Merge pull request #13332 from rails/pixeltrix/mail_view

WIP: Integration of mail_view gem

21 files changed, 765 insertions, 26 deletions

diff --git a/actionmailer/CHANGELOG.md b/actionmailer/CHANGELOG.md
index 857cde399a..fc9aefd416 100644
--- a/actionmailer/CHANGELOG.md
+++ b/actionmailer/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Add mailer previews feature based on 37 Signals mail_view gem
+
+    *Andrew White*
+
 *   Calling `mail()` without arguments serves as getter for the current mail
     message and keeps previously set headers.
 
diff --git a/actionmailer/lib/action_mailer.rb b/actionmailer/lib/action_mailer.rb
index 5b6960c8fc..557eec4728 100644
--- a/actionmailer/lib/action_mailer.rb
+++ b/actionmailer/lib/action_mailer.rb
@@ -41,6 +41,8 @@ module ActionMailer
   autoload :Base
   autoload :DeliveryMethods
   autoload :MailHelper
+  autoload :Preview
+  autoload :Previews, 'action_mailer/preview'
   autoload :TestCase
   autoload :TestHelper
 end
diff --git a/actionmailer/lib/action_mailer/base.rb b/actionmailer/lib/action_mailer/base.rb
index 5723b2cc1b..e1f3fd03e2 100644
--- a/actionmailer/lib/action_mailer/base.rb
+++ b/actionmailer/lib/action_mailer/base.rb
@@ -308,6 +308,25 @@ module ActionMailer
   # Note that unless you have a specific reason to do so, you should prefer using before_action
   # rather than after_action in your ActionMailer classes so that headers are parsed properly.
   #
+  # = Previewing emails
+  #
+  # You can preview your email templates visually by adding a mailer preview file to the
+  # <tt>ActionMailer::Base.preview_path</tt>. Since most emails do something interesting
+  # with database data, you'll need to write some scenarios to load messages with fake data:
+  #
+  #   class NotifierPreview < ActionMailer::Preview
+  #     def welcome
+  #       Notifier.welcome(User.first)
+  #     end
+  #   end
+  #
+  # Methods must return a Mail::Message object which can be generated by calling the mailer
+  # method without the additional <tt>deliver</tt>. The location of the mailer previews
+  # directory can be configured using the <tt>preview_path</tt> option which has a default
+  # of <tt>test/mailers/previews</tt>:
+  #
+  #     config.action_mailer.preview_path = "#{Rails.root}/lib/mailer_previews"
+  #
   # = Configuration options
   #
   # These options are specified on the class level, like
@@ -362,6 +381,7 @@ module ActionMailer
   #   <tt>delivery_method :test</tt>. Most useful for unit and functional testing.
   class Base < AbstractController::Base
     include DeliveryMethods
+    include Previews
 
     abstract!
 
