Spring data 弹簧&x2B；Google数据存储：@引用未持久化

我正在与Kotlin一起使用

org.springframework.cloud:springcloud gcp starter数据存储

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

@Entity(name = "books")
data class Book(
    @Reference var writer: Writer?, //Accepting NULL values
    var name: String,
    @Id val id: Key? = null
)

当我保存一个书实体时，引用一个保存的编写器，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。知道为什么会发生这种情况吗？
我发现答案不是因为关系没有持久化，而是因为存储库在实例化之后设置了关系实体。存储库首先尝试实例化实体，在关系（用@References注释）属性上分配NULL

因此，实体应如下所示：

@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

@Entity(name = "books")
data class Book(
    @Reference var writer: Writer?, //Accepting NULL values
    var name: String,
    @Id val id: Key? = null
)

所有这些都很好。
我发现答案不是因为关系没有持久化，而是因为存储库在实例化之后设置了关系实体。存储库首先尝试实例化实体，在关系（用@References注释）属性上分配NULL

因此，实体应如下所示：

@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

@Entity(name = "books")
data class Book(
    @Reference var writer: Writer?, //Accepting NULL values
    var name: String,
    @Id val id: Key? = null
)

一切都很好