active_record_gem.rb

# Activate the gem you are reporting the issue against.
gem 'activerecord', '4.2.0'
require 'active_record'
require 'minitest/autorun'
require 'logger'

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

# 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 do |t|
  end

  create_table :comments do |t|
    t.integer :post_id
    t.integer :comment_author_id
    t.integer :order
  end

  create_table :comment_authors do |t|
    t.string :name
  end
end

class Post < ActiveRecord::Base
  has_many :comments, -> { order(order: :asc) }
  has_many :comment_authors, through: :comments
end

class Comment < ActiveRecord::Base
  belongs_to :post
  belongs_to :comment_author
end

class CommentAuthor < ActiveRecord::Base
  has_many :comments
  has_many :posts, through: :comments
end

class BugTest < Minitest::Test
  def test_association_stuff
    post  = Post.create!
    bob   = CommentAuthor.create!(name: "Bob")
    alice = CommentAuthor.create!(name: "Alice")
    post.comments << Comment.create!(order: 1, comment_author: alice)
    post.comments << Comment.create!(order: 2, comment_author: bob)

    post_without_includes = Post.first
    assert_equal(post_without_includes.comments.map(&:comment_author).map(&:name), post_without_includes.comment_authors.map(&:name))

    post_with_includes = Post.includes(:comment_authors).first
    assert_equal(post_with_includes.comments.map(&:comment_author).map(&:name), post_with_includes.comment_authors.map(&:name))
  end
end

Gemfile

source "https://rubygems.org"

ruby "2.2.0"

gem "activerecord", "4.2.0"
gem "sqlite3"