create_or_find_by_mysql.rb

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "rails", github: "rails/rails"
  gem "mysql2"
end

require "active_record"
require "minitest/autorun"
require "logger"

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "mysql2", database: "rails")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
  end

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

  add_index :comments, :post_id, unique: true
end

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class BugTest < Minitest::Test

  # PROBLEM
  # When create fails, the autoincremented id does not roll back
  def test_primary_key_growth
    post_one = Post.create!

    comment_for_post_one = Comment.create_or_find_by(post: post_one)

    # Create fails for Post 1's second comment and it finds the same record for
    # first_comment_for_post_one
    1000.times do
      second_comment_for_post_one = Comment.create_or_find_by(post: post_one)
      assert_equal second_comment_for_post_one, comment_for_post_one
    end

    post_two = Post.create!
    # This fails
    # Failure:
    #   BugTest#test_primary_key_growth [create_or_find_by.rb:56]:
    #   Expected: 2
    #     Actual: #<Comment id: 1002, post_id: 2>
    comment_for_post_two = Comment.create_or_find_by(post: post_two)
    assert_equal comment_for_post_one.id + 1, comment_for_post_two.id
  end

  # This is nothing new, as the same thing can happen with `create`
  def test_create
    post_one = Post.create!

    comment_for_post_one = Comment.create(post: post_one)

    1000.times do
      assert_raises ActiveRecord::RecordNotUnique do
        second_comment_for_post_one = Comment.create(post: post_one)
      end
    end

    post_two = Post.create!
    comment_for_post_two = Comment.create(post: post_two)
    assert_equal comment_for_post_one.id + 1, comment_for_post_two.id
  end
end