Guardfile

guard :minitest do
  # with Minitest::Unit
  watch(%r{^test/(.*)\/?test_(.*)\.rb$})
  watch(%r{^lib/(.*/)?([^/]+)\.rb$})     { |m| "test/#{m[1]}test_#{m[2]}.rb" }
  watch(%r{^test/test_helper\.rb$})      { 'test' }

  # with Minitest::Spec
  # watch(%r{^spec/(.*)_spec\.rb$})
  # watch(%r{^lib/(.+)\.rb$})         { |m| "spec/#{m[1]}_spec.rb" }
  # watch(%r{^spec/spec_helper\.rb$}) { 'spec' }

  # Rails 4
   watch(%r{^app/(.+)\.rb$})                               { |m| "test/#{m[1]}_test.rb" }
   watch(%r{^app/controllers/application_controller\.rb$}) { 'test/controllers' }
   watch(%r{^app/controllers/(.+)_controller\.rb$})        { |m| "test/integration/#{m[1]}_test.rb" }
   watch(%r{^app/views/(.+)_mailer/.+})                    { |m| "test/mailers/#{m[1]}_mailer_test.rb" }
   watch(%r{^lib/(.+)\.rb$})                               { |m| "test/lib/#{m[1]}_test.rb" }
   watch(%r{^test/.+_test\.rb$})
   watch(%r{^test/test_helper\.rb$}) { 'test' }

  # Rails < 4
  # watch(%r{^app/controllers/(.*)\.rb$}) { |m| "test/functional/#{m[1]}_test.rb" }
  # watch(%r{^app/helpers/(.*)\.rb$})     { |m| "test/helpers/#{m[1]}_test.rb" }
  # watch(%r{^app/models/(.*)\.rb$})      { |m| "test/unit/#{m[1]}_test.rb" }
end