Option [String]没有Json格式化程序？

I am trying to marshall and un-marshall an Option[String] field to and from JSON. For my use-case, a None value should be marshaled as "null". Here is the code I have:

我正在尝试对JSON中的Option [String]字段进行编组和取消编组。对于我的用例,None值应该被封送为“null”。这是我的代码:

import org.scalatest.{FlatSpec, Matchers}

import play.api.libs.json._
import play.api.libs.json.Reads._
import play.api.libs.functional.syntax._


case class Person(
  id: Int,
  firstName: Option[String],
  lastName: Option[String]
)

object Person {
  implicit lazy val personFormat = (
    (__ \ "id").format[Int] and
    (__ \ "first_name").format[Option[String]] and
    (__ \ "last_name").format[Option[String]]
  )(Person.apply, unlift(Person.unapply))
}

class PersonSpec extends FlatSpec with Matchers {
  "When Person instance is marshaled None fields " should
    "be serialized as \"null\" values" in {
    val person = Person(1, None, None)
    import Person._
    val json = Json.toJson(person)
    println(json)
    (json \ "id").as[Int] should be (1)
    (json \ "first_name").get should be (JsNull)
    (json \ "last_name").get should be (JsNull)
  }
}

This results in the following compiler error:

这会导致以下编译器错误:

PersonSpec.scala:19: No Json formatter found for type Option[String]. Try to implement an implicit Format for this type.
[error]     (__ \ "first_name").format[Option[String]] and
[error]                               ^

These are some of the things I have tried:

这些是我尝试过的一些事情:

Replacing (__ \ "first_name").format[Option[String]] with (__ \ "first_name").formatNullable[String] makes the compiler happy, but the test fails (""java.util.NoSuchElementException: None.get"") with the following output (from println(json))

用[__ \“first_name”)替换(__ \“first_name”)。format [Option [String]]。formatNullable [String]使编译器满意,但测试失败(“”java.util.NoSuchElementException:None.get “”)具有以下输出(来自println(json))

{"id":1}

This confirms with formatNullable's behavior (don't render None valued fields).

这证实了formatNullable的行为(不渲染无值字段)。

Next, I replaced the format with a writes. Like so:

接下来,我用写入替换了格式。像这样:

object Person {
  implicit lazy val personWrite = (
    (__ \ "id").write[Int] and
    (__ \ "first_name").write[Option[String]] and
    (__ \ "last_name").write[Option[String]]
  )(unlift(Person.unapply))
}

Now, the compiler is happy and the test passes.

现在,编译器很高兴并且测试通过了。

But I now need to implement a separate Reads. If I could, I would rather not as it violates DRY principle.

但我现在需要实现单独的Reads。如果可以的话,我宁愿不要因为它违反DRY原则。

What am I doing wrong and when write[Option[...]] works perfectly why not format[Option[...]]?

我做错了什么,当写[Option [...]]完全有效时为什么不格式化[Option [...]]?

2 个解决方案

#1

Adding this code so that it is implicit-visible from your PersonFormat will make it work.

添加此代码以使其从PersonFormat隐式可见将使其工作。

implicit def optionFormat[T: Format]: Format[Option[T]] = new Format[Option[T]]{
    override def reads(json: JsValue): JsResult[Option[T]] = json.validateOpt[T]

    override def writes(o: Option[T]): JsValue = o match {
      case Some(t) ⇒ implicitly[Writes[T]].writes(t)
      case None ⇒ JsNull
    }
  }

I think that in play it is assumed that option-valued fields should be treated optional at all, hence the behaviour you observed with formatNullable.

我认为在游戏中假设选项值字段应该被视为可选的,因此您使用formatNullable观察到的行为。

#2

You can use:

您可以使用:

(__ \ "first_name").formatNullable[String]

(__ \“first_name”)。formatNullable [String]

#1