Fix and test for #490

lib/active_model/serializer/association/has_one.rb

@@ -12,7 +12,7 @@ module ActiveModel
         end
 
         def serializer_class(object, options = {})
-          serializer_from_options || serializer_from_object(object, options) || default_serializer
+          (serializer_from_options unless object.nil?) || serializer_from_object(object, options) || default_serializer
         end
 
         def build_serializer(object, options = {})

test/fixtures/poro.rb

@@ -88,7 +88,7 @@ class UserSerializer < ActiveModel::Serializer
 end
 
 class UserInfoSerializer < ActiveModel::Serializer
-  has_one :user
+  has_one :user, serializer: UserSerializer
 end
 
 class ProfileSerializer < ActiveModel::Serializer

test/unit/active_model/serializer/has_one_test.rb

@@ -176,6 +176,20 @@ module ActiveModel
         }, @user_serializer.as_json)
       end
 
+      def test_associations_embedding_objects_with_nil_values
+        user_info = UserInfo.new
+        user_info.instance_eval do
+          def user
+            nil
+          end
+        end
+        user_info_serializer = UserInfoSerializer.new(user_info)
+
+        assert_equal({
+          'user_info' => { user: nil }
+        }, user_info_serializer.as_json)
+      end
+
       def test_associations_embedding_ids_using_embed_namespace
         @association.embed_namespace = :links
         @association.embed = :ids