如何获得父嵌入的struct字段值？

Question

I have the following struct in my package.

type Animal struct {
  Id string
  // and some more common fields
}

A user who uses my package should be able to implement own Animals. As I need Id in one of my methods the user has to embed my struct.

type Dog struct {
  Animal
  Name string
  Legs int
}

In my package I have a save() function that saves Animals to database. As I don't know the user's type I have to use interface{} as argument type. My question is: How do I get the Id (from the parent struct Animal)? At the moment I use some JSON parsing and unmarshalling, but is this the way to go/Go?

func put(doc interface{}) error {
  res, err := json.Marshal(doc)
  if err != nil {
    return err
  }
  var animal *Animal
  err = json.Unmarshal(res, &animal)
  if err != nil {
    return err
  }
  fmt.Println(animal.Id) // finally I have the Id
  // ...
}

Usage

func main() {
  bello := Dog{
    Animal: Animal{
      Id: "abcd1234",
    },
    Name: "bello",
    Legs: 4,
  }
  err := put(bello)
  // ...
}

Answer 1

Maybe you can add an interface in order to be sure to get a reference to the parent struct:

type AnimalGetter interface {
    GetAnimal() *Animal
}

func (dog *Dog) GetAnimal() *Animal {
    return &dog.Animal
}

That would allow your save method to do a type assertion (AnimalGetter):

func save(obj interface{}) {
    animal := obj.(AnimalGetter)
    fmt.Printf("%v
", animal.GetAnimal())
}

See a complete example in <kbd>play.golang.org</kbd>.

Output:

&{{dogid} wouf 0}
&{dogid}

Simpler:

func save(animal AnimalGetter) {
    fmt.Printf("%v
", animal.GetAnimal())
}

(<kbd>play.golang.org</kbd>)