support custom types in query result scanning

internal/value/cast.go

@@ -10,5 +10,15 @@ func CastTo(v Value, dst interface{}) error {
 		return nil
 	}
 
+	if scanner, has := dst.(ValueScanner); has {
+		scanner.UnmarshalYDBValue(v)
+
+		return nil
+	}
+
 	return v.castTo(dst)
 }
+
+type ValueScanner interface {
+	UnmarshalYDBValue(Value) error
+}

internal/value/cast_test.go

@@ -32,6 +32,14 @@ func loadLocation(t *testing.T, name string) *time.Location {
 	return loc
 }
 
+type testStringValueScanner struct {
+	field string
+}
+
+func (s *testStringValueScanner) UnmarshalYDBValue(v Value) error {
+	return CastTo(v, &s.field)
+}
+
 func TestCastTo(t *testing.T) {
 	testsCases := []struct {
 		name  string
@@ -428,6 +436,13 @@ func TestCastTo(t *testing.T) {
 			exp:   DateValueFromTime(time.Date(2024, 1, 2, 0, 0, 0, 0, time.UTC)),
 			err:   nil,
 		},
+		{
+			name:  xtest.CurrentFileLine(),
+			value: TextValue("text-string"),
+			dst:   ptr[testStringValueScanner](),
+			exp:   testStringValueScanner{field: "text-string"},
+			err:   nil,
+		},
 	}
 	for _, tt := range testsCases {
 		t.Run(tt.name, func(t *testing.T) {

tests/integration/query_range_test.go

@@ -17,8 +17,17 @@ import (
 	"github.com/ydb-platform/ydb-go-sdk/v3/internal/value"
 	"github.com/ydb-platform/ydb-go-sdk/v3/internal/xtest"
 	"github.com/ydb-platform/ydb-go-sdk/v3/query"
+	"github.com/ydb-platform/ydb-go-sdk/v3/table/types"
 )
 
+type testStringValueScanner struct {
+	field string
+}
+
+func (v *testStringValueScanner) UnmarshalYDBValue(value types.Value) error {
+	return types.CastTo(value, &v.field)
+}
+
 func TestQueryRange(t *testing.T) {
 	ctx, cancel := context.WithCancel(xtest.Context(t))
 	defer cancel()
@@ -84,19 +93,22 @@ func TestQueryRange(t *testing.T) {
 			p1 string
 			p2 uint64
 			p3 time.Duration
+			p4 testStringValueScanner
 		)
 		err := db.Query().Do(ctx, func(ctx context.Context, s query.Session) error {
 			r, err := s.Query(ctx, `
 				DECLARE $p1 AS Text;
 				DECLARE $p2 AS Uint64;
 				DECLARE $p3 AS Interval;
-				SELECT $p1, $p2, $p3;
+				DECLARE $p4 AS Text;
+				SELECT $p1, $p2, $p3, $p4;
 			`,
 				query.WithParameters(
 					ydb.ParamsBuilder().
 						Param("$p1").Text("test").
 						Param("$p2").Uint64(100500000000).
 						Param("$p3").Interval(time.Duration(100500000000)).
+						Param("$p4").Text("test2").
 						Build(),
 				),
 				query.WithSyntax(query.SyntaxYQL),
@@ -112,7 +124,7 @@ func TestQueryRange(t *testing.T) {
 					if err != nil {
 						return err
 					}
-					err = row.Scan(&p1, &p2, &p3)
+					err = row.Scan(&p1, &p2, &p3, &p4)
 					if err != nil {
 						return err
 					}
@@ -126,6 +138,9 @@ func TestQueryRange(t *testing.T) {
 					if p3 != time.Duration(100500000000) {
 						return fmt.Errorf("unexpected p3 value: %v", p3)
 					}
+					if p4.field != "test2" {
+						return fmt.Errorf("unexpected p4 value: %v", p4)
+					}
 				}
 			}
 			return nil