batch_fetchable_manual.scala

def generateNotifications(activity: Seq[Event]): Seq[BatchFetchable[String]] = {
  activity.map {
    case NewPost(userId, groupId, postId) => BatchFetchable[String](
        keys = Keys(Set(userId), Set(groupId), Set(postId), Set()),
        entities => 
          val user = entities.users(userId)
          val group = entities.groups(groupId)
          val post = entities.posts(postId)
          s"${user.fullName} shared ${post.title} with ${group.name}"
      )
    case DirectMessage(fromId, toId, msgId) => BatchFetchable[String](
        keys = Keys(Set(fromId, toId), Set(), Set(), Set(msgId)),
        entities =>
          val (from, to) = (entities.users(fromId), entities.users(toId))
          val msg = entities.msgs(msgId)
          s"${from.fullName} said ${msg.text} to ${to.fullName}"
      )
  })
}