Implement has_one's embed objects

lib/active_model/serializer.rb

@@ -75,8 +75,8 @@ module ActiveModel
         if association.embed_ids?
           hash[association.key] = serialize_ids association
         elsif association.embed_objects?
-          # TODO
-          hash
+          associated_data = send(association.name)
+          hash[association.embedded_key] = association.build_serializer(associated_data).serializable_hash
         end
       end
     end

lib/active_model/serializer/associations.rb

@@ -37,6 +37,10 @@ module ActiveModel
         def key
           "#{name}_id"
         end
+
+        def embedded_key
+          name
+        end
       end
     end
   end