Converting map to struct - go

Question

I am trying to create a generic method in Go that will fill a struct using data from a map[string]interface{}. For example, the method signature and usage might look like:
func FillStruct(data map[string]interface{}, result interface{}) {
...
}
type MyStruct struct {
Name string
Age int64
}
myData := make(map[string]interface{})
myData["Name"] = "Tony"
myData["Age"] = 23
result := &MyStruct{}
FillStruct(myData, result)
// result now has Name set to "Tony" and Age set to 23
I know this can be done using JSON as an intermediary; is there another more efficient way of doing this?

Answer 1

The simplest way would be to use https://github.com/mitchellh/mapstructure
import "github.com/mitchellh/mapstructure"
mapstructure.Decode(myData, &result)
If you want to do it yourself, you could do something like this:
http://play.golang.org/p/tN8mxT_V9h
func SetField(obj interface{}, name string, value interface{}) error {
structValue := reflect.ValueOf(obj).Elem()
structFieldValue := structValue.FieldByName(name)
if !structFieldValue.IsValid() {
return fmt.Errorf("No such field: %s in obj", name)
}
if !structFieldValue.CanSet() {
return fmt.Errorf("Cannot set %s field value", name)
}
structFieldType := structFieldValue.Type()
val := reflect.ValueOf(value)
if structFieldType != val.Type() {
return errors.New("Provided value type didn't match obj field type")
}
structFieldValue.Set(val)
return nil
}
type MyStruct struct {
Name string
Age int64
}
func (s *MyStruct) FillStruct(m map[string]interface{}) error {
for k, v := range m {
err := SetField(s, k, v)
if err != nil {
return err
}
}
return nil
}
func main() {
myData := make(map[string]interface{})
myData["Name"] = "Tony"
myData["Age"] = int64(23)
result := &MyStruct{}
err := result.FillStruct(myData)
if err != nil {
fmt.Println(err)
}
fmt.Println(result)
}

Answer 2

Hashicorp's https://github.com/mitchellh/mapstructure library does this out of the box:
import "github.com/mitchellh/mapstructure"
mapstructure.Decode(myData, &result)
The second result parameter has to be an address of the struct.

Answer 3

the simplest way to do that is using encoding/json package
just for example:
package main
import (
"fmt"
"encoding/json"
)
type MyAddress struct {
House string
School string
}
type Student struct {
Id int64
Name string
Scores float32
Address MyAddress
Labels []string
}
func Test() {
dict := make(map[string]interface{})
dict["id"] = 201902181425 // int
dict["name"] = "jackytse" // string
dict["scores"] = 123.456 // float
dict["address"] = map[string]string{"house":"my house", "school":"my school"} // map
dict["labels"] = []string{"aries", "warmhearted", "frank"} // slice
jsonbody, err := json.Marshal(dict)
if err != nil {
// do error check
fmt.Println(err)
return
}
student := Student{}
if err := json.Unmarshal(jsonbody, &student); err != nil {
// do error check
fmt.Println(err)
return
}
fmt.Printf("%#v\n", student)
}
func main() {
Test()
}

Answer 4

You can do it ... it may get a bit ugly and you'll be faced with some trial and error in terms of mapping types .. but heres the basic gist of it:
func FillStruct(data map[string]interface{}, result interface{}) {
t := reflect.ValueOf(result).Elem()
for k, v := range data {
val := t.FieldByName(k)
val.Set(reflect.ValueOf(v))
}
}
Working sample: http://play.golang.org/p/PYHz63sbvL

Answer 5

There are two steps:
Convert interface to JSON Byte
Convert JSON Byte to struct
Below is an example:
dbByte, _ := json.Marshal(dbContent)
_ = json.Unmarshal(dbByte, &MyStruct)

Answer 6

You can roundtrip it through JSON:
package main
import (
"bytes"
"encoding/json"
)
func transcode(in, out interface{}) {
buf := new(bytes.Buffer)
json.NewEncoder(buf).Encode(in)
json.NewDecoder(buf).Decode(out)
}
Example:
package main
import "fmt"
type myStruct struct {
Name string
Age int64
}
func main() {
myData := map[string]interface{}{
"Name": "Tony",
"Age": 23,
}
var result myStruct
transcode(myData, &result)
fmt.Printf("%+v\n", result) // {Name:Tony Age:23}
}

