RSpec best practices

Posted on by in Development, Process

Rspec is a great tool in the behavior driven design process of writing human readable specifications that direct and validate the development of your application. We’ve found the following practices helpful in writing elegant and maintainable specifications.

First #describe what you are doing …

Begin by using a #describe for each of the methods you plan on defining, passing the method’s name as the argument. For class method specs prefix a “.” to the name, and for instance level specs prefix a “#”. This follows standard Ruby documenting practices and will read well when output by the spec runner.

describe User do

  describe '.authenticate' do
  end

  describe '.admins' do
  end

  describe '#admin?' do
  end

  describe '#name' do
  end

end

…Then establish the #context

Write a #context for each execution path through a method; literally specifying the behavior of a method in a given context.

For example, the following method has 2 execution paths:

class SessionsController < ApplicationController

  def create
    user = User.authenticate :email => params[:email],
                             :password => params[:password]
    if user.present?
      session[:user_id] = user.id
      redirect_to root_path
    else
      flash.now[:notice] = 'Invalid email and/or password'
      render :new
    end
  end

end

Create two contexts in the corresponding spec:

describe '#create' do
  context 'given valid credentials' do
  end

  context 'given invalid credentials' do
  end
end

Note the use of “given” in the argument to each #context. It communicates the context of receiving input. Another great word to use in a context for describing conditional driven behavior is “when”.

describe '#destroy' do
   context 'when logged in' do
   end

   context 'when not logged in' do
   end
 end

By following this style, you can then nest #contexts to clearly define further execution paths.

#it only expects one thing

By striving to only having one expectation per example, you increase the readability of your specs.

A spec with multiple un-related expectations in a single example:

describe UsersController do

  describe '#create' do
    ...

    it 'creates a new user' do
      User.count.should == @count + 1
      flash[:notice].should be
      response.should redirect_to(user_path(assigns(:user)))
    end
  end

end

Breaking out the expectations into separate examples clearly defines the behavior and results in easier to maintain examples:

describe UsersController do

  describe '#create' do
    ...

    it 'creates a new user' do
      User.count.should == @count + 1
    end

    it 'sets a flash message' do
      flash[:notice].should be
    end

    it "redirects to the new user's profile" do
      response.should redirect_to(user_path(assigns(:user)))
    end
  end

end

Write examples by starting with a present tense verb that describes the behavior.


    it 'creates a new user' do
    end

    it 'sets a flash message' do
    end

    it 'redirects to the home page' do
    end

    it 'finds published posts' do
    end

    it 'enqueues a job' do
    end

    it 'raises an error' do
    end

Finally, don’t begin examples names with the word ‘should’.  It’s redundant and results in hard to read spec output. Likewise, don’t hesitate to use words like ‘the’ or ‘a’ or ‘an’ in your examples when they improve readability.

Prefer explicitness

#it, #its and #specify may cut down on the amount of typing but they sacrifice readability.  You now have to read the body of the example in order to determine what its specifying.  Use these sparingly if at all.

Let’s compare the documentation formatter output of the following:

describe PostsController do

  describe '#new' do
    context 'when not logged in' do
      ...

      subject do
        response
      end

      it do
        should redirect_to(sign_in_path)
      end

      its :body do
        should match(/sign in/i)
      end
    end
  end

end

With the explicit behavior descriptions below:

describe PostsController do

  describe '#new' do
    context 'when not logged in' do
      ...

      it 'redirects to the sign in page' do
        response.should redirect_to(sign_in_path)
      end

      it 'displays a message to sign in' do
        response.body.should match(/sign in/i)
      end
    end
  end

end

The first example results in blunt, code-like output with redundancy from using the word ‘should’ multiple times:

$ rspec spec/controllers/posts_controller_spec.rb --format documentation

PostsController
  #new
    when not logged in
      should redirect to "/sign_in"
      should match /sign in/i

The second results in a very clearly, readable specification:

$ rspec spec/controllers/posts_controller_spec.rb --format documentation

PostsController
  #new
    when not logged in
      redirects to the sign in page
      displays a message to sign in

Run specs to confirm readability

Always run your specs with the ‘–format’ option set to ‘documentation’ (in RSpec 1.x the –format options are ‘nested’ and ‘specdoc’)

$ rspec spec/controllers/users_controller_spec.rb --format documentation

UsersController
  #create
    creates a new user
    sets a flash message
    redirects to the new user's profile
  #show
    finds the given user
    displays its profile
  #show.json
    returns the given user as JSON
  #destroy
    deletes the given user
    sets a flash message
    redirects to the home page

Continue to rename your examples until this output reads like clear conversation.

Use the right matcher

RSpec comes with a lot of useful matchers to help your specs read more like language.  When you feel there is a cleaner way … there usually is!

Here are some of our favorites matchers, before and after they are applied:

# before: double negative
object.should_not be_nil
# after: without the double negative
object.should be

# before: 'lambda' is too low level
lambda { model.save! }.should raise_error(ActiveRecord::RecordNotFound)
# after: for a more natural expectation replace 'lambda' and 'should' with 'expect' and 'to'
expect { model.save! }.to raise_error(ActiveRecord::RecordNotFound)

# before: straight comparison
collection.size.should == 4
# after: a higher level size expectation
collection.should have(4).items

Check out the docs and ask around.

Formatting

Use ‘do..end’ style multiline blocks for all blocks, even for one-line examples. Further improve readability with a single blank line between all blocks and at the beginning and end of the top level #describe.

Again compare:

describe PostsController do
  describe '#new' do
    context 'when not logged in' do
      ...
      subject { response }
      it { should redirect_to(sign_in_path) }
      its(:body) { should match(/sign in/i) }
    end
  end
end

With the clearly structured code below:

describe PostsController do

  describe '#new' do
    context 'when not logged in' do
      ...

      it 'redirects to the sign in page' do
        response.should redirect_to(sign_in_path)
      end

      it 'displays a message to sign in' do
        response.body.should match(/sign in/i)
      end
    end
  end

end

A consistent formatting style is hard to achieve with a team of developers but the time saved from having to learn to visually parse each teammate’s style makes it worthwhile.

Conclusion

As you can see, all these practices revolve around writing clear specifications readable by all developers. The ideal is to run all specs to not only pass but to have their output completely define your application. Every little step towards that goal helps, and we’re still learning better ways to get there. What are some of your best RSpec practices?

Update: This post is now available in Japanese thanks to Makoto Kuwata: http://jp.rubyist.net/magazine/?0032-TranslationArticle