Scala Play Framework JSON JsNull using json4s - json

Question

I'm new to Scala. How do I handle the JsNull value in my code?
I'm using json4s to convert the JSON to a map.
Should I somehow be converting JsNull to an Option?
Example:
Play JSON : creating json
val jsonA: JsValue = Json.obj(
"name" -> "Bob",
"location" -> "Irvine",
"resident" -> "No",
"nick-name" -> "Bigwig",
"age" -> "6",
"role" -> JsNull,
"car" -> "BMW",
"multiple-residents" -> JsArray(Seq(
JsObject(Seq(
"name" -> JsString("Fiver"),
"age" -> JsNumber(4),
"role" -> JsObject(Seq(
"position" -> JsString("Fiver"),
"" -> JsNumber(4),
"role" -> JsString("janitor")
))
))
))
)
json4s : parsing the json
var jsonAMap:Map[String, Any] = Map()
val jsonAString: String = Json.stringify(jsonA)
jsonAMap = jsonStrToMap(jsonAString)
After the JsValue is converted to a Map using json4s it looks like this:
Map(name -> Bob, location -> Irvine, role -> null, resident -> No, car -> BMW, multiple-residents -> List(Map(name -> Fiver, age -> 4, role -> Map(position -> Fiver, -> 4, role -> janitor))), age -> 6, nick-name -> Bigwig)
When I create a create a List of the values, I end up with a null value in my list. Once I'm pattern matching all the values of the list I end up trying to patter match a null which is not possible (I'm sure I'm not supposed to use a will card for all my cases, but I'm learning) :
for(i <- 0 to beforeValsList.length - 1){
beforeValsList(i) match {
case _ : Map[_,_] =>
compareJson(
beforeValsList(i).asInstanceOf[Map[String,Any]],
afterValsList(i).asInstanceOf[Map[String,Any]],
rdeltaBefore, rdeltaAfter, sameKeyList(i).toString()
)
case _ if (beforeValsList(i) != afterValsList(i)) =>
// if i'm from a recursion, build a new map and add me
// to the deltas as a key->value pair
rdeltaBefore += sameKeyList(i).toString -> beforeValsList(i)
rdeltaAfter += sameKeyList(i).toString -> afterValsList(i)
case _ =>
println("catch all: " + beforeValsList(i).toString
+ " " + afterValsList(i).toString)
}
}
json4s converts JsNull to a null. Should I do a null check:
if(!beforeValsList(i) == null){
beforeValsList(i) match{...}
}
Or is there a way for me to change the null to an Option when I'm putting the values from the Map to a List?
I'm not sure what best practices are and why jsno4s changes JsNull to null instead of an Option, and whether or not that's possible.
Cheers.

Answer 1

I'm still not sure how you would like to handle JsNull, null (or None if you use Option), but you function to get the difference between the Map[String, Any] before and after can be simplified :
type JsonMap = Map[String, Any]
def getMapDiffs(mapBefore: JsonMap, mapAfter: JsonMap) : (JsonMap, JsonMap) = {
val sameKeys = mapBefore.keySet intersect mapAfter.keySet
val startAcc = (Map.empty[String, Any], Map.empty[String, Any])
sameKeys.foldLeft(startAcc){ case (acc # (deltaBefore, deltaAfter), key) =>
(mapBefore(key), mapAfter(key)) match {
// two maps -> add map diff recursively to before diff and after diff
case (beforeMap: Map[_, _], afterMap: Map[_, _]) =>
val (deltaB, deltaA) =
getMapDiffs(beforeMap.asInstanceOf[JsonMap], afterMap.asInstanceOf[JsonMap])
(deltaBefore + (key -> deltaB), deltaAfter + (key -> deltaA))
// values before and after are different
// add values to before diff and after diff
case (beforeValue, afterValue) if beforeValue != afterValue =>
(deltaBefore + (key -> beforeValue), deltaAfter + (key -> afterValue))
// keep existing diff
case _ => acc
}
}
}
Which can be used as:
val (mapBefore, mapAfter) = (
Map("a" -> "alpha", "b" -> "beta", "c" -> "gamma", "d" -> Map("e" -> "epsilon")),
Map("a" -> "alpha", "b" -> List("beta"), "c" -> null, "d" -> Map("e" -> 3))
)
val (deltaBefore, deltaAfter) = getMapDiffs(mapBefore, mapAfter)
// deltaBefore: JsonMap = Map(b -> beta, c -> gamma, d -> Map(e -> epsilon))
// deltaAfter: JsonMap = Map(b -> List(beta), c -> null, d -> Map(e -> 3))
deltaBefore.toList
// List[(String, Any)] = List((b,beta), (c,gamma), (d,Map(e -> epsilon)))