Play JSON formatter for Map[Int,_] - json

Question

I am attempting to migrate a Rails/Mongodb application to Play 2.3 using play-reactivemongo and reactivemongo-extensions. In modeling my data I am running across a problem serializing and deserializing a Map[Int,Boolean].
When I try to define my formats via macro like so
implicit val myCaseClass = Json.format[MyCaseClass]
where MyCaseClass has a few string fields, a BSONObjectID field, and a Map[Int,Boolean] field the compiler complains with:
No Json serializer found for type Map[Int,Boolean]. Try to implement an implicit Writes or Format for this type.
No Json deserializer found for type Map[Int,Boolean]. Try to implement an implicit Reads or Format for this type.
Looking at the source code for Play in Reads.scala I see a Reads defined for Map[String,_] but none for Map[Int,_].
Is there a reason why Play has default Read/Writes for string maps but not for other simple types?
I don't fully understand the Map[String,_] defined by play because I am fairly new to scala. How would I go about translating that into a Map[Int,_]? If that is not possible for some technical reason how would I define a Reads/Writes for Map[Int,Boolean]?

Answer 1

you can write your own reads and writes in play.
in your case, this would look like this:
implicit val mapReads: Reads[Map[Int, Boolean]] = new Reads[Map[Int, Boolean]] {
def reads(jv: JsValue): JsResult[Map[Int, Boolean]] =
JsSuccess(jv.as[Map[String, Boolean]].map{case (k, v) =>
Integer.parseInt(k) -> v .asInstanceOf[Boolean]
})
}
implicit val mapWrites: Writes[Map[Int, Boolean]] = new Writes[Map[Int, Boolean]] {
def writes(map: Map[Int, Boolean]): JsValue =
Json.obj(map.map{case (s, o) =>
val ret: (String, JsValueWrapper) = s.toString -> JsBoolean(o)
ret
}.toSeq:_*)
}
implicit val mapFormat: Format[Map[Int, Boolean]] = Format(mapReads, mapWrites)
I have tested it with play 2.3. I'm not sure if it's the best approach to have a Map[Int, Boolean] on server side and a json object with string -> boolean mapping on the client side, though.

Answer 2

JSON only allows string keys (a limitation it inherits from JavaScript).

Answer 3

Play Json provides built-in mapReads and mapWrites for reading and writing Maps.
mapReads takes a (String => JsResult[K]) to let you convert the key to your custom type.
mapWrites returns a Writes[Map[String, Boolean]], and you can use contramap to modify that writer into one that works with a Map[Int, Boolean]
import play.api.libs.json.{JsResult, Reads, Writes}
import scala.util.Try
import play.api.libs.json.Reads.mapReads
import play.api.libs.json.MapWrites.mapWrites
object MapExample {
implicit val reads: Reads[Map[Int, Boolean]] =
mapReads[Int, Boolean](s => JsResult.fromTry(Try(s.toInt)))
implicit val writes: Writes[Map[Int, Boolean]] =
mapWrites[Boolean].contramap(_.map { case (k, v) => k.toString -> v})
}

Answer 4

Thanks to Seth Tisue. This is my "generics" (half) way.
"half" because it does not handle a generic key. one can copy paste and replace the "Long" with "Int"
"Summary" is a type I've wanted to serialize (and it needed its own
serializer)
/** this is how to create reader and writer or format for Maps*/
// implicit val mapReads: Reads[Map[Long, Summary]] = new MapLongReads[Summary]
// implicit val mapWrites: Writes[Map[Long, Summary]] = new MapLongWrites[Summary]
implicit val mapLongSummaryFormat: Format[Map[Long, Summary]] = new MapLongFormats[Summary]
This is the required implementation:
class MapLongReads[T]()(implicit reads: Reads[T]) extends Reads[Map[Long, T]] {
def reads(jv: JsValue): JsResult[Map[Long, T]] =
JsSuccess(jv.as[Map[String, T]].map{case (k, v) =>
k.toString.toLong -> v .asInstanceOf[T]
})
}
class MapLongWrites[T]()(implicit writes: Writes[T]) extends Writes[Map[Long, T]] {
def writes(map: Map[Long, T]): JsValue =
Json.obj(map.map{case (s, o) =>
val ret: (String, JsValueWrapper) = s.toString -> Json.toJson(o)
ret
}.toSeq:_*)
}
class MapLongFormats[T]()(implicit format: Format[T]) extends Format[Map[Long, T]]{
override def reads(json: JsValue): JsResult[Map[Long, T]] = new MapLongReads[T].reads(json)
override def writes(o: Map[Long, T]): JsValue = new MapLongWrites[T].writes(o)
}

