A single file that can run minitest within a full Rails environment - good for reproducing simple bugs

one_file_rails_test.rb

# frozen_string_literal: true

#
# to run this file:
#  - bundle install
#  - ruby one_file_rails_test.rb
#
# original source: https://discuss.rubyonrails.org/t/is-it-ever-ok-to-validate-the-presence-of-a-primary-key-of-a-belongs-to-association/84904
#

require "bundler/inline"

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

  gem "rails"
  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:")

# uncomment this if you want logs visible in stdout
# 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
end

class Post < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

class TestCommentCreation < Minitest::Test
  def test_create_post_and_comment_together
    post = Post.new
    comment = post.comments.build
    post.save!
    assert_equal Post.last, comment.reload.post
  end
end