Serialize polymorphic id like a composite FK

lib/active_model/serializer.rb

@@ -112,7 +112,16 @@ module ActiveModel
         end
 
         def serialize_ids(object, scope)
-          object && object.read_attribute_for_serialization(:id)
+          return unless object
+
+          if polymorphic?
+            { 
+              :id => object.read_attribute_for_serialization(:id),
+              "#{name}_type".to_sym => object.class.to_s.split('::').last
+            }
+          else
+            object.read_attribute_for_serialization(:id)
+          end
         end
       end
     end

test/serializer_test.rb

@@ -710,4 +710,28 @@ class SerializerTest < ActiveModel::TestCase
       }
     }, serializer.as_json)
   end
+
+  def test_polymorphic_has_one_with_ids
+    polymorphic_blog = Class.new do
+      attr_accessor :writer
+    end
+
+    polymorphic_serializer = Class.new(ActiveModel::Serializer) do
+      embed :ids
+      has_one :writer, :polymorphic => true
+    end
+
+    user = PolymorphicUser.new :id => 1
+    blog = polymorphic_blog.new
+    blog.writer = user
+
+    serializer = polymorphic_serializer.new(blog, user)
+
+    assert_equal({
+      :writer => {
+        :writer_type => 'PolymorphicUser',
+        :id => 1
+      }
+    }, serializer.as_json)
+  end
 end