diff --git a/src/dataframe/old/datatable.cpp b/src/dataframe/old/datatable.cpp
index 33ab6d87e..d3baca7cf 100644
--- a/src/dataframe/old/datatable.cpp
+++ b/src/dataframe/old/datatable.cpp
@@ -85,6 +85,15 @@ void SeriesIndex::setAggr(const std::string &aggr)
 	aggregator = Refl::get_enum<dataframe::aggregator_type>(aggr);
 }
 
+std::string SeriesIndex::toJSON() const
+{
+	std::string res;
+	Conv::JSONObj obj{res};
+	obj("name", name);
+	if (aggregator) obj("aggregator", Conv::toString(*aggregator));
+	return res;
+}
+
 DataCube::iterator_t DataCube::begin() const
 {
 	iterator_t res{this,
diff --git a/src/dataframe/old/types.h b/src/dataframe/old/types.h
index 3625277bb..5684a7c2c 100644
--- a/src/dataframe/old/types.h
+++ b/src/dataframe/old/types.h
@@ -69,11 +69,7 @@ class SeriesIndex
 
 	[[nodiscard]] bool isDimension() const { return !aggregator; }
 
-	[[nodiscard]] consteval static auto members()
-	{
-		return std::tuple{&SeriesIndex::name,
-		    &SeriesIndex::aggregator};
-	}
+	[[nodiscard]] std::string toJSON() const;
 };
 
 using SeriesList = Type::UniqueList<SeriesIndex>;