Mapped projection with <> to a case class with companion object in Slick

Question

With Slick, I am trying to project database table entries directly to the case class they represent. Following the example in the documentation, I set up a mapped projection usi

Answer 1

Companion objects of case classes usually are a function from the case class' first argument list to the case class. So if you had

case class Fnord(a: A, b: B, c: C)(d: D)

the Scala compiler would autogenerate the companion object similar to

object Fnord extends ((A, B, C) => Fnord) {
  ...
}

Now, as soon as you explicitly spell out something about the companion object yourself, the compiler no longer generates the FunctionN extending thingy. Thus, most of the time it is a good idea to add it yourself. In your case that would mean defining the companion of SomeEntity3 like so:

object SomeEntity3 extends ((Int, Int, Int) => SomeEntity3) {
  ...
}

There's a (long open) issue for this behaviour, too: https://issues.scala-lang.org/browse/SI-3664