Understanding Rails 3's respond_with

Utilizing ActionController's new respond_with method...how does it determine what to render when action (save) is successful and when it's not?

I ask because I'm trying to get a scaffold generated spec (included below) to pass, if only so that I can understand it. The app is working fine but, oddly, it appears to be rendering /carriers (at least that's what the browser's URL says) when a validation fails. Yet, the spec is expecting "new" (and so am I, for that matter) but instead is receiving <"">. If I change the spec to expect "" it still fails.

When it renders /carriers that page shows the error_messages next to the fields that failed validation as one would expect.

Can anyone familiar with respond_with see what's happening here?

#carrier.rb
  validates :name, :presence => true 

#carriers_controller.rb
class CarriersController < ApplicationController
  respond_to :html, :json

...

  def new
    respond_with(@carrier = Carrier.new)
  end

  def create
     @carrier = Carrier.new(params[:carrier])
     flash[:success] = 'Carrier was successfully created.' if @carrier.save
     respond_with(@carrier) 
  end

Spec that's failing:

#carriers_controller_spec.rb
require 'spec_helper'

describe CarriersController do

  def mock_carrier(stubs={})
    (@mock_carrier ||= mock_model(Carrier).as_null_object).tap do |carrier|
      carrier.stub(stubs) unless stubs.empty?
    end
  end


  describe "POST create" do
    describe "with invalid params" do
      it "re-renders the 'new' template" do
        Carrier.stub(:new) { mock_carrier(:save => false) }
        post :create, :carrier => {}
        response.should render_template("new")
      end
    end
  end
end

with this error:

  1) CarriersController POST create with invalid params re-renders the 'new' template
     Failure/Error: response.should render_template("new")
     expecting <"new"> but rendering with <"">.
     Expected block to return true value.
     # (eval):2:in `assert_block'
     # ./spec/controllers/carriers_controller_spec.rb:81:in `block (4 levels) in <top (required)>'

tl:dr

Add an error hash to the mock:

Carrier.stub(:new) { mock_carrier(:save => false, 
                       :errors => { :anything => "any value (even nil)" })}

This will trigger the desired behavior in respond_with.

What is going on here

Add this after the post :create

response.code.should == "200"

It fails with expected: "200", got: "302". So it is redirecting instead of rendering the new template when it shouldn't. Where is it going? Give it a path we know will fail:

response.should redirect_to("/")

Now it fails with Expected response to be a redirect to <http://test.host/> but was a redirect to <http://test.host/carriers/1001>

The spec is supposed to pass by rendering the new template, which is the normal course of events after the save on the mock Carrier object returns false. Instead respond_with ends up redirecting to show_carrier_path. Which is just plain wrong. But why?

After some digging in the source code, it seems that the controller tries to render 'carriers/create'. There is no such template, so an exception is raised. The rescue block determines the request is a POST and there is nothing in the error hash, upon which the controller redirects to the default resource, which is the mock Carrier.

That is puzzling, since the controller should not assume there is a valid model instance. This is a create after all. At this point I can only surmise that the test environment is somehow taking shortcuts.

So the workaround is to provide a fake error hash. Normally something would be in the hash after save fails, so that kinda makes sense.