Using RSpec Ordered Message Expectations to Tighten your Specs

I quite enjoy the competitive undercurrent of ping pong pair programming. As the person writing the implementation code, it is fun to write something that will turn a test green, but still not necessarily do what my partner was expecting. Taking this approach has also been helpful for improving our specs. Take this example controller spec:

describe ArticlesController do
  describe "handling create" do
    before(:each) do
      @article = mock_model(Article, :save => nil)
      Article.stub(:new).and_return(@article)

      @user = mock_model(User)
      controller.stub(:current_user).and_return(@user)
    end

    it "should build a new article from posted data" do
      Article.should_receive(:new).with('title' => 'Test Post')
      post :create, :article => {:title => 'Test Post'}
    end

    it "should assign the current user as the article's author" do
      @article.should_receive(:author=).with(@user)
      post :create
    end

    it "should save the article"
      @article.should_receive(:save)
      post :create
    end
  end
end

This looks like a reasonable set of concise, clear examples, but you can easily make them all pass and without building a controller action that does what you expect:

class ArticlesController < ApplicationController
  def create
    @article = Article.new(params[:article])
    @article.save
    @article.author = current_user
  end
end

This satisfies the examples, but saving the article before assigning the current user as author isn’t what we would have intended. Enter RSpec’s ordered message expectations. These allow you to specify the order in which you expect an object to receive message calls.

describe ArticlesController do
  describe "handling create" do
    it "should save the article after assigning the current user as author"
      @article.should_receive(:author=).with(@user).ordered
      @article.should_receive(:save).ordered
      post :create
    end
  end
end

This example would fail with the above controller action, and force us to write it properly:

class ArticlesController < ApplicationController
  def create
    @article = Article.new(params[:article])
    @article.author = current_user
    @article.save
  end
end

The result is a controller that does what you expect, a stronger set of specs, and an increased capacity for true behaviour driven development. Win, win, win!