播放框架（Scala）-获取包含数组的json子集

Question

I have many very large json-objects that I return from Play Framework with Scala.

In most cases the user doesn't need all the data in the objects, only a few fields. So I want to pass in the paths I need (as query parameters), and return a subset of the json object.

I have looked at using JSON Transformers for this task.

Filter code

def filterByPaths(paths: List[JsPath], inputObject: JsObject) : JsObject = {
  paths
    .map(_.json.pick)
    .map(inputObject.transform)
    .filter(_.isSuccess)
    .map { case JsSuccess(value, path) => (value, path) }
    .foldLeft(Json.obj()) { (obj, jsValueAndPath) =>
      val(jsValue, path) = jsValueAndPath
      val transformer = __.json.update(path.json.put(jsValue))
      obj.transform(transformer).get
    }
}

Usage:

val input = Json.obj(
  "field1" -> Json.obj(
    "field2" -> "right result"
  ),
  "field4" -> Json.obj(
    "field5" -> "not included"
  ),
)

val result = filterByPaths(List(JsPath \ "field1" \ "field2"), input)
// {"field1":{"field2":"right result"}}

Problem

This code works fine for JsObjects . But I can't make it work if there are JsArrays in the strucure. I had hoped that my JsPath could contain an index to look up the field, but that's not the case. (Don't know why I assumed that, maybe my head was too far in the JavaScript-world)

So this would fail to return the first entry in the Array:

val input: JsObject = Json.parse("""
  {
    "arr1" : [{
      "field1" : "value1"
    }]
  }
  """).as[JsObject]

val result = filterByPaths(List(JsPath \ "arr1" \ "0"), input)
// {}

Question

My question is: How can I return a subset of a json structure that contains arrays?

Alternative solution

I have the data as a case class first, and I serialize it to Json, and then run filterByPaths on it. Having a Reader that only creates the json I need in the first place might be a better solution, but creating a Reader on the fly, with configuration from queryparams seamed a more difficult task, then just stripping down the json afterwards.

Answer 1

The example of the returning array element:

val input: JsValue = Json.parse("""
  {
    "arr1" : [{
      "field1" : "value1"
    }]
  }
  """)

val firstElement = (input \ "arr1" \ 0).get
val firstElementAnotherWay = input("arr1")(0) 

More about this in the Play Framework documentation: https://www.playframework.com/documentation/2.6.x/ScalaJson

Update

It looks like you got the old issue RuntimeException: expected KeyPathNode . JsPath.json.put , JsPath.json.update can't past an object to a nesting array.

https://github.com/playframework/playframework/issues/943

https://github.com/playframework/play-json/issues/82

What you can do:

1. Use the JSZipper: https://github.com/mandubian/play-json-zipper
2. Create a script to update arrays "manually"
3. If you can afford it, strip array in a resulting object

Example of stripping array (point 3):

def filterByPaths(paths: List[JsPath], inputObject: JsObject) : JsObject = {
  paths
    .map(_.json.pick)
    .map(inputObject.transform)
    .filter(_.isSuccess)
    .map { case JsSuccess(value, path) => (value, path)}
    .foldLeft(Json.obj()) { (obj, jsValueAndPath) =>
      val (jsValue, path) = jsValueAndPath
      val arrayStrippedPath = JsPath(path.path.filter(n => !(n.toJsonString matches """\[\d+\]""")))
      val transformer = __.json.update(arrayStrippedPath.json.put(jsValue))
      obj.transform(transformer).get
    }
}

val result = filterByPaths(List(JsPath \ "arr1" \ "0"), input)
// {"arr1":{"field1":"value1"}}

The example

Answer 2

1. The best to handle JSON objects is by using case classes and create implicit Reads and Writes, by that you can handle errors every fields directly. Don't make it complicated.

2. Don't use .get() much recommended to use .getOrElse() because scala is a type-safe programming language.

3. Don't just use any Libraries except you know the process behind it, much better to create your own parsing method with simplified solution to save memory.

I hope it will help you..