diff --git a/actionmailer/lib/action_mailer/preview.rb b/actionmailer/lib/action_mailer/preview.rb
new file mode 100644
index 0000000000..43d9ec4bb5
--- /dev/null
+++ b/actionmailer/lib/action_mailer/preview.rb
@@ -0,0 +1,67 @@
+require 'active_support/descendants_tracker'
+
+module ActionMailer
+  module Previews #:nodoc:
+    extend ActiveSupport::Concern
+
+    included do
+      # Set the location of mailer previews through app configuration:
+      #
+      #     config.action_mailer.preview_path = "#{Rails.root}/lib/mailer_previews"
+      #
+      class_attribute :preview_path, instance_writer: false
+    end
+  end
+
+  class Preview
+    extend ActiveSupport::DescendantsTracker
+
+    class << self
+      # Returns all mailer preview classes
+      def all
+        load_previews if descendants.empty?
+        descendants
+      end
+
+      # Returns the mail object for the given email name
+      def call(email)
+        preview = self.new
+        preview.public_send(email)
+      end
+
+      # Returns all of the available email previews
+      def emails
+        public_instance_methods(false).map(&:to_s).sort
+      end
+
+      # Returns true if the email exists
+      def email_exists?(email)
+        emails.include?(email)
+      end
+
+      # Returns true if the preview exists
+      def exists?(preview)
+        all.any?{ |p| p.preview_name == preview }
+      end
+
+      # Find a mailer preview by its underscored class name
+      def find(preview)
+        all.find{ |p| p.preview_name == preview }
+      end
+
+      # Returns the underscored name of the mailer preview without the suffix
+      def preview_name
+        name.sub(/Preview$/, '').underscore
+      end
+
+      protected
+        def load_previews #:nodoc:
+          Dir["#{preview_path}/**/*_preview.rb"].each{ |file| require_dependency file }
+        end
+
+        def preview_path #:nodoc:
+          Base.preview_path
+        end
+    end
+  end
+end
diff --git a/actionmailer/lib/action_mailer/railtie.rb b/actionmailer/lib/action_mailer/railtie.rb
index 7677ff3a7c..af8009ba97 100644
--- a/actionmailer/lib/action_mailer/railtie.rb
+++ b/actionmailer/lib/action_mailer/railtie.rb
@@ -40,5 +40,13 @@ module ActionMailer
         config.compile_methods! if config.respond_to?(:compile_methods!)
       end
     end
+
+    initializer "action_mailer.configure_mailer_previews", before: :set_autoload_paths do |app|
+      if Rails.env.development?
+        options = app.config.action_mailer
+        options.preview_path ||= defined?(Rails.root) ? "#{Rails.root}/test/mailers/previews" : nil
+        app.config.autoload_paths << options.preview_path
+      end
+    end
   end
 end
diff --git a/actionpack/lib/action_dispatch/routing/inspector.rb b/actionpack/lib/action_dispatch/routing/inspector.rb
index 120bc54333..03ac42285d 100644
--- a/actionpack/lib/action_dispatch/routing/inspector.rb
+++ b/actionpack/lib/action_dispatch/routing/inspector.rb
@@ -69,7 +69,7 @@ module ActionDispatch
       end
 
       def internal?
-        controller.to_s =~ %r{\Arails/(info|welcome)} || path =~ %r{\A#{Rails.application.config.assets.prefix}}
+        controller.to_s =~ %r{\Arails/(info|mailers|welcome)} || path =~ %r{\A#{Rails.application.config.assets.prefix}}
       end
 
       def engine?
diff --git a/railties/lib/rails.rb b/railties/lib/rails.rb
index ea82327365..be7570a5ba 100644
--- a/railties/lib/rails.rb
+++ b/railties/lib/rails.rb
@@ -25,6 +25,7 @@ module Rails
 
   autoload :Info
   autoload :InfoController
+  autoload :MailersController
   autoload :WelcomeController
 
   class << self
diff --git a/railties/lib/rails/application/finisher.rb b/railties/lib/rails/application/finisher.rb
index 7a1bb1e25c..5b8509b2e9 100644
--- a/railties/lib/rails/application/finisher.rb
+++ b/railties/lib/rails/application/finisher.rb
@@ -22,6 +22,8 @@ module Rails
       initializer :add_builtin_route do |app|
         if Rails.env.development?
           app.routes.append do
+            get '/rails/mailers'         => "rails/mailers#index"
+            get '/rails/mailers/*path'   => "rails/mailers#preview"
             get '/rails/info/properties' => "rails/info#properties"
             get '/rails/info/routes'     => "rails/info#routes"
             get '/rails/info'            => "rails/info#index"
diff --git a/railties/lib/rails/application_controller.rb b/railties/lib/rails/application_controller.rb
new file mode 100644
index 0000000000..9a29ec21cf
--- /dev/null
+++ b/railties/lib/rails/application_controller.rb
@@ -0,0 +1,16 @@
+class Rails::ApplicationController < ActionController::Base # :nodoc:
+  self.view_paths = File.expand_path('../templates', __FILE__)
+  layout 'application'
+
+  protected
+
+  def require_local!
+    unless local_request?
+      render text: '<p>For security purposes, this information is only available to local requests.</p>', status: :forbidden
+    end
+  end
+
+  def local_request?
+    Rails.application.config.consider_all_requests_local || request.local?
+  end
+end
diff --git a/railties/lib/rails/generators/test_unit/mailer/mailer_generator.rb b/railties/lib/rails/generators/test_unit/mailer/mailer_generator.rb
index 3334b189bf..85dee1a066 100644
--- a/railties/lib/rails/generators/test_unit/mailer/mailer_generator.rb
+++ b/railties/lib/rails/generators/test_unit/mailer/mailer_generator.rb
@@ -4,11 +4,18 @@ module TestUnit # :nodoc:
   module Generators # :nodoc:
     class MailerGenerator < Base # :nodoc:
       argument :actions, type: :array, default: [], banner: "method method"
-      check_class_collision suffix: "Test"
+
+      def check_class_collision
+        class_collisions "#{class_name}Test", "#{class_name}Preview"
+      end
 
