无法将字节数组存储在Google的数据存储区中

Question

I'm using Google's datastore in my Go app. I have a Song struct, which has a uuid.UUID field.

type Song struct {
    ID: uuid.UUID
    Title: string
    ...
}

This UUID is taken from github.com/satori/go.uuid and is defined as

type UUID [16]byte

It seems that datastore can't handle byte arrays but in this use case only byte slices or strings. In the json package I can use a tag to interpret it as a string

type Song struct {
    ID: uuid.UUID `json:"id,string"`
    ....
}

Is there a way of telling datastore to interpret the UUID as a slice/string or do I either have to give up "type"-safety and just store a string or use a custom PropertyLoadSaver?

Answer 1

Per Google's Documentation:

Valid value types are:

• signed integers (int, int8, int16, int32 and int64),
• bool,
• string,
• float32 and float64,
• []byte (up to 1 megabyte in length),
• any type whose underlying type is one of the above predeclared types,
• ByteString,
• *Key,
• time.Time (stored with microsecond precision),
• appengine.BlobKey,
• appengine.GeoPoint,
• structs whose fields are all valid value types,
• slices of any of the above.

So, you will have to use a byte slice or string. You could do some behind the scenes manipulation when you need to do your setting or getting like (Playground Example):

type uuid [16]byte

type song struct {
    u []byte
}

func main() {
    var b [16]byte
    copy(b[:], "0123456789012345")

    var u uuid = uuid(b) //this would represent when you get the uuid

    s := song{u: []byte(u[:])}

    copy(b[:], s.u)
    u = uuid(b)
    fmt.Println(u)
}

This could also be done through methods. (Playground example)

Alternatively, you could have an entity specific to the datastore that carries the byte slice, and the transformers that go to and from that entity know how to do the conversion.