Answer 5

We can generalize the solution of 3x14159265 and Seth Tisue thanks to 2 small type classes:
import play.api.libs.json.Json.JsValueWrapper
import play.api.libs.json._
import simulacrum._
object MapFormat {
#typeclass trait ToString[A] {
def toStringValue(v: A): String
}
#typeclass trait FromString[A] {
def fromString(v: String): A
}
implicit final def mapReads[K: FromString, V: Reads]: Reads[Map[K, V]] =
new Reads[Map[K, V]] {
def reads(js: JsValue): JsResult[Map[K, V]] =
JsSuccess(js.as[Map[String, V]].map { case (k, v) => FromString[K].fromString(k) -> v })
}
implicit final def mapWrites[K: ToString, V: Writes]: Writes[Map[K, V]] =
new Writes[Map[K, V]] {
def writes(map: Map[K, V]): JsValue =
Json.obj(map.map {
case (s, o) =>
val ret: (String, JsValueWrapper) = ToString[K].toStringValue(s) -> o
ret
}.toSeq: _*)
}
implicit final def mapFormat[K: ToString: FromString, V: Format]: Format[Map[K, V]] = Format(mapReads, mapWrites)
}
Note that I use Simulacrum (https://github.com/mpilquist/simulacrum) to define my type classes.
Here is an example of how to use it:
final case class UserId(value: String) extends AnyVal
object UserId {
import MapFormat._
implicit final val userToString: ToString[UserId] =
new ToString[UserId] {
def toStringValue(v: UserId): String = v.value
}
implicit final val userFromString: FromString[UserId] =
new FromString[UserId] {
def fromString(v: String): UserId = UserId(v)
}
}
object MyApp extends App {
import MapFormat._
val myMap: Map[UserId, Something] = Map(...)
Json.toJson(myMap)
}
if IntelliJ says that your import MapFormat._ is never used, you can and this: implicitly[Format[Map[UserId, Something]]] just below the import. It'll fix the pb. ;)

Answer 6

A specific KeyWrites and KeyReads is available in play-json 2.9.x
private implicit val longKeyWrites = KeyWrites[Int](_.toString)
private implicit val longKeyReads =
KeyReads[Int](str => Try(str.toInt).fold(e => JsError(e.getMessage), JsSuccess(_)))
Json.obj("1" -> "test").validate[Map[Int,String]] // JsSuccess(Map(1 -> test))

Answer 7

Like the accepted answer - a bit shorter:
implicit val mapReads: Reads[Map[Int, Boolean]] = (jv: JsValue) =>
JsSuccess(jv.as[Map[String, Boolean]].map { case (k, v) =>
k.toInt -> v
})
implicit val mapWrites: Writes[Map[Int, Boolean]] = (map: Map[Int, Boolean]) =>
Json.toJson(map.map { case (s, o) =>
s.toString -> o
})
implicit val jsonMapFormat: Format[Map[Int, Boolean]] = Format(mapReads, mapWrites)
Here a little test:
val json = Json.toJson(Map(1 -> true, 2 -> false))
println(json) // {"1":true,"2":false}
println(json.validate[Map[Int, Boolean]]) // JsSuccess(Map(1 -> true, 2 -> false),)

Answer 8

https://gist.github.com/fancellu/0bea53f1a1dda712e179892785572ce3
Here is a way to persist a Map[NotString,...]