Getting a datastore entity into an interface in Go

Question

I have a couple of datastore Kinds that have the same field Id. Is it possible to create one generic function that can get me this value? Something similar to this?

var i interface{}
err = datastore.Get(c, key, &i)
v := reflect.ValueOf(i)
id := v.FieldByName("Id").String()

The above code, as it is, gives me a "datastore: invalid entity type" error.

Answer 1

var i interface{} isn't of any concrete type. The appengine datastore requires a concrete type to deserialize the data into since it uses reflection. It looks like from the documentation that missing fields or fields of a different type than the data was stored from will cause an error to be returned as well so you can't create a struct with just the ID field defined.

Even so it's possible you could work something out using a custom type that implements the PropertyLoadSaver interface like so:

type IdField struct {
  Id string
}

function (f *IdField) Load(ch <-chan Property) error {
  for p := range ch {
    if p.Name == "Id" {
      f.Id = p.Value.(string)
    }
  }
  return nil
}

function (f *IdField) Save(ch chan<- Property) error {
   return fmt.Errorf("Not implemented")
}

var i = &IdField{}
err := datastore.Get(c, key, i)
id := i.Id

It's probably not as concise as you were hoping but it's a little more typesafe doesn't require reflection and illustrates the general approach you could use to get partial data out of the datastore.