Mapping an ADT to multiple columns in Slick

Question

I'm trying to map an ADT (case classes inheriting from a sealed trait) as multiple columns of the same table, using Slick, something like:

sealed trait OrValue
case class IntValue(value: Int)
case class StringValue(value: String)

case class Thing(id: Int, value: OrValue)

class Things(tag: Tag) extends Table[Thing](tag, "things") {
  def id = column[Int]("id", O.PrimaryKey)
  def intValue = column[Option[Int]]("intValue")
  def stringValue = column[Option[String]]("stringValue")

  def toThing(_id: String, _intValue: Option[Int], _stringValue: Option[String]): Thing = Thing(id, ((_intValue, _stringValue) match {
    case (Some(a), None) => IntValue(a)
    case (None, Some(a)) => StringValue(a)
  }

  def fromThing(t: Thing): (String, Option[Int], Option[String]) = ??? // elided

  def * = (id, intValue, stringValue) <> ((toThing _).tupled, fromThing)
}

This is not compling:

[error]  found   : [U]slick.lifted.MappedProjection[Thing,U]
[error]  required: slick.lifted.ProvenShape[Thing]
[error]            ) <> ((toThing _).tupled, fromThing _)

Am I approaching this the wrong way? What's the idiomatic way of representing an ADT?

Answer 1

The problem is in signature of methods toThing and fromThing.

It should look so:

sealed trait OrValue
case class IntValue(value: Int)
case class StringValue(value: String)

case class Thing(id: Int, value: OrValue)

class Things(tag: Tag) extends Table[Thing](tag, "things") {
  def id = column[Int]("id", O.PrimaryKey)
  def intValue = column[Option[Int]]("intValue")
  def stringValue = column[Option[String]]("stringValue")

  def toThing(source: (Int, Option[Int], Option[String])): Thing = source match {
    case (id, intValue, stringValue) => ??? //TODO implement
  }
  def fromThing(t: Thing): Option[(Int, Option[Int], Option[String])] = ??? //TODO implement

  def * = (id, intValue, stringValue) <> (toThing _, fromThing _)
}