Mutation testing of Rails views

Thanks to mutation testing we can get much higher confidence while working with Ruby/Rails apps. There is one area, though, where I’ve been struggling to make mutant to work - the Rails views. People use erb or haml and they’re both not a proper Ruby code, they’re templating languages.

In this post, I’m showing a trick which can help make Rails views covered by mutation testing coverage.

It was 7 or 8 years ago, when I was using another approach to Rails views, called Erector. We wrote a whole project using it. It had some issues, but overall I loved the idea that I can have views written in Ruby, given how elegant our language is.

Since then, I haven’t used Erector much, but today I’ve tried to use it, just to learn that there’s another library which sounds like a better version of Erector - it’s called fortitude.

I have created a simple Rails 5 app and then created a simple Capybara test which will be used by mutant:

class HappyTest < ActionDispatch::IntegrationTest

  def test_happy_path
    visit("/")
    assert(page.has_text?("Hello world"))
  end
end

with the following routes:

Rails.application.routes.draw do
  resource :root, only: [:index]
  root to: "root#index"
end

and the controller:

class RootController < ApplicationController
  def index
  end
end

Then I created a fortitude view:

class Views::Root::Index < Views::Base

  def content
    p(:class => 'content') {
      text "Hello world"
    }
  end
end

Now when I run mutant with:

RAILS_ENV=test bundle exec mutant -r ./config/environment -r ./test/integration/happy_test.rb --use minitest "Views::Root::Index"

I get a nice mutation coverage report:

(more here...)

Subjects:        1
Mutations:       26
Results:         26
Kills:           16
Alive:           10
Runtime:         3.76s
Killtime:        10.74s
Overhead:        -64.96%
Mutations/s:     6.91
Coverage:        61.54%
Expected:        100.00%

and the mutated code:

def content
  -  p(class: "content") do
  -    text("Hello world")
  -  end
  +  text("Hello world")
end

This means, that the view did get covered by mutant and it was mutated to see what’s the coverage. With this example, it showed me, that I have no test requiring that it’s a <p> tag. I’m not sure if that’s really useful, but at least this technique can be applied in situations where we need to take care of Rails views as well :)

Obviously, if you want to apply it to existing Rails views, they need to be converted to Fortitude first, which may not be the best choise for every project…

Frontend friendly Rails

You can struggle with Rails views or … you can make your Rails more frontend-friendly and go with JavaScript-based applications. That’s one of our favourite ways at Arkency in the last years.

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