       def create_test_files
         template "functional_test.rb", File.join('test/mailers', class_path, "#{file_name}_test.rb")
       end
+
+      def create_preview_files
+        template "preview.rb", File.join('test/mailers/previews', class_path, "#{file_name}_preview.rb")
+      end
     end
   end
 end
diff --git a/railties/lib/rails/generators/test_unit/mailer/templates/preview.rb b/railties/lib/rails/generators/test_unit/mailer/templates/preview.rb
new file mode 100644
index 0000000000..ac14644145
--- /dev/null
+++ b/railties/lib/rails/generators/test_unit/mailer/templates/preview.rb
@@ -0,0 +1,11 @@
+<% module_namespacing do -%>
+class <%= class_name %>Preview < ActionMailer::Preview
+<% actions.each do |action| -%>
+
+  def <%= action %>
+    <%= class_name %>.<%= action %>
+  end
+<% end -%>
+
+end
+<% end -%>
diff --git a/railties/lib/rails/info_controller.rb b/railties/lib/rails/info_controller.rb
index 85ea11b4e7..908c4ce65e 100644
--- a/railties/lib/rails/info_controller.rb
+++ b/railties/lib/rails/info_controller.rb
@@ -1,9 +1,9 @@
+require 'rails/application_controller'
 require 'action_dispatch/routing/inspector'
 
-class Rails::InfoController < ActionController::Base # :nodoc:
-  self.view_paths = File.expand_path('../templates', __FILE__)
+class Rails::InfoController < Rails::ApplicationController # :nodoc:
   prepend_view_path ActionDispatch::DebugExceptions::RESCUES_TEMPLATE_PATH
-  layout -> { request.xhr? ? nil : 'application' }
+  layout -> { request.xhr? ? false : 'application' }
 
   before_filter :require_local!
 
