Library, Shelf and Book Model using Ruby/Active Record (in a typical ruby/rack application)

book.rb

class Book < ActiveRecord::Base
  attr_accessible :title, :author, :lang, :genre, :shelf_id
  belongs_to :shelves
  
  def enshelf shelf_id
    @book.shelf_id = shelf_id
  end
  
  def unshelf
    # setting shelf_id to nil means book is not on the shelf
    @book.shelf_id = nil
  end
   
end

library.rb

class Library < ActiveRecord::Base
  has_many :shelves

  def count_shelves
    @no_of_shelves = Shelf.find(:all)
  end
  
  def all_books
    @all_books = Book.find(:all)
  end

end

shelf.rb

class Shelf < ActiveRecord::Base
  attr_accessible :library_id
  has_many :books

  def list_books
    @shelf_books = Book.where(:shelf_id => @shelf.id)
  end
  
  ### sample examples for getting the list for a self e.g self no. 2 ###
  # @shelf = Shelf.find(:shelf_id => 2)
  # @shelf.list_books

end