Specify type of singular association during serialization

When serialising a class, specify the type of any singular associations, if
necessary. Rails already correctly specifies the :type of any enumerable
association (e.g. a has_many association), but made no attempt to do so for
non-enumerables (e.g. a has_one association).
We must specify the :type of any STI association. A has_one
association to a class which uses single-table inheritance is an example of
this type of association.

Fixes #7471

activemodel/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Specify type of singular association during serialization *Steve Klabnik*
+
 *   Fixed length validator to correctly handle nil values. Fixes #7180.
 
     *Michal Zima*

activemodel/lib/active_model/serializers/xml.rb

@@ -149,7 +149,12 @@ def add_associations(association, records, opts)
             end
           else
             merged_options[:root] = association.to_s
-            records.to_xml(merged_options)
+
+            unless records.class.to_s.underscore == association.to_s
+              merged_options[:type] = records.class.name
+            end
+
+            records.to_xml merged_options
           end
         end
 

activemodel/test/cases/serializers/xml_serialization_test.rb

@@ -6,12 +6,12 @@
 class Contact
   include ActiveModel::Serializers::Xml
 
-  attr_accessor :address, :friends
+  attr_accessor :address, :friends, :contact
 
   remove_method :attributes if method_defined?(:attributes)
 
   def attributes
-    instance_values.except("address", "friends")
+    instance_values.except("address", "friends", "contact")
   end
 end
 
@@ -56,6 +56,9 @@ def setup
     @contact.address.zip = 11111
     @contact.address.apt_number = 35
     @contact.friends = [Contact.new, Contact.new]
+    @related_contact = SerializableContact.new
+    @related_contact.name = "related"
+    @contact.contact = @related_contact
   end
 
   test "should serialize default root" do
@@ -256,4 +259,9 @@ def to_ary
     assert_match %r{<address>}, xml
     assert_match %r{<apt-number type="integer">}, xml
   end
+
+  test "association with sti" do
+    xml = @contact.to_xml(include: :contact)
+    assert xml.include?(%(<contact type="SerializableContact">))
+  end
 end