@@ -20,16 +20,4 @@ class Rails::InfoController < ActionController::Base # :nodoc:
     @routes_inspector = ActionDispatch::Routing::RoutesInspector.new(_routes.routes)
     @page_title = 'Routes'
   end
-
-  protected
-
-  def require_local!
-    unless local_request?
-      render text: '<p>For security purposes, this information is only available to local requests.</p>', status: :forbidden
-    end
-  end
-
-  def local_request?
-    Rails.application.config.consider_all_requests_local || request.local?
-  end
 end
diff --git a/railties/lib/rails/mailers_controller.rb b/railties/lib/rails/mailers_controller.rb
new file mode 100644
index 0000000000..dd318f52e5
--- /dev/null
+++ b/railties/lib/rails/mailers_controller.rb
@@ -0,0 +1,73 @@
+require 'rails/application_controller'
+
+class Rails::MailersController < Rails::ApplicationController # :nodoc:
+  prepend_view_path ActionDispatch::DebugExceptions::RESCUES_TEMPLATE_PATH
+
+  before_filter :require_local!
+  before_filter :find_preview, only: :preview
+
+  def index
+    @previews = ActionMailer::Preview.all
+    @page_title = "Mailer Previews"
+  end
+
+  def preview
+    if params[:path] == @preview.preview_name
+      @page_title = "Mailer Previews for #{@preview.preview_name}"
+      render action: 'mailer'
+    else
+      email = File.basename(params[:path])
+
+      if @preview.email_exists?(email)
+        @email = @preview.call(email)
+
+        if params[:part]
+          part_type = Mime::Type.lookup(params[:part])
+
+          if part = find_part(part_type)
+            response.content_type = part_type
+            render text: part.respond_to?(:decoded) ? part.decoded : part
+          else
+            raise AbstractController::ActionNotFound, "Email part '#{part_type}' not found in #{@preview.name}##{email}"
+          end
+        else
+          @part = find_preferred_part(request.format, Mime::HTML, Mime::TEXT)
+          render action: 'email', layout: false, formats: %w[html]
+        end
+      else
+        raise AbstractController::ActionNotFound, "Email '#{email}' not found in #{@preview.name}"
+      end
+    end
+  end
+
+  protected
+    def find_preview
+      candidates = []
+      params[:path].to_s.scan(%r{/|$}){ candidates << $` }
+      preview = candidates.detect{ |candidate| ActionMailer::Preview.exists?(candidate) }
+
+      if preview
+        @preview = ActionMailer::Preview.find(preview)
+      else
+        raise AbstractController::ActionNotFound, "Mailer preview '#{params[:path]}' not found"
+      end
+    end
+
+    def find_preferred_part(*formats)
+      if @email.multipart?
+        formats.each do |format|
+          return find_part(format) if @email.parts.any?{ |p| p.mime_type == format }
+        end
+      else
+        @email
+      end
+    end
+
+    def find_part(format)
+      if @email.multipart?
+        @email.parts.find{ |p| p.mime_type == format }
+      elsif @email.mime_type == format
+        @email
+      end
+    end
+end
\ No newline at end of file
diff --git a/railties/lib/rails/templates/layouts/application.html.erb b/railties/lib/rails/templates/layouts/application.html.erb
index 50a4755e45..4cd74c3f48 100644
--- a/railties/lib/rails/templates/layouts/application.html.erb
+++ b/railties/lib/rails/templates/layouts/application.html.erb
@@ -29,7 +29,12 @@
   </style>
 </head>
 <body>
-<h2>Your App: <%= link_to 'properties', '/rails/info/properties' %> | <%= link_to 'routes', '/rails/info/routes' %></h2>
+<h2>
+  Your App:
+  <%= link_to 'mailers', '/rails/mailers' %> |
+  <%= link_to 'properties', '/rails/info/properties' %> |
+  <%= link_to 'routes', '/rails/info/routes' %>
+</h2>
 <%= yield %>
 
