How can I use the new Slick 2.0 HList to overcome 22 column limit?

Question

I\'m currently writing Slick code to target an old schema with two tables > 22 columns. How do I use the new HList code? I\'ve got 2.0-M3 working fine in other respects under Sc

Answer 1

Definition

With Scala >= 2.10.4-RC2 (also emitted by the Slick 2.0.0 code generator):

import scala.slick.collection.heterogenous._
import syntax._
class Joiners(tag: Tag) extends Table[
    Int :: Option[String] :: Option[String] :: HNil
](tag, "joiner") {
  ...
  def * = id :: name :: contact :: HNil
}

The above leads to exponential compilation times in Scala 2.10.3 / 2.10.4-RC1. Not feasible for more than 26 columns due to extremely long compilation.

Workaround for Scala <= 2.10.3 / 2.10.4-RC1 (also emitted by the Slick 2.0.1 code generator)

import scala.slick.collection.heterogenous._
import syntax._
class Joiners(tag: Tag) extends Table[
    HCons[Int, HCons[Option[String], HCons[Option[String], HNil]]]
](tag, "joiner") {
  ...
  def * = id :: name :: contact :: HNil
}

Tested by us with 30-40 columns without problems.

There currently still seem to be a problem with occasional sporadic compilation errors in Scala 2.10.4-RC2, which looks like it will be fixed in the upcoming 2.10.4-RC3. See https://issues.scala-lang.org/browse/SI-8146

Example usage

Joiners.run.map( r => r(2) ) // Gets column contact. It's typesafe. .apply is a macro. Only works for literals not for variables as positions.

Use tuples for < 22 to be able to map them to a case class. Use HLists for > 22 without mapping to a case class (max field limit in Scala 2.10 is 22).

Also: Do NOT use O.Nullable. Use column[Option[String]] instead. It infers nullability.