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

So, I would stub out the find call to the model and return an object that the test controls, then use an "expect to change" assertion on the attribute in question.

describe "my request" do
  let(:my_model) { Model.new }
  it "should update the model" do
    Model.should_receive(:find).and_return(my_model)

    expect {
      xhr :put, model_path(model), {attribute: 'value'}
    }.to change { my_model.attribute }.to('value')
  end
end

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