 </body>
diff --git a/railties/lib/rails/templates/rails/mailers/email.html.erb b/railties/lib/rails/templates/rails/mailers/email.html.erb
new file mode 100644
index 0000000000..977feb922b
--- /dev/null
+++ b/railties/lib/rails/templates/rails/mailers/email.html.erb
@@ -0,0 +1,98 @@
+<!DOCTYPE html>
+<html><head>
+<meta name="viewport" content="width=device-width" />
+<style type="text/css">
+  header {
+    width: 100%;
+    padding: 10px 0 0 0;
+    margin: 0;
+    background: white;
+    font: 12px "Lucida Grande", sans-serif;
+    border-bottom: 1px solid #dedede;
+    overflow: hidden;
+  }
+
+  dl {
+    margin: 0 0 10px 0;
+    padding: 0;
+  }
+
+  dt {
+    width: 80px;
+    padding: 1px;
+    float: left;
+    clear: left;
+    text-align: right;
+    color: #7f7f7f;
+  }
+
+  dd {
+    margin-left: 90px; /* 80px + 10px */
+    padding: 1px;
+  }
+
+  iframe {
+    border: 0;
+    width: 100%;
+    height: 800px;
+  }
+</style>
+</head>
+
+<body>
+<header>
+  <dl>
+    <% if @email.respond_to?(:smtp_envelope_from) && Array(@email.from) != Array(@email.smtp_envelope_from) %>
+      <dt>SMTP-From:</dt>
+      <dd><%= @email.smtp_envelope_from %></dd>
+    <% end %>
+
+    <% if @email.respond_to?(:smtp_envelope_to) && @email.to != @email.smtp_envelope_to %>
+      <dt>SMTP-To:</dt>
+      <dd><%= @email.smtp_envelope_to %></dd>
+    <% end %>
+
+    <dt>From:</dt>
+    <dd><%= @email.header['from'] %></dd>
+
+    <% if @email.reply_to %>
+      <dt>Reply-To:</dt>
+      <dd><%= @email.header['reply-to'] %></dd>
+    <% end %>
+
+    <dt>To:</dt>
+    <dd><%= @email.header['to'] %></dd>
+
+    <% if @email.cc %>
+      <dt>CC:</dt>
+      <dd><%= @email.header['cc'] %></dd>
+    <% end %>
+
+    <dt>Date:</dt>
+    <dd><%= Time.current.rfc2822 %></dd>
+
+    <dt>Subject:</dt>
+    <dd><strong><%= @email.subject %></strong></dd>
+
+    <% unless @email.attachments.nil? || @email.attachments.empty? %>
+      <dt>Attachments:</dt>
+      <dd>
+        <%= @email.attachments.map { |a| a.respond_to?(:original_filename) ? a.original_filename : a.filename }.inspect %>
+      </dd>
+    <% end %>
+
+    <% if @email.multipart? %>
+      <dd>
+        <select onchange="document.getElementsByName('messageBody')[0].src=this.options[this.selectedIndex].value;">
+          <option <%= request.format == Mime::HTML ? 'selected' : '' %> value="?part=text%2Fhtml">View as HTML email</option>
+          <option <%= request.format == Mime::TEXT ? 'selected' : '' %> value="?part=text%2Fplain">View as plain-text email</option>
+        </select>
+      </dd>
+    <% end %>
+  </dl>
+</header>
+
+<iframe seamless name="messageBody" src="?part=<%= Rack::Utils.escape(@part.mime_type) %>"></iframe>
+
+</body>
+</html>
\ No newline at end of file
diff --git a/railties/lib/rails/templates/rails/mailers/index.html.erb b/railties/lib/rails/templates/rails/mailers/index.html.erb
new file mode 100644
index 0000000000..c4c9757d57
--- /dev/null
+++ b/railties/lib/rails/templates/rails/mailers/index.html.erb
@@ -0,0 +1,8 @@
+<% @previews.each do |preview| %>
+<h3><%= link_to preview.preview_name.titleize, "/rails/mailers/#{preview.preview_name}" %></h3>
+<ul>
+<% preview.emails.each do |email| %>
+<li><%= link_to email, "/rails/mailers/#{preview.preview_name}/#{email}" %></li>
+<% end %>
+</ul>
+<% end %>
diff --git a/railties/lib/rails/templates/rails/mailers/mailer.html.erb b/railties/lib/rails/templates/rails/mailers/mailer.html.erb
new file mode 100644
index 0000000000..607c8d1677
--- /dev/null
+++ b/railties/lib/rails/templates/rails/mailers/mailer.html.erb
@@ -0,0 +1,6 @@
+<h3><%= @preview.preview_name.titleize %></h3>
+<ul>
+<% @preview.emails.each do |email| %>
+<li><%= link_to email, "/rails/mailers/#{@preview.preview_name}/#{email}" %></li>
+<% end %>
+</ul>
diff --git a/railties/lib/rails/welcome_controller.rb b/railties/lib/rails/welcome_controller.rb
index 45b764fa6b..de9cd18b01 100644
--- a/railties/lib/rails/welcome_controller.rb
+++ b/railties/lib/rails/welcome_controller.rb
@@ -1,6 +1,7 @@
-class Rails::WelcomeController < ActionController::Base # :nodoc:
-  self.view_paths = File.expand_path('../templates', __FILE__)
-  layout nil
+require 'rails/application_controller'
+
+class Rails::WelcomeController < Rails::ApplicationController # :nodoc:
+  layout false
 
