controller_macros.rb

module ControllerMacros
  
  module InstanceMethods
    def should_render template
      it "should render the #{template} template" do
        do_request
        response.should render_template(template)
      end
    end
    
    def should_assign var_name
      it "should assign #{var_name}" do
        do_request
        assigns(var_name).should == @mock_model
      end
    end
    
    def expecting type, method, opts, &block
      args = opts[:with]
      before(:each) do
        @mock_model = mock_model(type) do |m|
          m.as_null_object
          other_method = method == :create ? :new : :create
          type.should_receive(method).with(args).and_return(m)
          type.should_not_receive(other_method).with do
            raise "#{type} is expecting #{method.inspect}, but it received #{other_method.inspect}"
          end
        end
      end
      instance_eval &block
    end
  end

  def on request_type, action, *args, &block
    describe "on #{request_type.to_s.upcase} #{action}" do
      extend InstanceMethods
      define_method :do_request do
        __send__(request_type, action, *args)
      end
  
      instance_eval &block
    end
  end
end

things_controller_spec.rb

describe ThingsController do
  extend ControllerMacros
  on :get, :index do
    should_render "index"
  end
  on :post, :create, :thing => {:these => 'params'} do
    expecting Thing, :to_receive => :new, :with => {'these' => 'params'} do
      should_assign :thing
    end
  end
end