copyobj.go

package main

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

type M map[string]interface{} // just an alias

var Record = []byte(`{ "bit_size": 8, "secret_key": false }`)

type DB struct {
    NumBits int  `json:"bit_size"`
    Secret  bool `json:"secret_key"`
}

type User struct {
    NumBits int `json:"num_bits"`
}

func main() {
    d := new(DB)
    e := json.Unmarshal(Record, d)
    if e != nil {
        panic(e)
    }
    m := mapFields(d)
    fmt.Println("Mapped fields: ", m)
    u := new(User)
    o := applyMap(u, m)
    fmt.Println("Applied map: ", o)
    j, e := json.Marshal(o)
    if e != nil {
        panic(e)
    }
    fmt.Println("Output JSON: ", string(j))
}

func applyMap(u *User, m M) M {
    t := reflect.TypeOf(u).Elem()
    o := make(M)
    for i := 0; i < t.NumField(); i++ {
        f := t.FieldByIndex([]int{i})
        // skip unexported fields
        if f.PkgPath != "" {
            continue
        }
        if x, ok := m[f.Name]; ok {
            k := f.Tag.Get("json")
            o[k] = x
        }
    }
    return o
}

func mapFields(x *DB) M {
    o := make(M)
    v := reflect.ValueOf(x).Elem()
    t := v.Type()
    for i := 0; i < v.NumField(); i++ {
        f := t.FieldByIndex([]int{i})
        // skip unexported fields
        if f.PkgPath != "" {
            continue
        }
        o[f.Name] = v.FieldByIndex([]int{i}).Interface()
    }
    return o
}