Spring+Google数据存储:@引用未持久化

我在用 org.springframework.cloud:spring-cloud-gcp-starter-data-datastore 和科特林在一起。

代码如下所示:

@Entity(name = "books")
data class Book(
    @Reference val writer: Writer,
    var name: String,
    @Id val id: Key? = null, //I leave the key as NULL so it that can be autogenerated
)

@Entity(name = "writers")
data class Writer(
    var name: String,
    @Id val id: Key? = null
)

//Also with Repositories

当我保存一个图书实体时,引用一个已保存的作者,当我检索它时,应该自动检索它,对吗?

示例代码:

var w = Writer("Shakespeare")
w = writerRepo.save(w)
var book = Book(w, "Macbeth")
book = bookRepo.save(book)

books = bookRepo.findByWriter(w) //Error happen here

上面的代码将抛出一个错误:无法用空Writer实例化Book。知道为什么会这样吗?