[json4s]:Extracting Array of different objects

问题

I am using the facebook graph API and the responses look similar to this:

{
  "data": [
    {
      "id": "311620272349920_311718615673419", 
      "from": {
        "id": "1456046457993048", 
        "name": "Richard Ettinson"
      }, 
      "to": {
        "data": [
          {
            "id": "311620272349920", 
            "name": "Barbara Fallerman"
          }
        ]
      }, 
      "with_tags": {
        "data": [
          {
            "id": "311620272349920", 
            "name": "Barbara Fallerman"
          }
        ]
      }, 
      "message": "I was gong out with her", 
      "actions": [
        {
          "name": "Comment", 
          "link": "https://www.facebook.com/311620272349920/posts/311718615673419"
        }, 
        {
          "name": "Like", 
          "link": "https://www.facebook.com/311620272349920/posts/311718615673419"
        }
      ]
}

I managed to for example extract the from field through

val extracted = (json \ "data" \"from").extract[PostFrom]

But I worry that if I use this technique I will need to pass over the Json multiple times to extract all the values I need which could lead to a bad performance.

How exactly could I extract these fields into case classes from the array of non similar objects?

I tried with the following case classes:

abstract class BaseResponse()
case class Data(list:List[Post])
case class Post(id: String, post: PostFrom) extends BaseResponse
case class PostFrom(id: String, name:String)

Which always lead to an empty List, is there a way to get back a Data class which has a list of certain classes which I am interested in? (As example the top level id, from and with_tags)

回答1:

A possibility I found was to use more case classes instead of inheritance:

case class Root[T](data:Option[T])
case class Post(id: String, from: From, message: String)
case class From(id: String, name:String)

Basically there has to be a root object which takes some kind of graphs response object, additionally it is optional so that it won't throw an exception if there was a problem with the parsing of the response.

I then used it in the following way:

val body = r.entity.asString
val json = parse(r.entity.asString)
val root = json.extract[Root[Post]]

root.data match {
  case Some(post) =>
      val tagger = Tagger(post.from.id, post.from.name, post.id, post.message)
      log.info(s"We received $tagger")
      originalSender ! RetrievedTagger(tagger)

  case None => originalSender ! NoTaggerFound
}

来源：https://stackoverflow.com/questions/25591859/json4sextracting-array-of-different-objects

标签

scala

facebook-graph-api

json4s