strange N..N mongoid relation behavior

relation.rb

require 'mongoid'

Mongoid.configure do |config|
  name = "gist_tests"
  host = "localhost"
  config.master = Mongo::Connection.new.db(name)
  config.persist_in_safe_mode = false
end

class Book
	include Mongoid::Document
	has_and_belongs_to_many :authors, class_name: 'Author', inverse_of: 'books'

	field :name
end


class Author
	include Mongoid::Document
	has_and_belongs_to_many :books, class_name: 'Book', inverse_of: 'authors'

	field :name
end

Book.destroy_all
Author.destroy_all

Book.create name: 'foo', authors: [Author.create(name: 'bar'), Author.create(name: 'buz') ]

Book.first.update_attributes authors: [Author.where(name: 'bar').one]

p Book.first.authors.size
p Author.where(book_ids: Book.first.id).count