Having a git workflow based on feature branches, where you create a new branch for every new story you are working on, allows you to have a strict barrier on what files changed.

But, when you have a global coverage ratio, it’s hard to know if you’re working towards increasing this ratio or making sure that every line (or critical parts) are covered by tests.

When coding in Ruby, the usual suspect for coverage reports is SimpleCov.

I won’t dig in SimpleCov features or in setup, as it’s best described in the project page, but how do we get the branch coverage as promised?

Configure SimpleCov to include a branch coverage report

We start with your helper file, like the documentation states:

require 'simplecov'
SimpleCov.start 'rails'

Assuming the reference branch is master, we want to report all the files that changed in current branch, against master branch. One way to achieve it, is to create a helper that would run a git diff command and gather the output. So we’ll end up with a configuration similar to this:

require 'simplecov'

class GitBranchFiles
  class << self
    def include?(filename)
      @changed_files ||= git_branch_changed_files
      @changed_files.include?(filename)
    end

    private

    def git_branch_changed_files
      `git diff --name-only master`.split(/\n/).map do |file|
        file_path = Pathname.new(Rails.root + file)
        file_path.to_s if file_path.extname =~ /.rb/
      end.compact
    end
  end
end


SimpleCov.start('rails') do
  add_group 'Current Branch' do |source_file
    GitBranchFiles.include?(source_file.filename)
  end
end

After running your tests, you’ll have a new tab with the changed files of your current branch, where you can check the coverage and improve it if necessary.