Can't Stub Rack

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In my Rack application, I will have a private method that generates thumbnails. An RSpec example that describes this:

      it "should generate a thumbnail" do
        app.should_receive(:mk_thumbnail)
        get "/foo.jpg", :thumbnail => 1
      end

In that example, app is the application defined for the benefit of Rack::Test:

def app
  target_app = mock("Target Rack Application")
  target_app.
    stub!(:call).
    and_return([200, { }, "Target app"])

  Rack::ThumbNailer.new(target_app)
end

The example fails with:

1)
Spec::Mocks::MockExpectationError in 'ThumbNailer Accessing an image with thumbnail param should generate a thumbnail'
# expected :mk_thumbnail with (any args) once, but received it 0 times
./thumbnailer_spec.rb:44:

Finished in 0.015466 seconds

4 examples, 1 failure

To make that pass, I add the private method mk_thumbnail and a call to mk_thumbnail:

    def call(env)
      req = Rack::Request.new(env)
      if !req.params['thumbnail'].blank?
        filename = "foo"
        mk_thumbnail(filename, env)
        thumbnail = ::File.new(filename).read
        [200, { }, thumbnail]
      else
        @app.call(env)
      end
    end

    private
    def mk_thumbnail(filename, env)
    end

That should make the example pass, but:

1)
Spec::Mocks::MockExpectationError in 'ThumbNailer Accessing an image with thumbnail param should generate a thumbnail'
# expected :mk_thumbnail with (any args) once, but received it 0 times
./thumbnailer_spec.rb:44:

Hunh? That totally should have been called!

After *much* experimentation, I finally conclude / guess that the app function must be getting evaluated in a different scope than the spec. Thus, when I place the expectation on the app in the code, I am placing it on a different app than Rack::Test is actually using. I must investigate this further, because I can see needing to do this often.

For tonight though, I side-step the issue, if only to make a little bit of progress. Rather than putting the expectation on an instance of the Rack::Thumbnailer class, I convert the method to a class method and put the expectation on the class itself:

      it "should generate a thumbnail" do
        Rack::ThumbNailer.should_receive(:mk_thumbnail)
        get "/foo.jpg", :thumbnail => 1
      end

Updating the code so that this is a class method makes this example pass:

    def call(env)
      req = Rack::Request.new(env)
      if !req.params['thumbnail'].blank?
        filename = "foo"
        Thumbnailer.mk_thumbnail(filename, env)
        thumbnail = ::File.new(filename).read
        [200, { }, thumbnail]
      else
        @app.call(env)
      end
    end

    private
    def self.mk_thumbnail(filename, env)
    end

I'll call it a night there and hope to make more progress tomorrow.