Passing :id param in link_to etc...

Hi --

Just need some more inside information...

Sample Code from the AWDWR book.

<%= link_to ( image_tag( product.image_url ),
  { :action => :add_to_cart, :id => product },
  :method => :post,
  :confirm => "Are you sure?" ) %>

Why is my ":id =>" reference using 'product' and not ''

Is the :id symbol 'dual purpose' and used as a key for an id param AND
it also infers that the FIELD to 'get' from the product object is named

It uses the to_param method to get the value, so product ends up being
product.to_param. By default, to_param returns the id, but you can
override it if you wish:

   class Book < ActiveRecord::Base
     def to_param

for example.