   def index
   end
diff --git a/railties/test/application/mailer_previews_test.rb b/railties/test/application/mailer_previews_test.rb
new file mode 100644
index 0000000000..14abb33cc6
--- /dev/null
+++ b/railties/test/application/mailer_previews_test.rb
@@ -0,0 +1,388 @@
+require 'isolation/abstract_unit'
+require 'rack/test'
+module ApplicationTests
+  class MailerPreviewsTest < ActiveSupport::TestCase
+    include ActiveSupport::Testing::Isolation
+    include Rack::Test::Methods
+
+    def setup
+      build_app
+      boot_rails
+    end
+
+    def teardown
+      teardown_app
+    end
+
+    test "/rails/mailers is accessible in development" do
+      app("development")
+      get "/rails/mailers"
+      assert_equal 200, last_response.status
+    end
+
+    test "/rails/mailers is not accessible in production" do
+      app("production")
+      get "/rails/mailers"
+      assert_equal 404, last_response.status
+    end
+
+    test "mailer previews are loaded from the default preview_path" do
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            mail to: "to@example.org"
+          end
+        end
+      RUBY
+
+      text_template 'notifier/foo', <<-RUBY
+        Hello, World!
+      RUBY
+
+      mailer_preview 'notifier', <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+        end
+      RUBY
+
+      app('development')
+
+      get "/rails/mailers"
+      assert_match '<h3><a href="/rails/mailers/notifier">Notifier</a></h3>', last_response.body
+      assert_match '<li><a href="/rails/mailers/notifier/foo">foo</a></li>', last_response.body
+    end
+
+    test "mailer previews are loaded from a custom preview_path" do
+      add_to_config "config.action_mailer.preview_path = '#{app_path}/lib/mailer_previews'"
+
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            mail to: "to@example.org"
+          end
+        end
+      RUBY
+
+      text_template 'notifier/foo', <<-RUBY
+        Hello, World!
+      RUBY
+
+      app_file 'lib/mailer_previews/notifier_preview.rb', <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+        end
+      RUBY
+
+      app('development')
+
+      get "/rails/mailers"
+      assert_match '<h3><a href="/rails/mailers/notifier">Notifier</a></h3>', last_response.body
+      assert_match '<li><a href="/rails/mailers/notifier/foo">foo</a></li>', last_response.body
+    end
+
+    test "mailer previews are reloaded across requests" do
+      app('development')
+
+      get "/rails/mailers"
+      assert_no_match '<h3><a href="/rails/mailers/notifier">Notifier</a></h3>', last_response.body
+
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            mail to: "to@example.org"
+          end
+        end
+      RUBY
+
+      text_template 'notifier/foo', <<-RUBY
+        Hello, World!
+      RUBY
+
+      mailer_preview 'notifier', <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+        end
+      RUBY
+
+      get "/rails/mailers"
+      assert_match '<h3><a href="/rails/mailers/notifier">Notifier</a></h3>', last_response.body
+
+      remove_file 'test/mailers/previews/notifier_preview.rb'
+      sleep(1)
+
+      get "/rails/mailers"
+      assert_no_match '<h3><a href="/rails/mailers/notifier">Notifier</a></h3>', last_response.body
+    end
+
+    test "mailer preview actions are added and removed" do
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            mail to: "to@example.org"
+          end
+        end
+      RUBY
+
+      text_template 'notifier/foo', <<-RUBY
+        Hello, World!
+      RUBY
+
+      mailer_preview 'notifier', <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+        end
+      RUBY
+
+      app('development')
+
+      get "/rails/mailers"
+      assert_match '<h3><a href="/rails/mailers/notifier">Notifier</a></h3>', last_response.body
+      assert_match '<li><a href="/rails/mailers/notifier/foo">foo</a></li>', last_response.body
+      assert_no_match '<li><a href="/rails/mailers/notifier/bar">bar</a></li>', last_response.body
+
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            mail to: "to@example.org"
+          end
+
+          def bar
+            mail to: "to@example.net"
+          end
+        end
+      RUBY
+
+      text_template 'notifier/foo', <<-RUBY
+        Hello, World!
+      RUBY
+
+      text_template 'notifier/bar', <<-RUBY
+        Goodbye, World!
+      RUBY
+
+      mailer_preview 'notifier', <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+
+          def bar
+            Notifier.bar
+          end
+        end
+      RUBY
+
+      sleep(1)
+
+      get "/rails/mailers"
+      assert_match '<h3><a href="/rails/mailers/notifier">Notifier</a></h3>', last_response.body
+      assert_match '<li><a href="/rails/mailers/notifier/foo">foo</a></li>', last_response.body
+      assert_match '<li><a href="/rails/mailers/notifier/bar">bar</a></li>', last_response.body
+
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            mail to: "to@example.org"
