Use simplecov in your Rails test suite

Use…

…simplecov in your tests.

Add to Gemfile

group :test do
  gem 'simplecov', require: false
end

And run bundle in the root directory of your project.

Add to .gitignore

Do not commit generated files in the coverage directory.

coverage/*

Add to top of spec/spec_helper.rb or test/test_helper.rb

require 'simplecov'
SimpleCov.start 'rails' do
  add_filter '/bin/'
  add_filter '/db/'
  add_filter '/spec/' # for rspec
  add_filter '/test/' # for minitest
end

Add this right at the top of the file. The add_filter lines means files matching the passed string are excluded from the results.

Then run your tests as normal, you’ll see a message that looks like:

Coverage report generated for RSpec to /PATH_TO_YOUR_APP. 584 / 1068 LOC (54.68%) covered.

But why?

Having an awareness of your level of coverage is useful as an input to understanding the stability of your application and protection against regressions.

Exploring the generated HTML reports (available in coverage/index.html) is a good way to see if there are areas of your code that are poorly tested.

It’s important to note that measuring your test coverage of your application’s code is a blunt instrument. In fact, aiming for 100% coverage is not a noble aim, or even useful.

Some codebases with 100% test coverage will still have bugs, and some codebases with much lower coverage will not.

Why not?

It could slow your test suite a little. But it’s only, at most, additional seconds in a multiple-minute test suite.