Explicit multipart messages respect :parts_order

As issue #7978, the order in which ActionMailer
sends multipart messages could be unintentionally
overwritten if a block is passed to the mail
method. This changes the mail method such that
:parts_order is always respected, regardless of
whether a block is passed to mail.

actionmailer/CHANGELOG.md

@@ -1,5 +1,8 @@
 ## Rails 4.0.0 (unreleased) ##
 
+* Explicit multipart messages no longer set the order of the MIME parts.
+  *Nate Berkopec*
+  
 * Do not render views when mail() isn't called.
   Fix #7761
 

actionmailer/lib/action_mailer/base.rb

@@ -698,7 +698,7 @@ module ActionMailer
       assignable.each { |k, v| m[k] = v }
 
       # Render the templates and blocks
-      responses, explicit_order = collect_responses_and_parts_order(headers, &block)
+      responses = collect_responses(headers, &block)
       create_parts_from_responses(m, responses)
 
       # Setup content type, reapply charset and handle parts order
@@ -706,8 +706,7 @@ module ActionMailer
       m.charset      = charset
 
       if m.multipart?
-        parts_order ||= explicit_order || headers[:parts_order]
-        m.body.set_sort_order(parts_order)
+        m.body.set_sort_order(headers[:parts_order])
         m.body.sort_parts!
       end
 
@@ -742,14 +741,13 @@ module ActionMailer
       I18n.t(:subject, scope: [mailer_scope, action_name], default: action_name.humanize)
     end
 
-    def collect_responses_and_parts_order(headers) #:nodoc:
-      responses, parts_order = [], nil
+    def collect_responses(headers) #:nodoc:
+      responses = []
 
       if block_given?
         collector = ActionMailer::Collector.new(lookup_context) { render(action_name) }
         yield(collector)
-        parts_order = collector.responses.map { |r| r[:content_type] }
-        responses  = collector.responses
+        responses = collector.responses
       elsif headers[:body]
         responses << {
           body: headers.delete(:body),
@@ -769,7 +767,7 @@ module ActionMailer
         end
       end
 
-      [responses, parts_order]
+      responses
     end
 
     def each_template(paths, name, &block) #:nodoc:

actionmailer/test/base_test.rb

@@ -322,19 +322,6 @@ class BaseTest < ActiveSupport::TestCase
     assert_not_nil(mail.content_type_parameters[:boundary])
   end
 
-  test "explicit multipart does not sort order" do
-    order = ["text/html", "text/plain"]
-    with_default BaseMailer, parts_order: order do
-      email = BaseMailer.explicit_multipart
-      assert_equal("text/plain", email.parts[0].mime_type)
-      assert_equal("text/html",  email.parts[1].mime_type)
-
-      email = BaseMailer.explicit_multipart(parts_order: order.reverse)
-      assert_equal("text/plain", email.parts[0].mime_type)
-      assert_equal("text/html",  email.parts[1].mime_type)
-    end
-  end
-
   test "explicit multipart with attachments creates nested parts" do
     email = BaseMailer.explicit_multipart(attachments: true)
     assert_equal("application/pdf", email.parts[0].mime_type)
@@ -349,10 +336,10 @@ class BaseTest < ActiveSupport::TestCase
     email = BaseMailer.explicit_multipart_templates
     assert_equal(2, email.parts.size)
     assert_equal("multipart/alternative", email.mime_type)
-    assert_equal("text/html", email.parts[0].mime_type)
-    assert_equal("HTML Explicit Multipart Templates", email.parts[0].body.encoded)
-    assert_equal("text/plain", email.parts[1].mime_type)
-    assert_equal("TEXT Explicit Multipart Templates", email.parts[1].body.encoded)
+    assert_equal("text/plain", email.parts[0].mime_type)
+    assert_equal("TEXT Explicit Multipart Templates", email.parts[0].body.encoded)
+    assert_equal("text/html", email.parts[1].mime_type)
+    assert_equal("HTML Explicit Multipart Templates", email.parts[1].body.encoded)
   end
 
   test "explicit multipart with format.any" do
@@ -387,10 +374,23 @@ class BaseTest < ActiveSupport::TestCase
     email = BaseMailer.explicit_multipart_with_one_template
     assert_equal(2, email.parts.size)
     assert_equal("multipart/alternative", email.mime_type)
-    assert_equal("text/html", email.parts[0].mime_type)
-    assert_equal("[:html]", email.parts[0].body.encoded)
-    assert_equal("text/plain", email.parts[1].mime_type)
-    assert_equal("[:text]", email.parts[1].body.encoded)
+    assert_equal("text/plain", email.parts[0].mime_type)
+    assert_equal("[:text]", email.parts[0].body.encoded)
+    assert_equal("text/html", email.parts[1].mime_type)
+    assert_equal("[:html]", email.parts[1].body.encoded)
+  end
+
+  test "explicit multipart with sort order" do
+    order = ["text/html", "text/plain"]
+    with_default BaseMailer, parts_order: order do
+      email = BaseMailer.explicit_multipart
+      assert_equal("text/html",  email.parts[0].mime_type)
+      assert_equal("text/plain", email.parts[1].mime_type)
+
+      email = BaseMailer.explicit_multipart(parts_order: order.reverse)
+      assert_equal("text/plain", email.parts[0].mime_type)
+      assert_equal("text/html",  email.parts[1].mime_type)
+    end
   end
 
   # Class level API with method missing

guides/source/action_mailer_basics.md

@@ -377,23 +377,7 @@ If you use this setting, you should pass the `only_path: false` option when usin
 
 Action Mailer will automatically send multipart emails if you have different templates for the same action. So, for our UserMailer example, if you have `welcome_email.text.erb` and `welcome_email.html.erb` in `app/views/user_mailer`, Action Mailer will automatically send a multipart email with the HTML and text versions setup as different parts.
 
-The order of the parts getting inserted is determined by the `:parts_order` inside of the `ActionMailer::Base.default` method. If you want to explicitly alter the order, you can either change the `:parts_order` or explicitly render the parts in a different order:
-
-```ruby
-class UserMailer < ActionMailer::Base
-  def welcome_email(user)
-    @user = user
-    @url  = user_url(@user)
-    mail(to: user.email,
-         subject: 'Welcome to My Awesome Site') do |format|
-      format.html
-      format.text
-    end
-  end
-end
-```
-
-Will put the HTML part first, and the plain text part second.
+The order of the parts getting inserted is determined by the `:parts_order` inside of the `ActionMailer::Base.default` method.
 
 ### Sending Emails with Attachments