has_and_belongs_to_many association bug

association_bug_reproduction.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"
  gem "rails", "5.1.3"
  gem "arel"
  gem "sqlite3"
end

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, force: true do |t|
    t.integer :comment_id
  end

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

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

class Post < ActiveRecord::Base
  has_and_belongs_to_many :comments

  def add_comment(reload:)
    comments << Comment.new
    comments.reload if reload
  end
end

class Comment < ActiveRecord::Base
  has_and_belongs_to_many :posts
  belongs_to :post
end

class BugTest < Minitest::Test
  def test_association_stuff
    # Reloading the association before the Post is saved breaks
    # the association
    post = Post.new
    post.add_comment(reload: true)
    post.save
    assert_equal 1, post.comments.count
    assert_nil post.comments.first

    # Not reloading the association before the Post is saved
    # works as expected
    post = Post.new
    post.add_comment(reload: false)
    post.save
    assert_equal 1, post.comments.count
    assert_kind_of Comment, post.comments.first
  end
end