gistfile1.rb

require 'mongoid'

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

class Person
    include Mongoid::Document
    embeds_many :addresses
    collection.remove
end
class Address
    include Mongoid::Document
    embedded_in :person, :inverse_of => :addresses
end

@person = Person.create
first = @person.addresses.create
@person.addresses.length # 1
@person.addresses << Address.new
@person.addresses.length # 2
@person.reload
@person.addresses.length # 1