object_id_fail.rb

require 'rubygems'
require 'bundler'
Bundler.require

Mongoid.configure do |config|
  config.master = Mongo::Connection.new.db('testing')
end

class Account
  include Mongoid::Document
  references_many :movies
  collection.remove
end

class Movie
  include Mongoid::Document
  referenced_in :account
  referenced_in :image
  collection.remove
end

class Image
  include Mongoid::Document
  references_many :movies
  collection.remove
end

a = Account.create
movie = a.movies.build(:image_id => "")