+          end
+        end
+      RUBY
+
+      remove_file 'app/views/notifier/bar.text.erb'
+
+      mailer_preview 'notifier', <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+        end
+      RUBY
+
+      sleep(1)
+
+      get "/rails/mailers"
+      assert_match '<h3><a href="/rails/mailers/notifier">Notifier</a></h3>', last_response.body
+      assert_match '<li><a href="/rails/mailers/notifier/foo">foo</a></li>', last_response.body
+      assert_no_match '<li><a href="/rails/mailers/notifier/bar">bar</a></li>', last_response.body
+    end
+
+    test "mailer preview not found" do
+      app('development')
+      get "/rails/mailers/notifier"
+      assert last_response.not_found?
+      assert_match "Mailer preview &#39;notifier&#39; not found", last_response.body
+    end
+
+    test "mailer preview email not found" do
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            mail to: "to@example.org"
+          end
+        end
+      RUBY
+
+      text_template 'notifier/foo', <<-RUBY
+        Hello, World!
+      RUBY
+
+      mailer_preview 'notifier', <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+        end
+      RUBY
+
+      app('development')
+
+      get "/rails/mailers/notifier/bar"
+      assert last_response.not_found?
+      assert_match "Email &#39;bar&#39; not found in NotifierPreview", last_response.body
+    end
+
+    test "mailer preview email part not found" do
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            mail to: "to@example.org"
+          end
+        end
+      RUBY
+
+      text_template 'notifier/foo', <<-RUBY
+        Hello, World!
+      RUBY
+
+      mailer_preview 'notifier', <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+        end
+      RUBY
+
+      app('development')
+
+      get "/rails/mailers/notifier/foo?part=text%2Fhtml"
+      assert last_response.not_found?
+      assert_match "Email part &#39;text/html&#39; not found in NotifierPreview#foo", last_response.body
+    end
+
+    test "message header uses full display names" do
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "Ruby on Rails <core@rubyonrails.org>"
+
+          def foo
+            mail to: "Andrew White <andyw@pixeltrix.co.uk>",
+                 cc: "David Heinemeier Hansson <david@heinemeierhansson.com>"
+          end
+        end
+      RUBY
+
+      text_template 'notifier/foo', <<-RUBY
+        Hello, World!
+      RUBY
+
+      mailer_preview 'notifier', <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+        end
+      RUBY
+
+      app('development')
+
+      get "/rails/mailers/notifier/foo"
+      assert_equal 200, last_response.status
+      assert_match "Ruby on Rails &lt;core@rubyonrails.org&gt;", last_response.body
+      assert_match "Andrew White &lt;andyw@pixeltrix.co.uk&gt;", last_response.body
+      assert_match "David Heinemeier Hansson &lt;david@heinemeierhansson.com&gt;", last_response.body
+    end
+
+    test "part menu selects correct option" do
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            mail to: "to@example.org"
+          end
+        end
+      RUBY
+
+      html_template 'notifier/foo', <<-RUBY
+        <p>Hello, World!</p>
+      RUBY
+
+      text_template 'notifier/foo', <<-RUBY
+        Hello, World!
+      RUBY
+
+      mailer_preview 'notifier', <<-RUBY
+        class NotifierPreview < ActionMailer::Preview
+          def foo
+            Notifier.foo
+          end
+        end
+      RUBY
+
+      app('development')
+
+      get "/rails/mailers/notifier/foo.html"
+      assert_equal 200, last_response.status
+      assert_match '<option selected value="?part=text%2Fhtml">View as HTML email</option>', last_response.body
+
+      get "/rails/mailers/notifier/foo.txt"
+      assert_equal 200, last_response.status
+      assert_match '<option selected value="?part=text%2Fplain">View as plain-text email</option>', last_response.body
+    end
+
+    private
+      def build_app
+        super
+        app_file 'config/routes.rb', "Rails.application.routes.draw do; end"
+      end
+
+      def mailer(name, contents)
+        app_file("app/mailers/#{name}.rb", contents)
+      end
+
+      def mailer_preview(name, contents)
+        app_file("test/mailers/previews/#{name}_preview.rb", contents)
+      end
+
+      def html_template(name, contents)
+        app_file("app/views/#{name}.html.erb", contents)
+      end
+
+      def text_template(name, contents)
+        app_file("app/views/#{name}.text.erb", contents)
+      end
+  end
+end
diff --git a/railties/test/generators/generators_test_helper.rb b/railties/test/generators/generators_test_helper.rb
index 32a3d072c9..77ec2f1c0c 100644
--- a/railties/test/generators/generators_test_helper.rb
+++ b/railties/test/generators/generators_test_helper.rb
@@ -1,10 +1,14 @@
 require 'abstract_unit'
+require 'active_support/core_ext/module/remove_method'
 require 'rails/generators'
 require 'rails/generators/test_case'
 
