Testing controller JSON responses in Rspec

I was recently rewriting some controller specs because they were way too heavy: all the models were saved to the database and there was no mocking. While trying to test the JSON response of an action, I got the following exception:

ActiveSupport::JSON::Encoding::CircularReferenceError

Here is the relevant controller code:

format.json do
  render :json => as_json(@questions)
end

Here is the test's mock:

question = mock_model(Question)

A little cryptic, right? After a little digging, I changed the mock to this:

question = mock_model(Question, :as_json => {'foo' => 'bar'})

Ah, there we go!

The spec ended up looking like this:

context "as json" do
  it "lists the questions" do
    question = mock_model(Question, :as_json => {'foo' => 'bar'})
    Question.should_receive(:find_ordered_subjects).and_return([question])
    get :index, :format => 'json'
    response.body.should == "[{\"foo\":\"bar\"}]"
  end
end

In our application_controller.rb, an as_json() method called to_json, which would call as_json in the test, resulting in a circular reference. Oops.

Don't forget to stub as_json()!