TDD Action Caching in Rails 3

brand_sweeper.rb

class BrandSweeper < ActionController::Caching::Sweeper
  observe Brand # Observers will introspect on the class, but Sweepers don't
 
  def after_update(brand)
    expire_action :controller => "brand", :action => :preview, :brand_id => brand.to_param
  end
  ...

brands_controller.rb

class BrandsController < ApplicationController
  caches_action :preview
  cache_sweeper :brand_sweeper

  def preview
  end

  def update
    ...
    @brand.save

brands_controller_spec.rb

describe BrandsController do
  describe "caching" do
    let(:brand) { Factory.create(:brand) }
    let(:preview_cache_path) {'views/test.host' + preview_brand_path(brand)}

    around do |example|
      caching, ActionController::Base.perform_caching = ActionController::Base.perform_caching, true
      store, ActionController::Base.cache_store = ActionController::Base.cache_store, :memory_store
      silence_warnings { Object.const_set "RAILS_CACHE", ActionController::Base.cache_store }

      example.run

      silence_warnings { Object.const_set "RAILS_CACHE", store }
      ActionController::Base.cache_store = store
      ActionController::Base.perform_caching = caching
    end

    it "should action cache #preview" do
      Rails.cache.clear
      BrandsController.caches_action :preview # must be recapitulated to get around load time weirdfullness
      
      get :preview, :brand_id => brand.to_param
      
      ActionController::Base.cache_store.exist?(preview_cache_path).should be_true
    end

    it "should clear the cache on #update" do
      ActionController::Base.cache_store.write(preview_cache_path, 'CACHED ACTION')

      put :update, id: brand.to_param, brand: {one: 'attribute', after: 'another'}

      ActionController::Base.cache_store.exist?(sign_up_cache_path).should be_false
    end
  end
end

sadly, when I tried this the spec only worked in isolation.
Any new insights as to how-to test the key generated for an overriden :cache_path (re: caches_action)?