generate application_mailer.rb if it is missing

actionmailer/lib/rails/generators/mailer/mailer_generator.rb

@@ -9,6 +9,10 @@ module Rails
 
       def create_mailer_file
         template "mailer.rb", File.join('app/mailers', class_path, "#{file_name}_mailer.rb")
+
+        unless File.exist?('app/mailers/application_mailer.rb')
+          template 'application_mailer.rb', 'app/mailers/application_mailer.rb'
+        end
       end
 
       hook_for :template_engine, :test_framework

actionmailer/lib/rails/generators/mailer/templates/application_mailer.rb

@@ -0,0 +1,4 @@
+class ApplicationMailer < ActionMailer::Base
+  default from: 'from@example.com'
+  layout :mailer
+end

railties/test/generators/mailer_generator_test.rb

@@ -12,6 +12,12 @@ class MailerGeneratorTest < Rails::Generators::TestCase
       assert_no_match(/default from: "from@example.com"/, mailer)
       assert_no_match(/layout :mailer_notifier/, mailer)
     end
+
+    assert_file 'app/mailers/application_mailer.rb' do |mailer|
+      assert_match(/class ApplicationMailer < ActionMailer::Base/, mailer)
+      assert_match(/default from: 'from@example.com'/, mailer)
+      assert_match(/layout :mailer/, mailer)
+    end
   end
 
   def test_mailer_with_i18n_helper