How to add generic versioning to Lift's MongoDB classes.

Content.scala

/**
 * Carries some page content.
 */
class Content extends MongoRecord[Content]
  with ObjectIdPk[Content]
  with Versioned[Content, ContentRevision] {
  def meta = Content
  def revisionMeta = ContentRevision
  // ...
}

ContentRevision.scala

/**
 * Stores revisions of the Content class.
 */
class ContentRevision
  extends Revision[Content, ContentRevision]
  with MongoRecord[ContentRevision]
  with ObjectIdPk[ContentRevision] {
  override def meta = ContentRevision
  override def versionedMeta = Content
}

object ContentRevision
  extends ContentRevision
  with MongoMetaRecord[ContentRevision] {}

Revision.scala

/**
 * Represents a revision of a certain versioned class (e.g., a ContentRevision).
 * Thanks to Tim Nelson for help with the Manifest parameter.
 */
abstract class Revision[T <: MongoRecord[T], TREV <: MongoRecord[TREV]](implicit mf: Manifest[T]) {
  self: MongoRecord[TREV] =>
  def versionedMeta: T with MongoMetaRecord[T]

  /** The object that this is a revision of */
  object of extends ObjectIdRefField(this.asInstanceOf[TREV], versionedMeta)
  /** The date when the revision was created */
  object created extends DateField(this.asInstanceOf[TREV]) {
    override def defaultValue = new Date()
  }
  /** The actual data of the archived object */
  object data extends BsonRecordField[TREV, T](this.asInstanceOf[TREV], versionedMeta)
}

Versioned.scala

/**
 * This is needed so that we can call super.save() from Versioned.
 */
trait PreVersioned[T <: MongoRecord[T], TREV <: MongoRecord[TREV] with ObjectIdPk[TREV] with Revision[T, TREV]] {
  def save: T
}

/**
 * Represents a class that is versioned, i.e., has revisions.
 */
trait Versioned[T <: MongoRecord[T], TREV <: MongoRecord[TREV] with ObjectIdPk[TREV] with Revision[T, TREV]]
  extends PreVersioned[T, TREV]
  with Loggable {
  self: MongoRecord[T] with ObjectIdPk[T] =>

  /**
   * To be overridden: points to the meta object of the class that we
   * want to have versioned.
   */
  def revisionMeta: TREV with MongoMetaRecord[TREV]

  /**
   * Store a version of this object in the history before save.
   */
  abstract override def save = {
    logger.debug("before saving " + id.is)
    meta.find(id.is).foreach(obj => {
      tryo {
        val hist = revisionMeta.createRecord.of(id.is).data(obj).asInstanceOf[TREV].save
        hist
      } match {
        case Failure(msg, _, _) =>
          logger.error("error while creating history item for " + id.is + ": " + msg)
        case _ =>
          logger.info("created history item for " + id.is)
      }
    })
    super.save
  }
}