const INI = `
[Section]
FieldInSubSection = 1
FieldInSection = 2
`

type File struct {
	Section `ini:"Section"`
}

type Section struct {
	SubSection
	FieldInSection string `ini:"FieldInSection"`
}
type SubSection struct {
	FieldInSubSection string `ini:"FieldInSubSection"`
}

func main() {
	iniFile, err := ini.Load(strings.NewReader(INI))
	if err != nil {
		log.Fatalf("load error: %v", err)
	}
	f := new(File)
	iniFile.MapTo(f)
	fmt.Printf("%+v", f)  // FieldInSubSection is not mapped
}