Golang reflection; assign to struct field by tag name

assign_struct_field_by_tag_name.go

package main

import (
	"fmt"
	"reflect"
	"strings"
)

// The goal: allow assignment to a go struct field based on the name that field
// is tagged with (specifically it's json-tagged name).

type Wham struct {
	Username string `json:"username,omitempty"`
	Password string `json:"password"`
	ID       int64  `json:"_id"`
	Homebase string `json:"homebase"`
}

func main() {
	w := Wham{
		Username: "maria",
		Password: "hunter2",
		ID:       42,
		Homebase: "2434 Main St",
	}
	fmt.Printf("%+v\n", w)
	SetField(&w, "username", "larry")
	fmt.Printf("%+v\n", w)
}

func SetField(item interface{}, fieldName string, value interface{}) error {
	v := reflect.ValueOf(item).Elem()
	if !v.CanAddr() {
		return fmt.Errorf("cannot assign to the item passed, item must be a pointer in order to assign")
	}
	// It's possible we can cache this, which is why precompute all these ahead of time.
	findJsonName := func(t reflect.StructTag) (string, error) {
		if jt, ok := t.Lookup("json"); ok {
			return strings.Split(jt, ",")[0], nil
		}
		return "", fmt.Errorf("tag provided does not define a json tag", fieldName)
	}
	fieldNames := map[string]int{}
	for i := 0; i < v.NumField(); i++ {
		typeField := v.Type().Field(i)
		tag := typeField.Tag
		jname, _ := findJsonName(tag)
		fieldNames[jname] = i
	}

	fieldNum, ok := fieldNames[fieldName]
	if !ok {
		return fmt.Errorf("field %s does not exist within the provided item")
	}
	fieldVal := v.Field(fieldNum)
	fieldVal.Set(reflect.ValueOf(value))
	return nil
}