playframework 2.3 - 22 fields limit in Scala 2.11 + Play Framework 2.3 Case classes and functions

Question

Scala 2.11 is out and the 22 fields limit for case classes seems to be fixed (Scala Issue, Release Notes).

This has been an issue for me for a while because I use case classes to model database entities that have more than 22 fields in Play + Postgres Async. My solution in Scala 2.10 was to break the models into multiple case classes, but I find this solution hard to maintain and extend, and I was hoping I could implement something as described below after switching to Play 2.3.0-RC1 + Scala 2.11.0:

package entities

case class MyDbEntity(
  id: String,
  field1: String,
  field2: Boolean,
  field3: String,
  field4: String,
  field5: String,
  field6: String,
  field7: String,
  field8: String,
  field9: String,
  field10: String,
  field11: String,
  field12: String,
  field13: String,
  field14: String,
  field15: String,
  field16: String,
  field17: String,
  field18: String,
  field19: String,
  field20: String,
  field21: String,
  field22: String,
  field23: String,
) 

object MyDbEntity {
  import play.api.libs.json.Json
  import play.api.data._
  import play.api.data.Forms._

  implicit val entityReads = Json.reads[MyDbEntity]
  implicit val entityWrites = Json.writes[MyDbEntity]
}

The code above fails to compile with the following message for both the "Reads" and the "Writes":

No unapply function found

Updating the "Reads" and "Writes" to:

  implicit val entityReads: Reads[MyDbEntity] = (
    (__  "id").read[Long] and
    (__  "field_1").read[String]
    ........
  )(MyDbEntity.apply _)  

  implicit val postWrites: Writes[MyDbEntity] = (
    (__  "id").write[Long] and
    (__  "user").write[String]
    ........
  )(unlift(MyDbEntity.unapply))

Also doesn't work:

  implementation restricts functions to 22 parameters

  value unapply is not a member of object models.MyDbEntity

My understanding is that Scala 2.11 still has some limitations on functions and that something like what I described above is not possible yet. This seems weird to me as I don't see the benefit of lifting the restrictions on case classes if one it's major users cases is still not supported, so I'm wondering if I'm missing something.

Pointers to issues or implementation details are more than welcome! Thanks!

Answer 1

This is not possible, out of the box, for several reasons:

• First, as gourlaysama pointed it out, play-json library used scala macro to avoid bolierplate code, and the current code relies of the unapply and apply methods to retrieve fields. This explains the first error message in your question.

• Secondly play-json library relies of a functional library which currently works only with a fixed number of parameters corresponding to previous case class fields arity limit. This explains the second error message in your question.

However it is possible to bypass the second point by either:

• using shapeless Automatic Typeclass Derivation feature. Naveen Gattu has written an excellent gist doing exaclty so.

• overridding default functiunal builder

First, creating the missing FunctionalBuilder:

class CustomFunctionalBuilder[M[_]](canBuild: FunctionalCanBuild[M]) extends FunctionalBuilder {

    class CustomCanBuild22[A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21, A22](m1: M[A1 ~ A2 ~ A3 ~ A4 ~ A5 ~ A6 ~ A7 ~ A8 ~ A9 ~ A10 ~ A11 ~ A12 ~ A13 ~ A14 ~ A15 ~ A16 ~ A17 ~ A18 ~ A19 ~ A20 ~ A21], m2: M[A22]) {
def ~[A23](m3: M[A23]) = new CustomCanBuild23[A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21, A22, A23](canBuild(m1, m2), m3)

  def and[A23](m3: M[A23]) = this.~(m3)

  def apply[B](f: (A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21, A22) => B)(implicit fu: Functor[M]): M[B] =
  fu.fmap[A1 ~ A2 ~ A3 ~ A4 ~ A5 ~ A6 ~ A7 ~ A8 ~ A9 ~ A10 ~ A11 ~ A12 ~ A13 ~ A14 ~ A15 ~ A16 ~ A17 ~ A18 ~ A19 ~ A20 ~ A21 ~ A22, B](canBuild(m1, m2), { case a1 ~ a2 ~ a3 ~ a4 ~ a5 ~ a6 ~ a7 ~ a8 ~ a9 ~ a10 ~ a11 ~ a12 ~ a13 ~ a14 ~ a15 ~ a16 ~ a17 ~ a18 ~ a19 ~ a20 ~ a21 ~ a22 => f(a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18, a19, a20, a21, a22) })

  def apply[B](f: B => (A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21, A22))(implicit fu: ContravariantFunctor[M]): M[B] =
  fu.contramap(canBuild(m1, m2), (b: B) => { val (a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18, a19, a20, a21, a22) = f(b); new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(a1, a2), a3), a4), a5), a6), a7), a8), a9), a10), a11), a12), a13), a14), a15), a16), a17), a18), a19), a20), a21), a22) })

  def apply[B](f1: (A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21, A22) => B, f2: B => (A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21, A22))(implicit fu: InvariantFunctor[M]): M[B] =
  fu.inmap[A1 ~ A2 ~ A3 ~ A4 ~ A5 ~ A6 ~ A7 ~ A8 ~ A9 ~ A10 ~ A11 ~ A12 ~ A13 ~ A14 ~ A15 ~ A16 ~ A17 ~ A18 ~ A19 ~ A20 ~ A21 ~ A22, B](
    canBuild(m1, m2), { case a1 ~ a2 ~ a3 ~ a4 ~ a5 ~ a6 ~ a7 ~ a8 ~ a9 ~ a10 ~ a11 ~ a12 ~ a13 ~ a14 ~ a15 ~ a16 ~ a17 ~ a18 ~ a19 ~ a20 ~ a21 ~ a22 => f1(a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18, a19, a20, a21, a22) },
    (b: B) => { val (a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18, a19, a20, a21, a22) = f2(b); new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(new ~(a1, a2), a3), a4), a5), a6), a7), a8), a9), a10), a11), a12), a13), a14), a15), a16), a17), a18), a19), a20), a21), a22) }
  )

