Categories like ebay

I think the category feature of eBay is very handy. I want to figure out
a way to to make this function work on rails.

What is so sepcial with ebay’s category feature?
Let’s say we are going to buy a Movie DVD.
HOME->BUY->DVDs & Movies
in ‘Categories within DVDs & Movies’, click the first item ‘DVD, HD DVD
& Blu-ray’. ok, now its fancy categories for dvd movie is ready, right
on the left sidebar.

isn’t it different?

  1. It has multiple category systems: movies are divided by their Format,
    Genre(with subgenres), Rating and Condition.
  2. This multiple category systems are interactive. If you go deep to one
    category system’s subcategory, for instance we click PG-13(under Rating
    category), the other category system stay on sidebar, only now all
    categories show movies that is rated PG-13. (if any category doesn’t
    have any movies rated PG-13, this category removed)
  3. A number is shown besides every category name to indicate how many
    movies’ are below this category.

This is a very useful system to help you pick out the very few movies
that you love from more than 200,000 choices…

Is it hard to do it on Rails?