I have a struct:
type Human struct {
Head string `json:"a1"`
Body string `json:"a2"`
Leg string `json:"a3"`
}
How can I get the struct's field name by providing JSON tag name? Probably something like this:
fmt.Println(getFieldName("a1")) // "Head"
fmt.Println(getFieldName("a2")) // "Body"
fmt.Println(getFieldName("a99")) // ""
func getFieldName(tag string) (fieldname string) {
/* ... */
}
How should I implement the getFieldName function? I read online, it seems I need to use the reflect package, hmmm... any helping hand? :)
You can use the reflect package to loop over a struct's fields and match against their tag values.
func getFieldName(tag, key string, s interface{}) (fieldname string) {
rt := reflect.TypeOf(s)
if rt.Kind() != reflect.Struct {
panic("bad type")
}
for i := 0; i < rt.NumField(); i++ {
f := rt.Field(i)
v := strings.Split(f.Tag.Get(key), ",")[0] // use split to ignore tag "options" like omitempty, etc.
if v == tag {
return f.Name
}
}
return ""
}
https://play.golang.com/p/2zCC7pZKJTz
Alternatively, as pointed out by @icza, you can build up a map and then use that for quicker lookups.
// Human json a1 Head
var fieldsByTag = make(map[reflect.Type]map[string]map[string]string)
func buildFieldsByTagMap(key string, s interface{}) {
rt := reflect.TypeOf(s)
if rt.Kind() != reflect.Struct {
panic("bad type")
}
if fieldsByTag[rt] == nil {
fieldsByTag[rt] = make(map[string]map[string]string)
}
if fieldsByTag[rt][key] == nil {
fieldsByTag[rt][key] = make(map[string]string)
}
for i := 0; i < rt.NumField(); i++ {
f := rt.Field(i)
v := strings.Split(f.Tag.Get(key), ",")[0] // use split to ignore tag "options"
if v == "" || v == "-" {
continue
}
fieldsByTag[rt][key][v] = f.Name
}
}