mongoissue.rb

#!/usr/bin/env ruby
require 'bundler'
Bundler.require
Mongoid.configure do |config|
  config.master = Mongo::Connection.new.db('foo')
end

class Author
  include Mongoid::Document
  references_many :books,:inverse_of => :author
  field :name
end

class Book
  include Mongoid::Document
  referenced_in :author
  field :title
end

a = Author.new name: 'Aristotle'
b = Book.new title: 'Rhetoric'

b.author = a
b.save
a.save

puts a.books.first.title