gistfile1.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    gem 'sqlite3'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

require 'active_record'
require 'minitest/autorun'
require 'logger'

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

# Schema
ActiveRecord::Schema.define do
  ActiveRecord::Base.connection.create_table(:posts, force: true) do |t|
    t.integer :author_id
  end

  ActiveRecord::Base.connection.create_table(:authors, force: true)
end

# Models
class Post < ActiveRecord::Base
  belongs_to :author
  belongs_to :read_only_author, -> { readonly }, class_name: 'Author', foreign_key: :author_id
end

class Author < ActiveRecord::Base
  has_many :posts
end

class BugTest < Minitest::Test
  def test_eager_load_read_only_association
    Post.create!(author: Author.create!)
    post = Post.includes(:read_only_author).first!
    assert_raises(ActiveRecord::ReadOnlyRecord) { post.read_only_author.save! }
  end
end