GurusQ: Metal calling disparity-How to install metal in code

I have a simple class that I want to install as Rails Metal. I don't
want to put it into a file in app/metal for reasons I don't want to go
into. It looks like this, and handler is defined elsewhere.

class UrlFilter
  ALLOW = [404]
  DENY = [403]
  def self.call(env)
    handler.call(env["PATH_INFO"]) ? ALLOW : DENY
  end
end

I've tried installing it like this :
ActionController::Dispatcher.middleware.use UrlFilter

but on the first request it crashes with :

Sat Apr 03 00:29:43 +0800 2010: Read error: #<ArgumentError: wrong
number of arguments (0 for 1)>
/Library/Ruby/Gems/1.8/gems/actionpack-2.3.5/lib/action_controller/middleware_stack.rb:31:in
`call'
/Library/Ruby/Gems/1.8/gems/actionpack-2.3.5/lib/action_controller/middleware_stack.rb:31:in
`klass'
/Library/Ruby/Gems/1.8/gems/actionpack-2.3.5/lib/action_controller/middleware_stack.rb:42:in
`active?'
/Library/Ruby/Gems/1.8/gems/actionpack-2.3.5/lib/action_controller/middleware_stack.rb:112:in
`active'
/Library/Ruby/Gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:108:in
`find_all'
/Library/Ruby/Gems/1.8/gems/actionpack-2.3.5/lib/action_controller/middleware_stack.rb:112:in
`each'
/Library/Ruby/Gems/1.8/gems/actionpack-2.3.5/lib/action_controller/middleware_stack.rb:112:in
`find_all'
/Library/Ruby/Gems/1.8/gems/actionpack-2.3.5/lib/action_controller/middleware_stack.rb:112:in
`active'
/Library/Ruby/Gems/1.8/gems/actionpack-2.3.5/lib/action_controller/middleware_stack.rb:116:in
`build'
/Library/Ruby/Gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:130:in
`build_middleware_stack'
/Library/Ruby/Gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:113:in
`call'
/Library/Ruby/Gems/1.8/gems/actionpack-2.3.5/lib/action_controller/reloader.rb:34:in
`run'
/Library/Ruby/Gems/1.8/gems/actionpack-2.3.5/lib/action_controller/dispatcher.rb:108:in
`call'
/Library/Ruby/Gems/1.8/gems/rails-2.3.5/lib/rails/rack/static.rb:31:in
`call'
/Library/Ruby/Gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:46:in `call'
/Library/Ruby/Gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:40:in `each'
/Library/Ruby/Gems/1.8/gems/rack-1.0.1/lib/rack/urlmap.rb:40:in `call'
/Library/Ruby/Gems/1.8/gems/rails-2.3.5/lib/rails/rack/log_tailer.rb:17:in
`call'
/Library/Ruby/Gems/1.8/gems/rack-1.0.1/lib/rack/content_length.rb:13:in
`call'
/Library/Ruby/Gems/1.8/gems/rack-1.0.1/lib/rack/chunked.rb:15:in `call'
/Library/Ruby/Gems/1.8/gems/rack-1.0.1/lib/rack/handler/mongrel.rb:64:in
`process'
/Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:159:in
`process_client'
/Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:158:in `each'
/Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:158:in
`process_client'
/Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `run'
/Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in
`initialize'
/Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `new'
/Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:285:in `run'
/Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:268:in
`initialize'
/Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:268:in `new'
/Library/Ruby/Gems/1.8/gems/mongrel-1.1.5/lib/mongrel.rb:268:in `run'
/Library/Ruby/Gems/1.8/gems/rack-1.0.1/lib/rack/handler/mongrel.rb:34:in
`run'
/Library/Ruby/Gems/1.8/gems/rails-2.3.5/lib/commands/server.rb:111
/Library/Ruby/Site/1.8/rubygems/custom_require.rb:31:in
`gem_original_require'
/Library/Ruby/Site/1.8/rubygems/custom_require.rb:31:in `require'
script/server:3

The rails calling code looks like this :

      def klass
        if @klass.respond_to?(:call)
          @klass.call
        elsif @klass.is_a?(Class)
          @klass
        else
          @klass.to_s.constantize
        end
      rescue NameError
        @klass
      end

      def active?
        return false unless klass

        if @conditional.respond_to?(:call)
          @conditional.call
        else
          @conditional
        end
      end

Notice when calling call no arguments are passed (twice!) ? Every metal
example I've seen defines the entry method as

  def self.call(env)

yet this calling code doesn't pass any.

Whats going on ? How do I define and install a metal class in code ?