spray-json serialization in spray-routing with custom JsonFormats - scala

Question

Using Spray with spray-json for a system, version:
"io.spray" %% "spray-json" % "1.2.6"
I cannot figure how to get custom JsonFormat definitions to work for serialization that is being handled by spray-routing.
I've had two separate circumstances that have failed.
1. Nested Case Classes
Basic case class JSON serialization has worked fine
case class Something(a: String, b: String)
implicit val something2Json = jsonFormat3(Something)
However if I have a nested case class in the case class to be serialized, I can resolve compile issues by providing another JsonFormat implicit, yet at run-time it refuses to serialize
case class Subrecord(value: String)
case class Record(a: String, b: String, subrecord: Subrecord)
object MyJsonProtocol extends DefaultJsonProtocol {
implicit object SubrecordJsonFormat extends JsonFormat[Subrecord] {
def write(sub: Subrecord) = JsString(sub.value)
def read(value: JsValue) = value match {
case JsString(s) => Subrecord(s)
case _ => throw new DeserializationException("Cannot parse Subrecord")
}
}
implicit val record2Json = jsonFormat3(Record)
}
This will throw a MappingException at runtime, explaining there is no usable value for subrecord
2. Trait with various 0-N case extensions
Here I have a trait that serves as a capturing type for a group of case classes. Some of the extending classes have vals while others have no vals and are objects. When serialization occurs, it seems like my implicit defined JsonFormat is completely ignored and I'm just give an empty JsObject, particularly when the actual underlying type was one of the case object's with no vals.
sealed trait Errors
sealed trait ErrorsWithReason extends Errors {
def reason: String
}
case class ValidationError(reason: String) extends ErrorsWithReason
case object EntityNotFound extends Errors
case class DatabaseError(reason: String) extends ErrorsWithReason
object MyJsonProtocol extends DefaultJsonProtocol {
implicit object ErrorsJsonFormat extends JsonFormat[Errors] {
def write(err: Errors) = failure match {
case e: ErrorsWithReason => JsString(e.reason)
case x => JsString(x.toString())
}
def read(value: JsValue) = {
value match {
//Really only intended to serialize to JSON for API responses
case _ => throw new DeserializationException("Can't reliably deserialize Error")
}
}
}
}
So given the above, if the actual type being serialized is EntityNotFound, then the serialization becomes a RootJsonFormat turning into {}. If it's an ErrorsWithReason then it becomes a RootJsonFormat turning into { "reason": "somevalue" }. I may be confused with how the JsonFormat definition is supposed to work, but it doesn't seem to be using my write method at all and instead has suddenly figured out how to serialize on its own.
EDIT
Specific serialization cases are using read/deserialization like:
entity(as[JObject]) { json =>
val extraction: A = json.extract[A]
}
And write/serialization with the complete directive.
I now am realizing thanks to the first answer posted here that my JsonDefaultProtocol and JsonFormat implementations are for spray-json classes, meanwhile the entity directive extraction in the deserialization is using json4s JObject as opposed to spray-json JsObject.

Answer 1

Another approach for clean JSON output
import spray.json._
import spray.json.DefaultJsonProtocol._
// #1. Subrecords
case class Subrecord(value: String)
case class Record(a: String, b: String, subrecord: Subrecord)
implicit object RecordFormat extends JsonFormat[Record] {
def write(obj: Record): JsValue = {
JsObject(
("a", JsString(obj.a)),
("b", JsString(obj.b)),
("reason", JsString(obj.subrecord.value))
)
}
def read(json: JsValue): Record = json match {
case JsObject(fields)
if fields.isDefinedAt("a") & fields.isDefinedAt("b") & fields.isDefinedAt("reason") =>
Record(fields("a").convertTo[String],
fields("b").convertTo[String],
Subrecord(fields("reason").convertTo[String])
)
case _ => deserializationError("Not a Record")
}
}
val record = Record("first", "other", Subrecord("some error message"))
val recordToJson = record.toJson
val recordFromJson = recordToJson.convertTo[Record]
println(recordToJson)
assert(recordFromJson == record)

Answer 2

If you need both reads and writes you can do it this way:
import spray.json._
import spray.json.DefaultJsonProtocol._
// #1. Subrecords
case class Subrecord(value: String)
case class Record(a: String, b: String, subrecord: Subrecord)
implicit val subrecordFormat = jsonFormat1(Subrecord)
implicit val recordFormat = jsonFormat3(Record)
val record = Record("a", "b", Subrecord("c"))
val recordToJson = record.toJson
val recordFromJson = recordToJson.convertTo[Record]
assert(recordFromJson == record)
// #2. Sealed traits
sealed trait Errors
sealed trait ErrorsWithReason extends Errors {
def reason: String
}
case class ValidationError(reason: String) extends ErrorsWithReason
case object EntityNotFound extends Errors
case class DatabaseError(reason: String) extends ErrorsWithReason
implicit object ErrorsJsonFormat extends JsonFormat[Errors] {
def write(err: Errors) = err match {
case ValidationError(reason) =>
JsObject(
("error", JsString("ValidationError")),
("reason", JsString(reason))
)
case DatabaseError(reason) =>
JsObject(
("error", JsString("DatabaseError")),
("reason", JsString(reason))
)
case EntityNotFound => JsString("EntityNotFound")
}
def read(value: JsValue) = value match {
case JsString("EntityNotFound") => EntityNotFound
case JsObject(fields) if fields("error") == JsString("ValidationError") =>
ValidationError(fields("reason").convertTo[String])
case JsObject(fields) if fields("error") == JsString("DatabaseError") =>
DatabaseError(fields("reason").convertTo[String])
}
}
val validationError: Errors = ValidationError("error")
val databaseError: Errors = DatabaseError("error")
val entityNotFound: Errors = EntityNotFound
assert(validationError.toJson.convertTo[Errors] == validationError)
assert(databaseError.toJson.convertTo[Errors] == databaseError)
assert(entityNotFound.toJson.convertTo[Errors] == entityNotFound)