problem with references_and_referenced_in_many

mongoid_test.rb

require "rubygems"
require "mongoid"
require "test/unit"

Mongoid.configure do |config|
  name = "ref_test"
  config.master = Mongo::Connection.new.db(name)
  config.logger = nil
end

Mongoid.master.collections.select do |collection|
      collection.name !~ /system/
end.each(&:drop)

class Person
  include Mongoid::Document
  field :name
  references_and_referenced_in_many :preferences
end

class Preference
  include Mongoid::Document
  field :name
  references_and_referenced_in_many :people
end

 
class TestPerson < Test::Unit::TestCase
  def setup
    @person = Person.create(:name => "Billy")
    @pref_a = Preference.create(:name => "aaa")
    @pref_b = Preference.create(:name => "bbb")
  end
 
  def teardown
    @person.destroy
    @pref_a.destroy
    @pref_b.destroy
  end

  def test_relation_foreign_keys
    @person.preferences = [ @pref_a ]
    @person.save

    @person.preferences = [ @pref_b ]
    @person.save

    @person.reload
    @pref_a.reload
    @pref_b.reload

    assert @pref_a.person_ids == [], "pref_a.people should be empty"
  end

  def test_clear_relations
    @person.preferences = [ @pref_a, @pref_b ]
    @person.save

    @person.preferences = []
    @person.save

    @person.reload

    assert @person.preference_ids == [], "preferences should be empty"
    assert @pref_a.person_ids == [], "pref_a.people should be empty"
    assert @pref_b.person_ids == [], "pref_b.people should be empty"
  end
end