Assigning ids for HABTM relation can't handle duplicates in ActiveRecord 4.2.8

habtm_failure_active_record_4_2_8.rb

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", "4.2.8"
  gem "sqlite3"
end

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, force: true do |t|
  end

  create_table :comments_posts, force: true do |t|
    t.integer :post_id
    t.integer :comment_id
  end

  create_table :comments, force: true do |t|
  end
end

class Post < ActiveRecord::Base
  has_and_belongs_to_many :comments
end

class Comment < ActiveRecord::Base
  has_and_belongs_to_many :posts
end

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

    post.comment_ids = post.comment_ids
  end
end