Untitled

multicolumn primary keys in rails
authors
  author_name,
  author_letter,
  author_nr1,
  author_nr2
  ...

titles
  titel_nr,
  titel_name,
  ...

author_titles
  titel_nr,
  author_letter,
  author_nr1,
  author_nr2

add_index :users, [:merchant_id, :email], :unique => true
    add_index :users, [:merchant_id, :login], :unique => true

validates_uniqueness_of :email,    :scope => :merchant_id
  validates_uniqueness_of :login,    :scope => :merchant_id

class Author
  set_primary_keys :author_letter, :author_nr1, :author_nr2
  has_many :titles, :through => :author_title
end

class Title
  set_primary_key :title_nr
end

class AuthorTitle
  belongs_to :title, :foreign_key => :title_nr
  belongs_to :authori, :foreign_key => [:author_letter, :author_nr1, :author_nr2]
end