JSON Marshal based on custom struct tags

struct_tag_manipulation

package main

import (
	"encoding/json"
	"fmt"
	"reflect"
)

type User struct {
	Username string `json:"user" db:"username"`
	DisplayName string `json:"displayName" db:"display_name"`
	Email  string `json:"email"`
}

// - Unmarshalled from UI <-- {"user": "user123", "displayName": "John Brown","email": "johnbrown@usa.com"}
// - Marshalled for DB --> {"username":"user123","display_name":"John Brown","email":"johnbrown@usa.com"}
func main() {
	var user User
	const userAsString = `{"user": "user123", "displayName": "John Brown","email": "johnbrown@usa.com"}`

	err := json.Unmarshal([]byte(userAsString), &user)
	if err != nil {
		fmt.Println(err)
	}

	jsonByte, err := json.Marshal(&user)
	if err != nil {
		fmt.Println(err)
	}
	fmt.Println(string(jsonByte))
}

func (u *User) MarshalJSON() ([]byte, error) {
	// Get old struct fields
	uType := reflect.TypeOf(u).Elem()

	var fields []reflect.StructField

	// Scan and collect all fields converting any that have a "db" tag
	for i := 0; i < uType.NumField(); i++ {
		f := uType.Field(i)
		if dbkey, ok := f.Tag.Lookup("db"); ok && dbkey != "" {
			f.Tag = reflect.StructTag(fmt.Sprintf(`json:"%s"`, dbkey))
		}
		fields = append(fields, f)
	}

	newType := reflect.StructOf(fields)
	newTypeInstance := reflect.New(newType).Elem()
	origInstance := reflect.ValueOf(u).Elem()

	// Copy over the values
	for i := 0; i < origInstance.NumField(); i++ {
		newTypeInstance.Field(i).Set(origInstance.Field(i))
	}

	return json.Marshal(newTypeInstance.Interface())

	// Manual method picking out a few fields
	// userNameField, ok := uType.FieldByName("Username")
	// if !ok { return nil, errors.New("could not find Username field") }
	// // Manipulate username field struct tag
	// dbTagUser, _ := userNameField.Tag.Lookup("db")
	// fmt.Printf("dbTagUser %#v\n", dbTagUser)
	// if dbTagUser != "" {
	// 	userNameField.Tag = reflect.StructTag(fmt.Sprintf(`json:"%s"`, dbTagUser))
	// }
	//
	// fldDisplayName, ok := uType.FieldByName("DisplayName")
	// if !ok { return nil, errors.New("could not find DisplayName field") }
	// // Manipulate username field struct tag
	// dbTagDisplayName, _ := fldDisplayName.Tag.Lookup("db")
	// if dbTagDisplayName != "" {
	// 	fldDisplayName.Tag = reflect.StructTag(fmt.Sprintf(`json:"%s"`, dbTagDisplayName))
	// }
	//
	// fldEmail, _ := uType.FieldByName("Email")
	//
	// // Make a new struct from the provided fields. Fields must match original so copy below can work
	//
	// newType := reflect.StructOf([]reflect.StructField{userNameField, fldDisplayName, fldEmail})
	//
	// // Copy over the values
	// origInstance := reflect.ValueOf(u).Elem()
	// newTypeInstance := reflect.New(newType).Elem()
	// for i := 0; i < origInstance.NumField(); i++ {
	// 	newTypeInstance.Field(i).Set(origInstance.Field(i))
	// }
}