JoelQ/simple-stubs

Intro

When testing one object, you may find that the other objects it interacts with get in the way. For example, when testing a controller, it can be cumbersome to ensure the model is used correctly:

describe PostsController do
  describe "#index" do
    it "finds the 10 latest published posts" do
      10.times { |i| create(:post, :published, title: "published#{i}") }
      create(:post, published: false, title: "unpublished")

      get :index

      expect(assigns[:posts].map(&:title)).to match_array(%w(
        published0 published1 published2 published3 published4 published5
        published6 published7 published8 published9
      ))
    end
  end
end

In this example, the setup and verification phases become complex and hard to follow because we need to re-test the logic to find 10 published posts, even though this logic is already implemented and tested in the model layer.

We can use stubs to clean it up:

describe PostsController do
  describe "#index" do
    it "finds the 10 latest published posts" do
      posts = double("latest_published")
      allow(Post).to receive(:latest_published).and_return(posts)

      get :index

      expect(assigns[:posts]).to eq(posts)
    end
  end
end

The posts variable above is called a "stub," which is a kind of "test double."

You can create test doubles in RSpec by using the double method:

posts = double("latest_published")

This creates an object that stands in for an actual list of published posts; it's like a stunt double for your actual object. Using the double method is very similar to calling Object.new, but tests that use doubles will fail with much clearer error messages. The "latest_published" string above is simply a name which will be used in test failure messages.

You can stub out a method on any object (including a test double) by using allow:

allow(Post).to receive(:latest_published).and_return(posts)

This changes the Post class to return posts whenever latest_published is called. Stubbed methods will revert to their regular behavior after each test runs.

Using the double and allow methods, you can create simple stubs.

In this exercise, you'll use simple stubs to clean up some tests.

If you'd like to see a quick intro to test doubles, check out the related episode of the Weekly Iteration.

Setup

After running bin/setup, edit spec/dashboard_spec.rb and look for test logic which is duplicated from spec/post_spec.rb. Use double and allow to create a stub for posts and eliminate this duplication.

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