rspec test that a request makes certain changes to a model. How can I do something like this?

request_spec.rb

describe "my request" do
  it "should update the model" do
    Model.any_instance.should_receive(:save) do
      self.changes.should include(:attribute)
      self.attribute.should == 'value'
    end

    xhr :put, model_path(model), {attribute: 'value'}
  end
end

I like that a lot. Having a direct reference to the object means I can easily assert a lot of things.