ScalaJson implicit Write, found: Any required: play.api.libs.json.Json.JsValueWrapper - json

Question

I am building a web app using Scala / Play Framework and Reactive Mongo and I want the models to be defined in the database instead of having them hardcoded.
To do so, I am writing a class EntityInstance taking a Sequence of FieldInstance :
case class EntityInstance(fields: Seq[FieldInstance])
I am trying to accept fields from any types and to convert them to Json : example
new FieldInstance("name", "John") | json: { "name": "John" }
new FieldInstance("age", 18) | json: { "age": 18 }
At the moment I am trying to accept Strings, Booleans and Integers and if the type is not supported I write some error :
new FieldInstance("profilePicture", new Picture("john.jpg") | json: { "profilePicture": "Unsupported type
I wrote a FieldInstance class taking a fieldName as a String and a value as any type. As soon as that class is instantiated I cast the value to a known type or to the String describing the error.
class FieldInstance(fieldNamec: String, valuec: Any) {
val fieldName = fieldNamec
val value = valuec match {
case v: Int => v
case v: String => v
case v: Boolean => v
case _ => "Unrecognized type"
}
}
object FieldInstance {
implicit val fieldInstanceWrites = new Writes[FieldInstance] {
def writes(fieldInstance: FieldInstance) = Json.obj(
fieldInstance.fieldName -> fieldInstance.value
)
}
}
I created a companion object with an implicit Write to json so I can call "Json.toJson()" on an instance of FieldInstance and get a json as described on my examples above.
I get an error : found: Any required: play.api.libs.json.Json.JsValueWrapper
I understand that it comes from the fact that my value is of type Any but I thought the cast would change that Any to String || Boolean || Int before hitting the Writer.
PS: Ignore the bad naming of the classes, I could not name EntityInstance and FieldInstance, Entity and Field because these as the classes I use to describe my models.

Answer 1

I found a fix to my problem :
The type matching that I was doing in the class should be done in the implicit Write !
class FieldInstance(fieldNamec: String, valuec: Any) {
val fieldName = fieldNamec
val value = valuec
override def toString(): String = "(" + fieldName + "," + value + ")";
}
object FieldInstance {
implicit val fieldInstanceWrites = new Writes[FieldInstance] {
def writes(fieldInstance: FieldInstance) =
fieldInstance.value match {
case v: Int => Json.obj(fieldInstance.fieldName -> v.asInstanceOf[Int])
case v: String => Json.obj(fieldInstance.fieldName -> v.asInstanceOf[String])
case v: Boolean => Json.obj(fieldInstance.fieldName -> v.asInstanceOf[Boolean])
case _ => Json.obj(fieldInstance.fieldName -> "Unsupported type")
}
}
}
This code now allows a user to create an EntityInstance with Fields of Any type :
val ei = new EntityInstance(Seq[FieldInstance](new FieldInstance("name", "George"), new FieldInstance("age", 25), new FieldInstance("married", true)))
println("-- TEST ENTITY INSTANCE TO JSON --")
println(Json.toJson(ei))
prints : {"entity":[{"name":"George"},{"age":25},{"married":true}]}
Here is my EntityInstance code if you are trying to test it :
case class EntityInstance(fields: Seq[FieldInstance])
object EntityInstance {
implicit val EntityInstanceWrites = new Writes[EntityInstance] {
def writes(entityInstance: EntityInstance) =
Json.obj("entity" -> entityInstance.fields)
}
}

Answer 2

It is returning a String, Int or Boolean but Json.obj is expecting the value parameter of type (String, JsValueWrapper)
def obj(fields: (String, JsValueWrapper)*): JsObject = JsObject(fields.map(f => (f._1, f._2.asInstanceOf[JsValueWrapperImpl].field)))
a quick fix could be to convert the matched value v with toJson provided the implicit Writes[T] for type T is available (which they are for String, Int and Boolean)
class FieldInstance(fieldNamec: String, valuec: Any) {
val fieldName = fieldNamec
val value = valuec match {
case v: Int => Json.toJson(v)
case v: String => Json.toJson(v)
case v: Boolean => Json.toJson(v)
case _ => Json.toJson("Unrecognized type")
}
}
If you'd like to see which DefaultWrites are available you can browse them in the play.api.libs.json package in trait DefaultWrites
for example:
/**
* Serializer for Boolean types.
*/
implicit object BooleanWrites extends Writes[Boolean] {
def writes(o: Boolean) = JsBoolean(o)
}