goldiloader109.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', '~> 6.0.3'
  gem 'goldiloader'
  gem 'sqlite3'
  gem 'minitest'
end

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

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

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

  create_table :people, force: true do |t|
    t.string :type
  end

  create_table :persons_nationalities, force: true do |t|
    t.integer :person_id
    t.string :person_type
    t.integer :nationality_id
  end
end

class Person < ActiveRecord::Base
  has_many :persons_nationalities, dependent: :destroy, inverse_of: :person, as: :person
end

class Candidate < Person
end

class PersonsNationality < ActiveRecord::Base
  belongs_to :person, polymorphic: true
  belongs_to :nationality, class_name: "Country"
end

class Country < ActiveRecord::Base
  has_many :persons_nationalities, foreign_key: :nationality_id
end

class BugTest < defined?(Minitest::Test) ? Minitest::Test : MiniTest::Unit::TestCase
  def test_issue109
    candidate = Candidate.create!
    country1 = Country.create!
    country2 = Country.create!
    candidate.persons_nationalities.create!(nationality: country1)
    candidate.persons_nationalities.create!(nationality: country2)

    found_candiate = Candidate.find(candidate.id)
    found_candiate.persons_nationalities[0].nationality

    found_candiate.persons_nationalities.each do |persons_nationality|
      assert(persons_nationality.association(:nationality).loaded?)
    end
  end
end