Add comments with urls for email previews

railties/lib/rails/generators/test_unit/mailer/templates/preview.rb

 <% module_namespacing do -%>
+# Preview all emails at http://localhost:3000/rails/mailers/<%= file_path %>
 class <%= class_name %>Preview < ActionMailer::Preview
 <% actions.each do |action| -%>
 
+  # Preview this email at http://localhost:3000/rails/mailers/<%= file_path %>/<%= action %>
   def <%= action %>
     <%= class_name %>.<%= action %>
   end

railties/test/generators/mailer_generator_test.rb

@@ -36,12 +36,15 @@ def test_invokes_default_test_framework
       assert_match(/test "foo"/, test)
       assert_match(/test "bar"/, test)
     end
-    assert_file "test/mailers/previews/notifier_preview.rb" do |mailer|
-      assert_match(/class NotifierPreview < ActionMailer::Preview/, mailer)
-      assert_instance_method :foo, mailer do |foo|
+    assert_file "test/mailers/previews/notifier_preview.rb" do |preview|
+      assert_match(/\# Preview all emails at http:\/\/localhost\:3000\/rails\/mailers\/notifier/, preview)
+      assert_match(/class NotifierPreview < ActionMailer::Preview/, preview)
+      assert_match(/\# Preview this email at http:\/\/localhost\:3000\/rails\/mailers\/notifier\/foo/, preview)
+      assert_instance_method :foo, preview do |foo|
         assert_match(/Notifier.foo/, foo)
       end
-      assert_instance_method :bar, mailer do |bar|
+      assert_match(/\# Preview this email at http:\/\/localhost\:3000\/rails\/mailers\/notifier\/bar/, preview)
+      assert_instance_method :bar, preview do |bar|
         assert_match(/Notifier.bar/, bar)
       end
     end
@@ -105,8 +108,10 @@ def test_mailer_with_namedspaced_mailer
       assert_match(/class Farm::Animal < ActionMailer::Base/, mailer)
       assert_match(/en\.farm\.animal\.moos\.subject/, mailer)
     end
-    assert_file "test/mailers/previews/farm/animal_preview.rb" do |mailer|
-      assert_match(/class Farm::AnimalPreview < ActionMailer::Preview/, mailer)
+    assert_file "test/mailers/previews/farm/animal_preview.rb" do |preview|
+      assert_match(/\# Preview all emails at http:\/\/localhost\:3000\/rails\/mailers\/farm\/animal/, preview)
+      assert_match(/class Farm::AnimalPreview < ActionMailer::Preview/, preview)
+      assert_match(/\# Preview this email at http:\/\/localhost\:3000\/rails\/mailers\/farm\/animal\/moos/, preview)
     end
     assert_file "app/views/farm/animal/moos.text.erb"
     assert_file "app/views/farm/animal/moos.html.erb"