Question

我有以下对象：

case class A(id: Long, name:String, lastName:Option[String], age:Int)

我想使用Writes convertor只写入此对象的一部分。所以我尝试了这段代码（尝试编写没有年龄的对象）：

implicit val aWrites: Writes[A] = (
  (JsPath \ "it").write[Long] and
  (JsPath \ "name").write[String] and 
  (JsPath \ "lastName"). writeNullable[String]
)(unlift(A.unapply))

但这显然不能编译。有没有办法使这项工作？

Answer 1

您可以执行此操作，但无法引用A.unapply。 JSON组合器的部分简洁性来自编译器为案例类自动生成的apply和unapply方法。但是这些方法使用了类中所有的参数。

A.unapply具有签名A => Option[(Long, String, Option[String], Int)]。这与仅覆盖三个字段（而不是全部四个字段）的组合器不兼容。你有两个选择。

1）编写另一个具有正确签名的unapply方法：

def unapplyShort(a: A): Option[(Long, String, Option[String], Int)] =
    Some((a.id, a.name, a.lastName))

implicit val aWrites: Writes[A] = (
  (JsPath \ "id").write[Long] and
  (JsPath \ "name").write[String] and 
  (JsPath \ "lastName").writeNullable[String]
)(unlift(unapplyShort))

2）手动创建Writes作为匿名类：

implicit val aWrites: Writes[A] = new Writes[A] {
  def writes(a: A): JsValue = Json.obj(
    "id" -> a.id,
    "name" -> a.name,
    "lastName" -> a.lastName
  )
}

Answer 2

来自@ m-z的

选项1）可以更简洁地表达为：

implicit val aWrites: Writes[A] = (
  (JsPath \ "id").write[Long] and
  (JsPath \ "name").write[String] and
  (JsPath \ "lastName").writeNullable[String]
)((a) => (a.id, a.name, a.lastName))

Answer 3

仅供参考，你可以像下面这样实现。（这只是一个建议。）

object noWrites extends OWrites[Any] {
  override def writes(o: Any): JsObject = JsObject(Seq())
}

implicit val aWrites: Writes[A] = (
  (JsPath \ "id").write[Long] and
  (JsPath \ "name").write[String] and
  (JsPath \ "lastName").writeNullable[String] and noWrites // <---
)(unlift(A.unapply))

Play框架 - 写入部分对象

3 个答案: