Make embed nil ids work

lib/active_model/serializer.rb

@@ -161,7 +161,7 @@ module ActiveModel
       if associated_data.respond_to?(:to_ary)
         associated_data.map { |elem| elem.send(association.embed_key) }
       else
-        associated_data.send(association.embed_key)
+        associated_data.send(association.embed_key) if associated_data
       end
     end
 

test/unit/active_model/serializer/has_one_test.rb

@@ -54,6 +54,18 @@ module ActiveModel
         }, @user_serializer.as_json)
       end
 
+      def test_associations_embedding_nil_ids_serialization_using_as_json
+        @user.instance_eval do
+          def profile
+            nil
+          end
+        end
+
+        assert_equal({
+          'user' => { name: 'Name 1', email: 'mail@server.com', 'profile_id' => nil }
+        }, @user_serializer.as_json)
+      end
+
       def test_associations_embedding_nil_objects_serialization_using_as_json
         @association.embed = :objects
         @user.instance_eval do