comment, null: false
  end
end

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  has_and_belongs_to_many :great_posts, join_table: :comments_posts, class_name: Post
end

class BugTest < Minitest::Test
  def test_association_stuff
    post = Post.create!
    post.comments << Comment.create!

    assert_equal 1, post.comments.count
    assert_equal 1, Comment.count
    assert_equal post.id, Comment.first.post.id
  end
end