how to remove outer array "[" of JSON response

[mkr...@trialfire.com]

Hi. 

I have the following code in my controller (based loosely on the example from the reactivemongo-demo-app ):

 def index = Action { implicit request =>

    Async {

      

      val cursor: Cursor[JsObject] = collection.find(Json.obj()).cursor[JsObject]

       val futureArticleList: Future[List[JsObject]] = cursor.toList

       val futureArticleListJsonArray: Future[JsArray] = futureArticleList.map { articles =>

        Json.arr(articles)        

      }

      futureArticleListJsonArray.map { articles =>

        Ok(articles)

      }

      

    }

  }

The above code returns a doubly nested JSON array as show below. Notice the doubly nested array within an array. How do I get rid of the outer array? Or rather what is causing it?

[

• [

  • {

    • _id: 

      {

      • $oid: "51c33bce865173290120d2b7"

      },
    • title: "How i became awesome",
    • content: "blah blah blah",
    • publisher: "Tyler Durden",
    • creationDate: 

      {

      • $date: 1371749326516

      },
    • updateDate: 

      {

      • $date: 1371749326516

      }

    },
  • {

    • _id: 

      {

      • $oid: "51c34853865173389320d2b8"

      },
    • title: "sgf",
    • content: "sdfsdf",
    • publisher: "sdfsdf",
    • creationDate: 

      {

      • $date: 1371752531157

      },
    • updateDate: 

      {

      • $date: 1371752531157

      }

    }

  ]

]

[Paul Dijou]

Hey there,

If you write Json.arr(articles), you are creating a JsArray with only one element, your list (which will be transformed in an array). So you will have an array of one array.

The solution is to write Json.arr(articles: _*) instead, so you will have only one array of all your elements.

Regards

2013/6/21 <mkr...@trialfire.com>

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[mkr...@trialfire.com]

Thanks Paul,

  I follow your logic and that totally makes sense. (I'm new a to Scala so excuse my syntactical ignorance) I tried Json.arr(articles: _*) and it won't compile, looks like the Json.arr method expects a Seq[JSValueWrapper] but instead finds a List[JsObject]. So what is Json.arr(articles: _*) actually doing? And how to make this work as you describe in order to have a single array of all my objects?

-Max

[Paul Dijou]

Which version of Play are you using? According to last API ( http://www.playframework.com/documentation/api/2.1.1/scala/index.html#play.api.libs.json.Json$ ), Json.arr is expecting an arbitrary number of JsValueWrapper (the syntax with a "*" after the type of the parameter).

Since it doesn't specify how many, if you give it a List[JsObject], by default, it will consider you are giving him only one element (which is a list). By writing Json.arr(articles: _*), you tell him to explode your list in several elements and pass them to Json.arr, so now it will return a real array with the same length as your list.

So it should compile in the last version...Otherwise, you can still do "JsArray(articles)" instead of Json.arr.

Regards.

2013/6/21 <mkr...@trialfire.com>

[mkr...@trialfire.com]

I am using the latest. Play 2.1.1

Thanks for the explanation. But I'm afraid I'm still a little lost. The map method on futureArticleList expects a function which takes a List[JsObject], this list is futureArticleList (its wrapped in a future but still its a List[JsObject]). Json.arr expects one or more JsValueWrapper and as I understand _* unpacks a list. But that list is of type  List[JsObject] and not List[JsValueWrapper ] and hence why the compiler is complaining.

So I guess the question is how do I turn a List[JsObject] into List[JsValueWrapper ] ??? (Maybe thats not the right question to ask but I'm a bit confused now)

Any help would be greatly appreciated.

-Max

[Paul Dijou]

JsValueWrapper is a bit anything that can be cast to a JsValue. For example, a String can be cast to a JsString which extends JsValue so it can be considered as a JsValueWrapper. I'm pretty sure any JsValue is also a JsValueWrapper.

I tried to write Json.arr(Json.obj("key" -> "value"), Json.obj("key" -> "value")), it compile just fine.

Could you copy/paste your code and the compilation error you got?

2013/6/24 <mkr...@trialfire.com>

[mkr...@trialfire.com]

package controllers

import org.joda.time.DateTime

import scala.concurrent.Future

import play.api.Logger

import play.api.Play.current

import play.api.mvc._

import play.modules.reactivemongo.{ MongoController, ReactiveMongoPlugin }

import reactivemongo.api.gridfs.GridFS

import reactivemongo.api.gridfs.Implicits.DefaultReadFileReader

import reactivemongo.api.collections.default.BSONCollection

import reactivemongo.bson._

import reactivemongo.api.Cursor

import play.api.libs.json._

import play.modules.reactivemongo.json.collection.JSONCollection

import models.Article

import models.Article._

object Articles extends Controller with MongoController {

  // get the collection 'articles'

  val collection = db[JSONCollection]("articles")

  // a GridFS store named 'attachments'

  //val gridFS = new GridFS(db, "attachments")

  val gridFS = new GridFS(db)

  // let's build an index on our gridfs chunks collection if none

  gridFS.ensureIndex().onComplete {

    case index =>

      Logger.info(s"Checked index, result is $index")

  }

  def index = Action { implicit request =>

    Async {

      

      val cursor: Cursor[JsObject] = collection.find(Json.obj()).cursor[JsObject]

       val futureArticleList: Future[List[JsObject]] = cursor.toList

       val futureArticleListJsonArray: Future[JsArray] = futureArticleList.map { articles =>

        Json.arr(articles: _*);        

      }

      futureArticleListJsonArray.map { articles =>

        Ok(articles)

      }

      

    }

  }

  

}

Error at line 44: "Json.arr(articles: _*);": type mismatch;  found   : List[play.api.libs.json.JsObject]  required: Seq[play.api.libs.json.Json.JsValueWrapper] Application.scala /reactivemongo-demo-app/app/controllers line 41 Scala Problem

[Paul Dijou]

Okay, I've worked a bit more on the problem and I'm quite frustrated.

Json.arr(Json.obj("key" -> "value"), Json.obj("key" -> "value")) is fine.

Json.arr(List(Json.obj("key"->"value"), Json.obj("key"->"value")): _*) fails to compile where it should be exactly the same.

It looks like you have reach one limit of the compiler. JsValueWrapper is a mechanism to have implicit conversion from "primitive" types (like String or Boolean) to JsValue so you can write Json.arr("test", true). But going with a List of JsValue seems to crash it.

So go for plan B: replace Json.arr(articles: _*) with JsArray(articles), it should works fine.

2013/6/25 <mkr...@trialfire.com>

[mkr...@trialfire.com]

Paul, thanks for your help and patience. That works and gives me the result I desire. 

There still some things about the Play Json libs that I find a bit confusing. I still don't fully comprehend the difference between JsValue and JsObject and when to use which... but i guess that is a discussion for the play group