Typus + HABTM

author.rb

class Author < ActiveRecord::Base
  has_and_belongs_to_many :books

  attr_accessible :name, :book_ids, as: [:default, :admin]
end

author.yml

Author:
  fields:
    default: name, books
  relationships: books
  application: Application

book.rb

class Book < ActiveRecord::Base
  has_and_belongs_to_many :authors

  attr_accessible :title, :author_ids, as: [:default, :admin]
end

book.yml

Book:
  fields:
    default: title, authors
  relationships: authors
  application: Application