Slick mapped tables using traits throws exception - scala

Question

I'm stuck trying to define mapped tables using traits in slick 3.1.0. Since there is nothing mentioned in the official docs, I'm not even sure whether this is possible or not. Here is what I have so far:
Table definition:
class PersonTable(tag: Tag) extends Table[PersonModel](tag, "person") {
def id = column[Int]("id", O.PrimaryKey, O.AutoInc)
def firstName = column[String]("first_name", O.Length(PersonDb.FirstNameColumnLength))
def lastName = column[String]("last_name", O.Length(PersonDb.LastNameColumnLength))
def * = (id.?, firstName, lastName) <> (PersonModelImpl.tupled, PersonModelImpl.unapply _)
}
PersonModel:
trait PersonModel {
def id: Option[Int]
def firstName: String
def lastName: String
}
PersonModelImpl:
case class PersonModelImpl(
override val id: Option[Int],
override val firstName: String,
override val lastName: String)
extends PersonModel
Compiling the code above causes an error:
Compilation error[type mismatch;
found : slick.lifted.MappedProjection[models.PersonModelImpl,(Option[Int], String, String]
required: slick.lifted.ProvenShape[models.PersonModel]]
However changing ...extends Table[PersonModel]... to ...extends Table[PersonModelImpl]... in the table definition works flawlessly.
So basically my question is:
Is it possible to use traits as TableElementType in mapped tables?
If yes, what am I doing wrong?

Answer 1

Answers:
Yes, but it requires having the correct projection function (*).
You have the wrong type on *. In order for the implicit to resolve to the right MappedProjection, the types have to match exactly.
You should be able to resolve both by doing:
def * = {
val applyAsPersonModel: (Option[Int], String, String) => PersonModel =
(PersonModelImpl.apply _)
val unapplyAsPersonModel: PersonModel => Option[(Option[Int], String, String)] =
{
// TODO: Handle any other subclasses of PersonModel you expect.
case personImpl: PersonModelImpl => PersonModelImpl.unapply(personImpl)
}
(id.?, firstName, lastName) <> (applyAsPersonModel.tupled, unapplyAsPersonModel)
}
Note the TODO. You'll get an exception if you try to insert any non-PersonModelImpl instances unless you add additional case statements to that partial function. Alternatively, you can just create a new PersonModelImpl instance to pass to unapply:
val unapplyAsPersonModel: PersonModel => Option[(Option[Int], String, String)] =
{ person: PersonModel =>
PersonModelImpl.unapply(
PersonModelImpl(person.id, person.firstName, person.lastName)
)
}