Auto numbering of childs for a parent

Rails
1

I have a Author model and Book model

field_names => [id, name]

class Author < AR::Base
has_many :books
end

field_names => [id, name]

class Book < AR::Base
belong_to :author,
attr_accessor :serial_number
end

Now for example I have inserted two books BOOK1(id:101) and
BOOK2(id:102) for a author AUTHOR1(id:30)

Now when I load the author with books like Author.find_by_id
(30, :include => :books)

I want a way to set the serial_number incrementally for each Book
according to the order of their ID field (or may be any other logic).
In above example BOOK1(serial_number:1) and BOOK2(serial_number:2)

Some more requirements:
I need it to work even when i find Book.find_by_id(102) and expecting
the serial_number to set 2

Any better recommendation?

Samiron paul

http://www.code71.com

2 
attr_accessor :serial_number

In above example BOOK1(serial_number:1) and BOOK2(serial_number:2)

Some more requirements:
I need it to work even when i find Book.find_by_id(102) and expecting
the serial_number to set 2

Any better recommendation?

You might look into acts_as_list at Ruby on Rails ยท GitHub
acts_as_list.

-philip