rails 4.1 cannot eager load polymorphic associations

polymorphic_eagerload.rb

gem 'activerecord', '4.1.0.rc1'
# gem 'activerecord', '4.0.2'

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

puts ActiveRecord::VERSION::STRING

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

ActiveRecord::Schema.define do
  create_table :feed_entries, :force => true do |t|
    t.integer :feedable_id
    t.string :feedable_type
  end

  create_table :trainings, :force => true do |t|
    t.integer :user_id
    t.boolean :featured, default: false, null: false
  end

  create_table :users, :force => true do |t|
  end
end

class FeedEntry < ActiveRecord::Base
  belongs_to :feedable, polymorphic: true

  def self.with_featured_training
    joins("LEFT JOIN trainings ON feed_entries.feedable_type = 'Training' AND feed_entries.feedable_id = trainings.id")
    .where(trainings: {featured: true})
  end
end

class Training < ActiveRecord::Base
  belongs_to :user
  has_many :feed_entries, as: :feedable
end

class User < ActiveRecord::Base
end

class TestMe < MiniTest::Unit::TestCase
  def test_polymorphic_eagerload
    user_1 = User.create!
    user_2 = User.create!
    training_1 = Training.create!(user: user_1, featured: true)
    training_2 = Training.create!(user: user_2, featured: true)
    FeedEntry.create!(feedable: training_1)
    FeedEntry.create!(feedable: training_2)

    scope = FeedEntry.with_featured_training.includes(feedable: [:user])

    scope.each { |fe| fe.feedable.user }
  end
end

This throws error in rails 4.1, but it was working fine in 4.0.x