how to expire cached category browse paginated pages

I have a set of categories. Each category has a set of products.

A user can browse by categories. The product listing by category
provides pagination.

so I am using rails' fragment cache to cache results like
/category/computers/page/1, /category/computers/page/2 in Memcached:

unless read_fragment({:page => params[:page] || 1, :category =>

so whenever the system updates a product, I need to expire the related
category's product listing pages. But I don't know which page the
product belongs to before it's updated( say the listing is sorted by
product name, and the product name got updated ). so I don't know
which pages to expire.

because of my use of memcached as the caching store, I can't use the
regular expression to find caches.

so what's a solution? keeping track of the page numbers that have been

The last solution I want would be expire caches automatically after like
10 mins.