NoMethodError: undefined method `substitute' for #<Address:0x007fd40bdb9c00>
    ~/.rvm/gems/ruby-2.1.5/gems/mongoid-4.0.2/lib/mongoid/relations/accessors.rb:231

require 'factory_girl'
require 'mongoid'
require 'minitest/autorun'

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

Mongoid.configure.connect_to("mongoid_test")

class Event
  include Mongoid::Document  
  field :name, type: String
  has_one :address, as: :addressable
  validates :address, presence: true  
end

class User
  include Mongoid::Document  
  field :email, type: String
  has_many :addresses, as: :addressable  
end

class Address
  include Mongoid::Document
  belongs_to :addressable, polymorphic: :true
  field :city, type: String  
end

FactoryGirl.define do
  factory :event do
    name 'My Awesome Event'
    
    after(:build) do |event|
      event.address ||= FactoryGirl.build(:event_address, addressable: event)
    end
  end
end

FactoryGirl.define do
  factory :user do
    email 'me@nowhere.com'
  end
end

FactoryGirl.define do
  factory :user_address, class: 'Address' do
    association :addressable, factory: :user
    city 'Pilsen'
  end
  
  factory :event_address, class: 'Address' do
    association :addressable, factory: :event
    city 'Prague'
  end
end

class BugTest < Minitest::Test
  def test_polymorphic
    e = FactoryGirl.build(:event)
    assert e.valid?, e.errors.full_messages
  end
end