Active record failure, 4.1.0.beta1

gistfile1.rb

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

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

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 :cards do |t|
  end

  create_table :animals do |t|
  end

  create_table :observations do |t|
    t.integer :animal_id
    t.integer :card_id
    t.integer :image_id
  end

  create_table :images do |t|
  end
end

class Image < ActiveRecord::Base
  has_many :observations
  has_many :animals, through: :observations
end

class Observation < ActiveRecord::Base
  belongs_to :animal
  belongs_to :card
  belongs_to :image
end

class Card < ActiveRecord::Base
  has_many :observations
end

class Animal < ActiveRecord::Base
  has_many :observations
end

class BugTest < MiniTest::Unit::TestCase
  def test_association_stuff
    sp = Animal.create!
    card = Card.create!

    obs = Observation.create!(animal: sp, card: card)
    im = Image.create!
    im.observations = [obs]
    im.save!

    # These three lines pass
    assert Image.preload(:animals, :observations).to_a
    assert Image.preload(:observations => :card).to_a
    assert Image.preload({:observations => :card}, :animals).to_a

    # This line fails
    assert Image.preload(:animals, :observations => :card).to_a
  end
end