播放框架:JSON读取单个属性的案例类 [英] Play framework: JSON Reads for a single-attribute case class

问题描述

我正在尝试为包含单个属性的案例类创建隐式的JSON读取，但出现错误读取[无]不符合预期类型".这是代码:

I'm trying to create a JSON Reads implicit for a case class that contains a single attribute, but I'm getting an error "Reads[Nothing] doesn't conform to expected type". Here's teh codes:

import play.api.libs.functional.syntax._
import play.api.libs.json.Reads._
import play.api.libs.json.{JsPath, Json, Reads}

case class Feedback(message: String)
object Feedback {
  implicit val reads: Reads[Feedback] = (
      (JsPath \ "message").read[String](maxLength[String](2000))
    )(Feedback.apply _)
}

为什么这不起作用?如果我将额外的属性添加到case类中，并且多个.read调用与and一起加入，那么它将起作用...

Why isn't this working? If I add extra attributes to the case class and multiple .read calls joined with and it works...

推荐答案

Json组合器不适用于单个字段案例类.

Json combinators doesn't work for single field case class.

您可以执行以下操作:

import play.api.libs.json.Reads._
import play.api.libs.json.{__, Reads}

case class Feedback(message: String)
object Feedback {
  implicit val reads: Reads[Feedback] = (__ \ "message")
    .read[String](maxLength[String](2000)).map {message => Feedback(message)}
}

这是由于当前Macro实施中的限制所致.您可以在此处了解更多信息: Pacal是此书的作者API

It's because of a limitation in the current Macro implementation. You can read more about it here: Pacal is the writer of this API

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