check out the problems in http://stackoverflow.com/questions/19290251/how-to-eager-load-a-nested-polymorphic-association

gistfile1.txt

unless File.exists?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', '4.0.0'
    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 :posts do |t|
    t.string :name
    t.integer :user_id
  end

  create_table :comments do |t|
    t.integer :post_id
    t.string  :ownable_type
  end

  create_table :users do |t|
    t.integer :post_id
  end
end

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

class Comment < ActiveRecord::Base
  belongs_to :ownable, polymorphic: true
end

class User < ActiveRecord::Base
  has_many :comments, as: :ownable
end

class EagerLoadTest < MiniTest::Unit::TestCase
  def test_association_stuff
    user = User.create!
    post = Post.create! user: user
    post.comments << Comment.create!

    assert_equal [1], Post.includes(comments: :ownable).map(&:id)
  end
end