Using simplecov code coverage with Rails & Minitest

Simplecov is a Ruby code coverage tool. This article documents the integration with a Rails 8 application, with the minitest test framework.

Setup

1. Include the simplecov gem in Gemfile.

   gem "rails", "~> 8.1"
   gem "minitest", "~> 5"
   gem "simplecov", require: false
   

2. Install the gem.

   bundle install
   

Integrate

1. Edit test/test_helper.rb and these codes at the top of the file.

   # frozen_string_literal: true
   
   require "simplecov"
   SimpleCov.start "rails" do
     enable_coverage :branch
     command_name "#{ENV['TEST_SUITE'] || 'unit'}-#{Process.pid}"
     merge_timeout 3600
   end
   

2. Optionally, you can add filters to exclude certain files. For example, any files with fewer than 5 lines of code are likely scaffolds.

   add_filter do |source_file|
     source_file.lines.count < 5
   end
   

3. While simplecov already supports default grouping (such as controllers, mailers, models, jobs), you can create new groups as needed. For example, you have a few classes that fall under the providers group, you can do so:

   add_group "Providers", "app/providers"
   

Execute

1. To combine the full coverage, you can run all the tests together. In the example below, we run both unit testing and system testing.

   bin/rails db:test:prepare && \
     TEST_SUITE=unit bin/rails test && \
     TEST_SUITE=system bin/rails test:system
   

2. The result can be located in the coverage directory.