Fix mailer previews with attachments

Use the mail gem's own API to locate the correct part.

Fixes #14435.

railties/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Fix mailer previews with attachments by using the mail gem's own API to
+    locate the first part of the correct mime type.
+
+    Fixes #14435.
+
+    *Andrew White*
+
 *   Remove sqlite support from `rails dbconsole`.
 
     *Andrew White*

railties/lib/rails/mailers_controller.rb

@@ -54,18 +54,20 @@ class Rails::MailersController < Rails::ApplicationController # :nodoc:
     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 }
+      formats.each do |format|
+        if part = @email.find_first_mime_type(format)
+          return part
         end
-      else
+      end
+
+      if formats.any?{ |f| @email.mime_type == f }
         @email
       end
     end
 
     def find_part(format)
-      if @email.multipart?
-        @email.parts.find{ |p| p.mime_type == format }
+      if part = @email.find_first_mime_type(format)
+        part
       elsif @email.mime_type == format
         @email
       end

railties/lib/rails/templates/rails/mailers/email.html.erb

@@ -103,7 +103,7 @@
   </dl>
 </header>
 
-<% if @part.mime_type %>
+<% if @part && @part.mime_type %>
   <iframe seamless name="messageBody" src="?part=<%= Rack::Utils.escape(@part.mime_type) %>"></iframe>
 <% else %>
   <p>

railties/test/application/mailer_previews_test.rb

@@ -1,5 +1,7 @@
 require 'isolation/abstract_unit'
 require 'rack/test'
+require 'base64'
+
 module ApplicationTests
   class MailerPreviewsTest < ActiveSupport::TestCase
     include ActiveSupport::Testing::Isolation
@@ -484,6 +486,190 @@ module ApplicationTests
       assert_match '<li><a href="/my_app/rails/mailers/notifier/foo">foo</a></li>', last_response.body
     end
 
+    test "plain text mailer preview with attachment" do
+      image_file "pixel.png", "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEWzIioc\na/JlAAAACklEQVQI12NgAAAAAgAB4iG8MwAAAABJRU5ErkJggg=="
+
+      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"
+      assert_equal 200, last_response.status
+      assert_match %r[<iframe seamless name="messageBody"], last_response.body
+
+      get "/rails/mailers/notifier/foo?part=text/plain"
+      assert_equal 200, last_response.status
+      assert_match %r[Hello, World!], last_response.body
+    end
+
+    test "multipart mailer preview with attachment" do
+      image_file "pixel.png", "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEWzIioc\na/JlAAAACklEQVQI12NgAAAAAgAB4iG8MwAAAABJRU5ErkJggg=="
+
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            attachments['pixel.png'] = File.read("#{app_path}/public/images/pixel.png")
+            mail to: "to@example.org"
+          end
+        end
+      RUBY
+
+      text_template 'notifier/foo', <<-RUBY
+        Hello, World!
+      RUBY
+
+      html_template 'notifier/foo', <<-RUBY
+        <p>Hello, World!</p>
+      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 %r[<iframe seamless name="messageBody"], last_response.body
+
+      get "/rails/mailers/notifier/foo?part=text/plain"
+      assert_equal 200, last_response.status
+      assert_match %r[Hello, World!], last_response.body
+
+      get "/rails/mailers/notifier/foo?part=text/html"
+      assert_equal 200, last_response.status
+      assert_match %r[<p>Hello, World!</p>], last_response.body
+    end
+
+    test "multipart mailer preview with inline attachment" do
+      image_file "pixel.png", "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABAQMAAAAl21bKAAAAA1BMVEWzIioc\na/JlAAAACklEQVQI12NgAAAAAgAB4iG8MwAAAABJRU5ErkJggg=="
+
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            attachments['pixel.png'] = File.read("#{app_path}/public/images/pixel.png")
+            mail to: "to@example.org"
+          end
+        end
+      RUBY
+
+      text_template 'notifier/foo', <<-RUBY
+        Hello, World!
+      RUBY
+
+      html_template 'notifier/foo', <<-RUBY
+        <p>Hello, World!</p>
+        <%= image_tag attachments['pixel.png'].url %>
+      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 %r[<iframe seamless name="messageBody"], last_response.body
+
+      get "/rails/mailers/notifier/foo?part=text/plain"
+      assert_equal 200, last_response.status
+      assert_match %r[Hello, World!], last_response.body
+
+      get "/rails/mailers/notifier/foo?part=text/html"
+      assert_equal 200, last_response.status
+      assert_match %r[<p>Hello, World!</p>], last_response.body
+    end
+
+    test "multipart mailer preview with attached email" do
+      mailer 'notifier', <<-RUBY
+        class Notifier < ActionMailer::Base
+          default from: "from@example.com"
+
+          def foo
+            message = ::Mail.new do
+              from    'foo@example.com'
+              to      'bar@example.com'
+              subject 'Important Message'
+
+              text_part do
+                body 'Goodbye, World!'
+              end
+
+              html_part do
+                body '<p>Goodbye, World!</p>'
+              end
+            end
+
+            attachments['message.eml'] = message.to_s
+            mail to: "to@example.org"
+          end
+        end
+      RUBY
+
+      text_template 'notifier/foo', <<-RUBY
+        Hello, World!
+      RUBY
+
+      html_template 'notifier/foo', <<-RUBY
+        <p>Hello, World!</p>
+      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 %r[<iframe seamless name="messageBody"], last_response.body
+
+      get "/rails/mailers/notifier/foo?part=text/plain"
+      assert_equal 200, last_response.status
+      assert_match %r[Hello, World!], last_response.body
+
+      get "/rails/mailers/notifier/foo?part=text/html"
+      assert_equal 200, last_response.status
+      assert_match %r[<p>Hello, World!</p>], last_response.body
+    end
+
     private
       def build_app
         super
@@ -505,5 +691,9 @@ module ApplicationTests
       def text_template(name, contents)
         app_file("app/views/#{name}.text.erb", contents)
       end
+
+      def image_file(name, contents)
+        app_file("public/images/#{name}", Base64.decode64(contents))
+      end
   end
 end