Set ID on a struct if it exists and hasn't already been set.

reflection.go

package reflectiontest

import "reflect"

type ThingWithId struct {
        Id   string
        Name string
}

type ThingWithoutId struct {
        Name string
}

func CopyOfWithId(id string, t interface{}) interface{} {
        val := reflect.ValueOf(t)
        if _, hasIdField := val.Type().FieldByName("Id"); !hasIdField {
                return t
        }

        if val.FieldByName("Id").Interface() != "" {
                return t
        }

        newbie := reflect.New(val.Type()).Elem()
        for i := 0; i < val.NumField(); i++ {
                newbie.Field(i).Set(val.Field(i))
        }

        newbie.FieldByName("Id").SetString(id)
        return newbie.Interface()
}

reflection_test.go

package reflectiontest_test

import (
        "testing"

        "github.com/dugancathal/reflection-test"
)

func BenchmarkWithIdAndNewThingReflect(b *testing.B) {
        for n := 0; n < b.N; n++ {
                reflectiontest.CopyOfWithId("imma id", reflectiontest.ThingWithId{
                        Name: "Other item",
                })
        }
}

func BenchmarkWithIdAndWithoutNewThingReflect(b *testing.B) {
        for n := 0; n < b.N; n++ {
                reflectiontest.CopyOfWithId("imma id", reflectiontest.ThingWithId{
                        Id:   "already set",
                        Name: "Other item",
                })
        }
}

func BenchmarkWithoutIdOrNewThingReflect(b *testing.B) {
        for n := 0; n < b.N; n++ {
                reflectiontest.CopyOfWithId("imma id", reflectiontest.ThingWithoutId{
                        Name: "Other item",
                })
        }
}

Some benchmarks on reflection in Golang. On my machine:

$ go test -bench=.
testing: warning: no tests to run
PASS
BenchmarkWithIdAndNewThingReflect        2000000               992 ns/op
BenchmarkWithIdAndWithoutNewThingReflect         3000000               430 ns/op
BenchmarkWithoutIdOrNewThingReflect     20000000               115 ns/op
ok      github.com/dugancathal/reflection-test  7.141s