has_one through not available on unpersisted instances

has_one_through_bug.rb

# frozen_string_literal: true

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"

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

  gem "rails", "5.1.2"
  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 :owner_id
    t.string :title
  end

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

  create_table :owners, force: true do |t|
    t.integer :post_id
    t.string :name
  end
end

class Post < ActiveRecord::Base
  belongs_to :owner
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
  has_one :owner, through: :post
end

class Owner < ActiveRecord::Base
end

class BugTest < Minitest::Test
  def test_association_stuff
    owner = Owner.new(name: "cool_post_owner")
    post = Post.new(owner: owner, title: "A cool post")
    comment = Comment.new
    post.comments << comment

    refute_nil comment.post.title
    assert_equal post.title, comment.post.title
    # passes when not using the has_one through association
    assert_equal owner.name, post.comments.first.post.owner.name
    # fails when using the has_one through association
    assert_equal owner.name, post.comments.first.owner.name
  end
end