Mapper inheritance

FilmTranslation.scala

import scala.xml.Text
import scala.annotation.tailrec
import net.liftweb.mapper._
import net.liftweb.sitemap._
import net.liftweb.common._
import net.liftweb.http._
import net.liftweb.util.Helpers._
import net.liftweb.sitemap.Menu.ParamExtractor
import eu.vismath.web.lib._

/*
 * This class inherits Translation and passes in its own type as a
 *  type parameter. It will have all fields that the translation class
 *  has, and it can add some new fields. There will be a table for this
 *  class, but not for the superclass, i.e. it is non-trivial to find
 *  "all other XyzTranslations" in the database.
 */
class FilmTranslation extends Translation[FilmTranslation, Film](Film)
                      with LongKeyedMapper[FilmTranslation] with IdPK
                      with ManyToMany {
  override def getSingleton = FilmTranslation

  object title extends MappedString(this, 100)
  object subtitle extends MappedString(this, 256)
}

object FilmTranslation extends FilmTranslation with LongKeyedMetaMapper[FilmTranslation] with CRUDify[Long, FilmTranslation] {
  override def dbIndexes = indexes ++ super.dbIndexes
}

Translation.scala

import net.liftweb.mapper._
import net.liftweb.common._
import net.liftweb.http.S

/*
 * This class is the base class for FilmTranslation, BookTranslation, ...
 *  It does *not* inherit LongKeyedMapper and there will *not* be a table
 *  for this class, but it will add common fields to all classes that
 *  inherit it.
 * The "secret" is that we pass in the "Owner" class as a type parameter
 *  and then can say MappedString(this.asInstanceOf[Owner], 100).
 */
abstract class Translation[Owner<:LongKeyedMapper[Owner], T<:LongKeyedMapper[T]](_foreignMeta: => LongKeyedMetaMapper[T]) {
  def getSingleton : LongKeyedMetaMapper[Owner]

  object translates extends MappedLongForeignKey(this.asInstanceOf[Owner], _foreignMeta) {
    override def validSelectValues =
      Full(_foreignMeta.findAll.map(p => (p.primaryKeyField.is, p.toString)))
  }
  object locale extends MappedLocale(this.asInstanceOf[Owner])
  object slug extends MappedString(this.asInstanceOf[Owner], 100)

  // slug and translated object are unique per language
  protected def indexes : List[BaseIndex[Owner]] =
    UniqueIndex(List(IndexField(translates), IndexField(locale))) ::
    UniqueIndex(List(IndexField(slug), IndexField(locale))) ::
    Nil

  // find translated objects from their slug
  protected def finder(param : String) : Box[Owner] = {
    getSingleton.find(By(slug, param),
                      By(locale, S.locale.getLanguage))
  }
}