OnlyOnePrimary.scala

import shapeless.{HList, LabelledGeneric, Poly1, Witness}
import shapeless.labelled.{FieldType, field}
import shapeless.ops.hlist
import shapeless.tag.@@

trait Primary {
  val primary: Option[Boolean]
}

object OnlyOnePrimary {

  sealed trait LowerPriorityFieldMapper extends Poly1 {

    implicit def c[K, V]: Case.Aux[FieldType[K, V], FieldType[K, V]] =
      at[FieldType[K, V]](identity)

  }

  object unflagPrimary extends LowerPriorityFieldMapper {

    type PrimaryKey = Symbol @@ Witness.`"primary"`.T

    implicit val primary: Case.Aux[FieldType[PrimaryKey, Option[Boolean]], FieldType[PrimaryKey, Option[Boolean]]] =
      at[FieldType[PrimaryKey, Option[Boolean]]] { _ =>
        field[PrimaryKey](Some(false))
      }

  }

  def apply[A <: Primary, Repr <: HList](
    list: List[A]
  )(implicit generic: LabelledGeneric.Aux[A, Repr],
    mapper: hlist.Mapper.Aux[unflagPrimary.type, Repr, Repr],
  ): List[A] =
    list match {
      case Nil => Nil
      case head :: tail if head.primary.exists(identity) =>
        head :: tail.map { elem =>
          generic.from(generic.to(elem).map(unflagPrimary))
        }
      case head :: tail => head :: apply(tail)
    }

}