request path_info being ignored?

[Mark Larimer <larimer@fantasybowl.com>]

I've got Rack fronting a Rails 2.3.2 app. And I wrote some Rack
middleware to do a simple rewrite of the request url. Here is a
simplified version of the call() method:

  def call(env)
    req = Rack::Request.new(env)
    req.path_info = "/people/12345/accounts.xml"
    @app.call(env)
  end

When Rack operates on this request and passes control through to the
Rails app, I expect the Rails router to call the index method of the
accounts controller...but instead the path_info that was set in the
middleware method is being completely ignored, and Rails is routing
based on the original path.

If I look at env['PATH_INFO'] after the req.path_info=() call, it
shows the new value.

What am I missing?

Thanks.
--mark