readme.md

Dropping this into lib/tasks/test_wip.rake will add rails test:diff command. This runs certain tests that relate to your changes. In particular, the following tests are picked:

• modified^* test files
• test files for modified model/controller/job/mailer/helper files (e.g. if app/models/user.rb has changed, test/models/user_test.rb is picked)
• test files for controllers/mailers whose views have been modified

^*modified file is a file that is either git modifed or appears in git diff master

The quality of the result relies on how well you stick to the rails conventions. E.g., changes in app/models/user.rb won't trigger app/models/account_test.rb, if that's where your user models tests happens to reside.

This works for minitest, but could be easily adapted to rspec.

test_wip.rake

# Runs tests that changed on current branch
class DiffTests
  def self.run(tests)
    return if tests.blank?

    require 'childprocess'

    puts "rails test #{tests.join(' ')}"

    p = ChildProcess.build('rails', 'test', *tests)
    p.io.inherit!
    p.start
    p.wait

    # rubocop:disable Style/NumericPredicate
    exit p.exit_code if p.exit_code > 0
    # rubocop:enable Style/NumericPredicate
  end

  def self.non_committed_changed
    `git status --porcelain | grep .rb`
      .split("\n")
      # skip deleted files
      .grep_v(/^ *D /)
      .map do |line|
        # e.g.
        #   'M app/models/user.rb' =>  'app/models/user.rb'
        #   'R app/models/user.rb -> app/models/user2.rb' => 'app/models/user2.rb'
        line.sub(/.*?([^ ]+)$/, '\1')
      end
  end

  def self.committed_changed
    `git diff --name-only master... | grep .rb`.split("\n")
  end

  def self.changed_files
    @changed_files ||= committed_changed.concat(non_committed_changed).uniq
  end

  def self.tests_to_run
    changed_test_files = changed_files.select { |file| file =~ %r{test/.+_test\.rb} && File.exist?(file) }

    (changed_test_files + test_files_for_changed_ruby_files + test_files_for_changed_erb_files).uniq
  end

  def self.changed_code_files
    changed_files.grep(/^app/)
  end

  def self.test_files_for_changed_ruby_files
    select_existing_files do
      changed_code_files.filter_map do |path|
        next unless path.match?(/\.rb$/)

        path.sub(%r{^app/(.*)\.rb$}, 'test/\1_test.rb')
      end
    end
  end

  def self.test_files_for_changed_erb_files
    select_existing_files do
      changed_code_files.filter_map do |path|
        next unless path.match?(/html.erb$/)

        if path.include?('_mailer/')
          path.sub(%r{app/views/(.+_mailer)/.*$}, 'test/mailers/\1_test.rb')
        else # controller
          path.sub(%r{app/views/(.+)/[^/]+$}, 'test/controllers/\1_controller_test.rb')
        end
      end
    end
  end

  def self.select_existing_files
    yield.select { |path| File.exist?(path) }
  end
end

namespace :test do
  task :diff do
    regular_tests_to_run, system_tests_to_run = DiffTests.tests_to_run.partition do |path|
      path !~ Regexp.new('test/system/')
    end

    # Run system tests last
    DiffTests.run(regular_tests_to_run)
    DiffTests.run(system_tests_to_run)
  end
end