(Newbie) routing question

Hi, I have a question about routing and nested resources. I have an app
with Photos that belong to Users via a many2one relationship. The URL:

http://localhost:3000/photos

lists all photos (from all users). Clicking on a random photo from this
listing should send one to f ex:

http://localhost:3000/users/2/photos/10

Because I have this in my routes.rb:

map.resources :users, :member => { :enable => :put } do |users|
    users.resources :photos, :name_prefix => 'user_',
                             :controller => 'user_photos',
                             :member => { :add_tag => :put, :remove_tag
=> :delete }
end

However when I load http://localhost:3000/photos I get this error
message:

user_photo_url failed to generate from {:controller=>"user_photos",
:action=>"show", :user_id=>nil, :id=>#<Photo id: 2, user_id: nil, title:
nil, body: nil, created_at: "2008-10-27 14:26:32", etc

Extracted source (around line #2):

1: <li>
2: <%= link_to image_tag(photo.public_filename('thumb')),
user_photo_path(:user_id => photo.user, :id => photo) %>
3: </li>

The above partial, _photo.rhtml, is what causes the error.

This code has worked fine up until I refactored it to function with
will_paginate instead of classic_pagination. I haven't touched the
routing declarations or the _photo.rhtml partial.

Any help will be appreciated!

/ Vahagn

Looking at the error message, it seems to be saying that user_id is
nil ... which tells me that you don't have a user set for that photo
and it can't generate the URL without knowing the id of the user.