RSpec: Can't convert Image to String when using Nested Resource

I'm having trouble with and RSpec view test. I'm using nested resources and the model with a belongs_to association.

Here's what I have so far:

describe "/images/edit.htm开发者_StackOverflow社区l.erb" do
  include ImagesHelper

  before(:each) do
    @image_pool = stub_model(ImagePool, :new_record => false,
                             :base_path => '/')
    assigns[:image] = @image =
      stub_model(Image,
                 :new_record? => false,
                 :source_name => "value for source_name",
                 :image_pool => @image_pool)
  end

  it "renders the edit image form" do
    render

    response.should have_tag("form[action=#{image_path(@image)}][method=post]") do
      with_tag('input#image_source_name[name=?]', "image[source_name]")
    end
  end
end

The error I'm receiving:

ActionView::TemplateError in '/images/edit.html.erb renders the edit image form'
can't convert Image into String
On line #3 of app/views/images/edit.html.erb

    1: <h1>Editing image</h1>
    2:
    3: <% form_for(@image) do |f| %>
    4:   <%= f.error_messages %>
    5:
    6:   <p>

    app/views/images/edit.html.erb:3
    /opt/dtcm/railstest/lib/ruby/gems/1.9.1/gems/rspec-rails-1.3.2/lib/spec/rails/extensions/action_view/base.rb:27:in `render_with_mock_proxy'
    /opt/dtcm/railstest/lib/ruby/gems/1.9.1/gems/rspec-rails-1.3.2/lib/spec/rails/example/view_example_group.rb:170:in `render'

Looking at the rails code where the exception occurs is not very revealing. Any ideas on how I can narrow down what is going on here?

One thing I tried was calling form_for directly from the example and I got a different error griping about lack of 'polymorphic_path' defined on Spec::Rails::Example::ViewExampleGroup::Subclass_4:0xblah. Not sure if that actually means anything.

Ok. This has nothing to do with Rspec and everything to do with proper use of nested resources and rails helpers.

Apparently the right way to handle nested resources in a view is:

<% form_for [@image_pool, @image] do |f| %>

It's just that the error message isn't helpful.