Answer 7

I adapt dave's answer, and add a recursive feature. I'm still working on a more user friendly version. For example, a number string in the map should be able to be converted to int in the struct.
package main
import (
"fmt"
"reflect"
)
func SetField(obj interface{}, name string, value interface{}) error {
structValue := reflect.ValueOf(obj).Elem()
fieldVal := structValue.FieldByName(name)
if !fieldVal.IsValid() {
return fmt.Errorf("No such field: %s in obj", name)
}
if !fieldVal.CanSet() {
return fmt.Errorf("Cannot set %s field value", name)
}
val := reflect.ValueOf(value)
if fieldVal.Type() != val.Type() {
if m,ok := value.(map[string]interface{}); ok {
// if field value is struct
if fieldVal.Kind() == reflect.Struct {
return FillStruct(m, fieldVal.Addr().Interface())
}
// if field value is a pointer to struct
if fieldVal.Kind()==reflect.Ptr && fieldVal.Type().Elem().Kind() == reflect.Struct {
if fieldVal.IsNil() {
fieldVal.Set(reflect.New(fieldVal.Type().Elem()))
}
// fmt.Printf("recursive: %v %v\n", m,fieldVal.Interface())
return FillStruct(m, fieldVal.Interface())
}
}
return fmt.Errorf("Provided value type didn't match obj field type")
}
fieldVal.Set(val)
return nil
}
func FillStruct(m map[string]interface{}, s interface{}) error {
for k, v := range m {
err := SetField(s, k, v)
if err != nil {
return err
}
}
return nil
}
type OtherStruct struct {
Name string
Age int64
}
type MyStruct struct {
Name string
Age int64
OtherStruct *OtherStruct
}
func main() {
myData := make(map[string]interface{})
myData["Name"] = "Tony"
myData["Age"] = int64(23)
OtherStruct := make(map[string]interface{})
myData["OtherStruct"] = OtherStruct
OtherStruct["Name"] = "roxma"
OtherStruct["Age"] = int64(23)
result := &MyStruct{}
err := FillStruct(myData,result)
fmt.Println(err)
fmt.Printf("%v %v\n",result,result.OtherStruct)
}

Answer 8

Here function to convert map to struct by tag. If tag not exist it will find by fieldByName.
Thanks to https://gist.github.com/lelandbatey/a5c957b537bed39d1d6fb202c3b8de06
type MyStruct struct {
Name string `json:"name"`
ID int `json:"id"`
}
myStruct := &MyStruct{}
for k, v := range mapToConvert {
err := MapToStruct(myStruct, k, v)
if err != nil {
fmt.Println(err)
}
}
func MapToStruct(s interface{}, k string, v interface{}) error {
var jname string
structValue := reflect.ValueOf(s).Elem()
fieldByTagName := func(t reflect.StructTag) (string, error) {
if jt, ok := t.Lookup("keyname"); ok {
return strings.Split(jt, ",")[0], nil
}
return "", fmt.Errorf("tag provided %s does not define a json tag", k)
}
fieldNames := map[string]int{}
for i := 0; i < structValue.NumField(); i++ {
typeField := structValue.Type().Field(i)
tag := typeField.Tag
if string(tag) == "" {
jname = toMapCase(typeField.Name)
} else {
jname, _ = fieldByTagName(tag)
}
fieldNames[jname] = i
}
fieldNum, ok := fieldNames[k]
if !ok {
return fmt.Errorf("field %s does not exist within the provided item", k)
}
fieldVal := structValue.Field(fieldNum)
fieldVal.Set(reflect.ValueOf(v))
return nil
}
func toMapCase(s string) (str string) {
runes := []rune(s)
for j := 0; j < len(runes); j++ {
if unicode.IsUpper(runes[j]) == true {
if j == 0 {
str += strings.ToLower(string(runes[j]))
} else {
str += "_" + strings.ToLower(string(runes[j]))
}
} else {
str += strings.ToLower(string(runes[j]))
}
}
return str
}

Answer 9

Simple way just marshal it json string
and then unmarshat it to struct
here is the link