## book.rb
class Book < ActiveRecord::Base
  has_many :book_people, :dependent => :destroy
  has_many :people, :through => :book_people, :uniq => true

  has_many :editors, :class_name => 'Person', :finder_sql => <<-SQL
  SELECT people.* FROM people
  INNER JOIN book_people ON (book_people.person_id = people.id AND people.type = 'Editor')
  WHERE book_people.book_id = #{id}'
  SQL

  has_many :editors, :class_name => 'Person', :finder_sql => <<-SQL
  SELECT people.* FROM people
  INNER JOIN book_people ON (book_people.person_id = people.id AND people.type = 'Author')
  WHERE book_people.book_id = #{id}'
  SQL
end

##person.rb
class Person < ActiveRecord::Base
  has_many :book_people, :dependent => :destroy
  has_many :books, :through => :book_people
end

##book_person.rb
class BookPerson < ActiveRecord::Base
  belongs_to :person
  belongs_to :book

  belongs_to :author, :class_name => "Person", :foreign_key => :person_id
  belongs_to :editor, :class_name => "Person", :foreign_key => :person_id
end