 module Rails
-  def self.root
-    @root ||= File.expand_path(File.join(File.dirname(__FILE__), '..', 'fixtures'))
+  class << self
+    remove_possible_method :root
+    def root
+      @root ||= File.expand_path(File.join(File.dirname(__FILE__), '..', 'fixtures'))
+    end
   end
 end
 Rails.application.config.root = Rails.root
diff --git a/railties/test/generators/mailer_generator_test.rb b/railties/test/generators/mailer_generator_test.rb
index 6b2351fc1a..120ff3a2df 100644
--- a/railties/test/generators/mailer_generator_test.rb
+++ b/railties/test/generators/mailer_generator_test.rb
@@ -1,7 +1,6 @@
 require 'generators/generators_test_helper'
 require 'rails/generators/mailer/mailer_generator'
 
-
 class MailerGeneratorTest < Rails::Generators::TestCase
   include GeneratorsTestHelper
   arguments %w(notifier foo bar)
@@ -23,8 +22,11 @@ class MailerGeneratorTest < Rails::Generators::TestCase
   end
 
   def test_check_class_collision
-    content = capture(:stderr){ run_generator ["object"] }
-    assert_match(/The name 'Object' is either already used in your application or reserved/, content)
+    Object.send :const_set, :Notifier, Class.new
+    content = capture(:stderr){ run_generator }
+    assert_match(/The name 'Notifier' is either already used in your application or reserved/, content)
+  ensure
+    Object.send :remove_const, :Notifier
   end
 
   def test_invokes_default_test_framework
@@ -34,6 +36,31 @@ class MailerGeneratorTest < Rails::Generators::TestCase
       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_match(/Notifier.foo/, foo)
+      end
+      assert_instance_method :bar, mailer do |bar|
+        assert_match(/Notifier.bar/, bar)
+      end
+    end
+  end
+
+  def test_check_test_class_collision
+    Object.send :const_set, :NotifierTest, Class.new
+    content = capture(:stderr){ run_generator }
+    assert_match(/The name 'NotifierTest' is either already used in your application or reserved/, content)
+  ensure
+    Object.send :remove_const, :NotifierTest
+  end
+
+  def test_check_preview_class_collision
+    Object.send :const_set, :NotifierPreview, Class.new
+    content = capture(:stderr){ run_generator }
+    assert_match(/The name 'NotifierPreview' is either already used in your application or reserved/, content)
+  ensure
+    Object.send :remove_const, :NotifierPreview
   end
 
   def test_invokes_default_template_engine
@@ -65,6 +92,9 @@ class MailerGeneratorTest < Rails::Generators::TestCase
       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)
+    end
     assert_file "app/views/farm/animal/moos.text.erb"
   end