玩！ Scala：Json 阅读器和附加类字段答案

【问题标题】：Play! Scala: Json reader and additional class field玩！ Scala：Json 阅读器和附加类字段
【发布时间】：2015-02-26 07:55:35
【问题描述】：

我正在使用 Play！ Scala 2.2，我正在阅读 json 如下，效果很好：

case class YoutubeTrack(//artistId: String,
                          videoId: String,
                          title: String,
                          thumbnail: Option[String] )
val youtubeTrackReads: Reads[YoutubeTrack] = (
      (__ \ "id" \ "videoId").read[String] and
        (__ \ "snippet" \ "title").read[String] and
        (__ \ "snippet" \ "thumbnails" \ "default" \ "url").readNullable[String]
      )(YoutubeTrack)

现在我想在我的YoutubeTrack class 中添加一个字段（在类声明中注释的artistId）。该字段是我在其他地方定义的变量。

如何在读取 json 的同时将此字段添加到我的 YoutubeTrack 中，即我想做类似的事情：

val youtubeTrackReads: Reads[YoutubeTrack] = (
      (__ \ "id" \ "videoId").read[String] and
        (__ \ "snippet" \ "title").read[String] and
        (__ \ "snippet" \ "thumbnails" \ "default" \ "url").readNullable[String]
      )((artistId, videoId, title, url) => YoutubeTrack(artistId, videoId, title, url))

【问题讨论】：

这取决于您是否认为这个额外的字段是可选的（可以在 init 之后定义，也可以不定义），或者在某些时候该字段是强制性的。

标签： json scala playframework-2.2

【解决方案1】：

给定函数

def toArtistId(
    videoId: String,
    title: String,
    thumbnail: Option[String]): String = ...

您甚至可以在不更改阅读器的情况下添加您的艺术家 ID

case class YoutubeTrack(
    videoId: String,
    title: String,
    thumbnail: Option[String]) {
    val artistId = toArtistId(videoId, title, thumbnail)
}

或者像这样改变你的阅读器

(
    (__ \ "id" \ "videoId").read[String] and 
    (__ \ "snippet" \ "title").read[String] and
    (__ \ "snippet" \ "thumbnails" \ "default" \ "url").readNullable[String]
)((videoId, title, thumbnail) => 
     YoutubeTrack(toArtistId(videoId, title, thumbnail), videoId, title, thumbnail)
)

甚至像这样

def artistReader(artistId: String): Reads[YoutubeTrack] = {
    (
        (__ \ "id" \ "videoId").read[String] and
        (__ \ "snippet" \ "title").read[String] and
        (__ \ "snippet" \ "thumbnails" \ "default" \ "url").readNullable[String]
    )((videoId, title, thumbnail) => 
        YoutubeTrack(artistId, videoId, title, thumbnail))
}

artistReader("A")

【讨论】：

【解决方案2】：

case class YoutubeTrackTemp(
  videoId: String,
  title: String,
  thumbnail: Option[ String ]
)

val youtubeTrackReads: Reads[ YoutubeTrack ] = (
  ( __ \ "id" \ "videoId" ).read[ String ] and
    ( __ \ "snippet" \ "title" ).read[ String ] and
    ( __ \ "snippet" \ "thumbnails" \ "default" \ "url" ).readNullable[ String ]
  )( YoutubeTrackTemp )

case class YoutubeTrack(
  artistId: String,
  videoId: String,
  title: String,
  thumbnail: Option[ String ]
)

object YoutubeTrack {
  def apply( yt: YoutubeTrackTemp ): YoutubeTrack = {
    val artistId = getArtistIdSomehow()
    YoutubeTrack( artistId, yt.videoId, yt.title, yt.thumbnail )
  } 
}

val youtubeTrack = YoutubeTrack( myjson.as[ YoutubeTrackTemp ] )

【讨论】：