Merge pull request #1516 from ClickHouse/issue_1515

fix: preserve nil elements in map

Author: SpencerTorres

lib/column/map.go

@@ -328,9 +328,19 @@ func (col *Map) row(n int) reflect.Value {
 		from = int(prev)
 	)
 	for next := 0; next < size; next++ {
+		mapValue := col.values.Row(from+next, false)
+		var mapReflectValue reflect.Value
+		if mapValue == nil {
+			// Convert interface{} nil to typed nil (such as nil *string) to preserve map element
+			// https://github.com/ClickHouse/clickhouse-go/issues/1515
+			mapReflectValue = reflect.New(value.Type().Elem()).Elem()
+		} else {
+			mapReflectValue = reflect.ValueOf(mapValue)
+		}
+
 		value.SetMapIndex(
 			reflect.ValueOf(col.keys.Row(from+next, false)),
-			reflect.ValueOf(col.values.Row(from+next, false)),
+			mapReflectValue,
 		)
 	}
 	return value

tests/issues/1515_test.go

@@ -0,0 +1,38 @@
+package issues
+
+import (
+	"context"
+	"github.com/ClickHouse/clickhouse-go/v2/tests"
+	"github.com/stretchr/testify/require"
+	"testing"
+)
+
+func TestIssue1515(t *testing.T) {
+	ctx := context.Background()
+
+	conn, err := tests.GetConnection("issues", nil, nil, nil)
+	require.NoError(t, err)
+	defer conn.Close()
+
+	row := conn.QueryRow(ctx, "SELECT CAST((['key', 'key2'], ['value', null]), 'Map(String, Nullable(String))')")
+	require.NoError(t, row.Err())
+
+	var resultStr map[string]*string
+	err = row.Scan(&resultStr)
+	require.NoError(t, err)
+	require.Len(t, resultStr, 2)
+	require.NotNil(t, resultStr["key"])
+	require.Equal(t, "value", *resultStr["key"])
+	require.Nil(t, resultStr["key2"])
+
+	row = conn.QueryRow(ctx, "SELECT CAST((['key', 'key2'], [42, null]), 'Map(String, Nullable(Int64))')")
+	require.NoError(t, row.Err())
+
+	var resultInt map[string]*int64
+	err = row.Scan(&resultInt)
+	require.NoError(t, err)
+	require.Len(t, resultInt, 2)
+	require.NotNil(t, resultInt["key"])
+	require.Equal(t, int64(42), *resultInt["key"])
+	require.Nil(t, resultInt["key2"])
+}