如何在Play Framework 2.x中实现嵌入对象的隐式Json Writes

Question

有两个班级Foo和Bar.Foo包含一个字段Bar.问题是,如何Writes为类实现隐式json Foo？

这是代码:

package models

import play.api.libs.json._

case class Foo(id: String, bar: Bar)

object Foo {
  implicit val implicitFooWrites = new Writes[Foo] {
    def writes(foo: Foo): JsValue = {
      Json.obj(
        "id" -> foo.id,
        "bar" -> foo.bar
      )
    }
  }
}

case class Bar(x: String, y: Int)

object Bar {
  implicit val implicitBarWrites = new Writes[Bar] {
    def writes(bar: Bar): JsValue = {
      Json.obj(
        "x" -> bar.x,
        "y" -> bar.y
      )
    }
  }
}

当我尝试编译时,我收到以下错误:

没有找到类型模型的Json反序列化器.Bar.尝试为此类型实现隐式写入或格式.

我不明白这个编译器错误,因为我为models.Bar类实现了一个隐式Writes.这里有什么问题？

Answer 1

这是一个可见性的问题,当声明隐式Writes [Foo]时,你没有看到它的隐式Writes [Bar]:

scala> :paste
// Entering paste mode (ctrl-D to finish)

import play.api.libs.json._

case class Bar(x: String, y: Int)

object Bar {
  implicit val implicitBarWrites = new Writes[Bar] {
    def writes(bar: Bar): JsValue = {
      Json.obj(
        "x" -> bar.x,
        "y" -> bar.y
      )
    }
  }
}

case class Foo(id: String, bar: Bar)

object Foo {

  import Bar._

  implicit val implicitFooWrites = new Writes[Foo] {
    def writes(foo: Foo): JsValue = {
      Json.obj(
        "id" -> foo.id,
        "bar" -> foo.bar
      ) 
    } 
  }     
}

// Exiting paste mode, now interpreting.

import play.api.libs.json._
defined class Bar
defined module Bar
defined class Foo
defined module Foo

scala> Json.prettyPrint(Json.toJson(Foo("23", Bar("x", 1))))
res0: String = 
{
  "id" : "23",
  "bar" : {
    "x" : "x",
    "y" : 1
  }
}

另外,如果你使用的播放2.1+确保检查出全新的使用2.10的宏:http://www.playframework.com/documentation/2.1.0/ScalaJsonInception

如果您对使用case类和val/vars的名称作为json输出中的键使用感到满意,就像你的BTW一样,那么你可以使用两个单行:

implicit val barFormat = Json.writes[Bar]
implicit val fooFormat = Json.writes[Foo]

这将给你完全相同的:

scala> import play.api.libs.json._
import play.api.libs.json._

scala> case class Bar(x: String, y: Int)
defined class Bar

scala> case class Foo(id: String, bar: Bar)
defined class Foo

scala> implicit val barWrites = Json.writes[Bar]
barWrites: play.api.libs.json.OWrites[Bar] = play.api.libs.json.OWrites$$anon$2@257cae95

scala> implicit val fooWrites = Json.writes[Foo]
fooWrites: play.api.libs.json.OWrites[Foo] = play.api.libs.json.OWrites$$anon$2@48f97e2a

scala> Json.prettyPrint(Json.toJson(Foo("23", Bar("x", 1))))
res0: String = 
{
  "id" : "23",
  "bar" : {
    "x" : "x",
    "y" : 1
  }
}