Rails form_for with nested resources: "No route matches" - ruby-on-rails-3

Question

I have the following nested resources in my routes.rb file. The inner resource specifies a controller name.
resources :batches, :except => [:new], :path => "sets" do
resources :tags, :controller => "batches_tags"
end
In the view for BatchesTags#new, I am trying to build a form:
<%= form_for [#batch, #tag], :url => batch_tag_path do |f| %>
...
<% end %>
Attempting to load this page (/sets/1/tags/new) gives me a ActionController::RoutingError with the message:
No route matches {:action=>"show", :controller=>"batches_tags"}
But when I run $ rake routes, it clearly shows this route does exist:
batch_tag GET /sets/:batch_id/tags/:id(.:format) {:action=>"show", :controller=>"batches_tags"}
Does anyone know how to fix this error?
Edit:
In the view for Batches#show, I use that same batch_tag_path function and it works perfectly:
<%= link_to "...", batch_tag_path(#batch, tag) %>

Answer 1

It turns out that, while batch_tag_path is a valid route (making the "No route matches" error message very confusing), the path I needed was the pluralized batch_tags_path, as seen in this $ rake routes output:
batch_tags GET /sets/:batch_id/tags(.:format) {:action=>"index", :controller=>"batches_tags"}
POST /sets/:batch_id/tags(.:format) {:action=>"create", :controller=>"batches_tags"}
Perhaps the error message meant that batch_tag_path wasn't a valid route for POST?