  def join[A >: A1](implicit witness1: <:<[A, A1], witness2: <:<[A, A2], witness3: <:<[A, A3], witness4: <:<[A, A4], witness5: <:<[A, A5], witness6: <:<[A, A6], witness7: <:<[A, A7], witness8: <:<[A, A8], witness9: <:<[A, A9], witness10: <:<[A, A10], witness11: <:<[A, A11], witness12: <:<[A, A12], witness13: <:<[A, A13], witness14: <:<[A, A14], witness15: <:<[A, A15], witness16: <:<[A, A16], witness17: <:<[A, A17], witness18: <:<[A, A18], witness19: <:<[A, A19], witness20: <:<[A, A20], witness21: <:<[A, A21], witness22: <:<[A, A22], fu: ContravariantFunctor[M]): M[A] =
  apply[A]((a: A) => (a: A1, a: A2, a: A3, a: A4, a: A5, a: A6, a: A7, a: A8, a: A9, a: A10, a: A11, a: A12, a: A13, a: A14, a: A15, a: A16, a: A17, a: A18, a: A19, a: A20, a: A21, a: A22))(fu)

  def reduce[A >: A1, B](implicit witness1: <:<[A1, A], witness2: <:<[A2, A], witness3: <:<[A3, A], witness4: <:<[A4, A], witness5: <:<[A5, A], witness6: <:<[A6, A], witness7: <:<[A7, A], witness8: <:<[A8, A], witness9: <:<[A9, A], witness10: <:<[A10, A], witness11: <:<[A11, A], witness12: <:<[A12, A], witness13: <:<[A13, A], witness14: <:<[A14, A], witness15: <:<[A15, A], witness16: <:<[A16, A], witness17: <:<[A17, A], witness18: <:<[A18, A], witness19: <:<[A19, A], witness20: <:<[A20, A], witness21: <:<[A21, A], witness22: <:<[A22, A], fu: Functor[M], reducer: Reducer[A, B]): M[B] =
  apply[B]((a1: A1, a2: A2, a3: A3, a4: A4, a5: A5, a6: A6, a7: A7, a8: A8, a9: A9, a10: A10, a11: A11, a12: A12, a13: A13, a14: A14, a15: A15, a16: A16, a17: A17, a18: A18, a19: A19, a20: A20, a21: A21, a22: A22) =>  reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.append(reducer.unit(a1: A), a2: A), a3: A), a4: A), a5: A), a6: A), a7: A), a8: A), a9: A), a10: A), a11: A), a12: A), a13: A), a14: A), a15: A), a16: A), a17: A), a18: A), a19: A), a20: A), a21: A), a22: A))(fu)

  def tupled(implicit v: VariantExtractor[M]): M[(A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21, A22)] =
  v match {
    case FunctorExtractor(fu) => apply { (a1: A1, a2: A2, a3: A3, a4: A4, a5: A5, a6: A6, a7: A7, a8: A8, a9: A9, a10: A10, a11: A11, a12: A12, a13: A13, a14: A14, a15: A15, a16: A16, a17: A17, a18: A18, a19: A19, a20: A20, a21: A21, a22: A22) => (a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18, a19, a20, a21, a22) }(fu)
    case ContravariantFunctorExtractor(fu) => apply[(A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21, A22)] { (a: (A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21, A22)) => (a._1, a._2, a._3, a._4, a._5, a._6, a._7, a._8, a._9, a._10, a._11, a._12, a._13, a._14, a._15, a._16, a._17, a._18, a._19, a._20, a._21, a._22) }(fu)
    case InvariantFunctorExtractor(fu) => apply[(A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21, A22)]({ (a1: A1, a2: A2, a3: A3, a4: A4, a5: A5, a6: A6, a7: A7, a8: A8, a9: A9, a10: A10, a11: A11, a12: A12, a13: A13, a14: A14, a15: A15, a16: A16, a17: A17, a18: A18, a19: A19, a20: A20, a21: A21, a22: A22) => (a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, a17, a18, a19, a20, a21, a22) }, { (a: (A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21, A22)) => (a._1, a._2, a._3, a._4, a._5, a._6, a._7, a._8, a._9, a._10, a._11, a._12, a._13, a._14, a._15, a._16, a._17, a._18, a._19, a._20, a._21, a._22) })(fu)
    }

  }

  class CustomCanBuild23[A1, A2, A3, A4, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14, A15, A16, A17, A18, A19, A20, A21, A22, A23](m1: M[A1 ~ A2 ~ A3 ~ A4 ~ A5 ~ A6 ~ A7 ~ A8 ~ A9 ~ A10 ~ A11 ~ A12 ~ A13 ~ A14 ~ A15 ~ A16 ~ A17 ~ A18 ~ A19 ~ A20 ~ A21 ~ A22], m2: M[A23]) {
  }

}

and then by providing your own FunctionalBuilderOps instance:

implicit def customToFunctionalBuilderOps[M[_], A](a: M[A])(implicit fcb: FunctionalCanBuild[M]) = new CustomFunctionalBuilderOps[M, A](a)(fcb)

Finally, regarding the first point, I have sent a pull request to try to simplify the current implementation.