从REST API调用更新对象-结构合并？

Question

I have a JSON REST API accepting sparse updates, but the pattern I've come up with seems exceptionally verbose. Am I going about this the wrong way?

(Assume this is using a data store with no sparse update support built in.)

func choose(a, b *string) *string {
    if a != nil {
        return a
    }
    return b
}

type Model {
    Id     *string `json:"id"`
    Field1 *string `json:"field1"`
    Field2 *string `json:"field2"`
    Field3 *string `json:"field3"`
    ...
}

func (m1 Model) Update(m2 Model) (m3 Model) {
    m3.Id = choose(m2.Id, m1.Id)
    m3.Field1 = choose(m2.Field1, m1.Field1)
    m3.Field2 = choose(m2.Field2, m1.Field2)
    m3.Field3 = choose(m2.Field3, m1.Field3)
    ...
    return
}

func UpdateController(input Model) error {
    previous, _ := store.Get(*input.Id)
    updated := previous.Update(input)
    return store.Put(updated)
}

Ideally I'd be able to write UpdateController like this instead:

func UpdateController(input Model) {
    previous, _ := store.Get(*input.Id)
    updated, _ := structs.Update(previous, input)
    return store.Put(updated)
}

(Error-handling elided for clarity.)

Answer 1

Well, if you are open to using reflection, the problem becomes fairly simple:

http://play.golang.org/p/dc-OnO1cZ4

func (m1 Model) Update(m2 Model) (m3 Model) {
    old := reflect.ValueOf(m1)
    new := reflect.ValueOf(m2)
    final := reflect.ValueOf(&m3).Elem()
    for i := 0; i < old.NumField(); i++ {
        if !new.Field(i).IsNil()  {
           final.Field(i).Set(new.Field(i))
        } else {
           final.Field(i).Set(old.Field(i))
        }      
    }
    return
}

The reason we do reflect.ValueOf(&m3).Elem() is that v3 needs to be settable, see http://blog.golang.org/laws-of-reflection

But basically, by using reflection, we can loop through the struct fields, see if the updated one is nil, and if so, we use the old value.