update or create nested association

book.rb

class Book < ActiveRecord::Base
  attr_accessible :authors_attributes

  has_and_belongs_to_many :authors
  accepts_nested_attributes_for :authors

  def authors_attributes=(new_authors)
    self.authors = new_authors.collect do |author_attrs|
      if author_id = author_attrs.delete(:id)
        author = Author.find(author_id) rescue nil
      end

      # new author or bad author_id
      author = Author.new unless author

      author.attributes = author_attrs
      author
    end
  end
end