batch_prefetch.scala

case class NewPost(userId: Id[User], groupId: Id[Group], postId: Id[Post])
def generateNotifications(activity: Seq[NewPost]): Seq[String] = {
  val (allUsers, allGroups, allPosts) = {
    val usersIds = activity.map(_.userId).toSet
    val groupIds = activity.map(_.groupId).toSet
    val postIds = activity.map(_.postId).toSet
    (getManyUsersFromDb(userIds), getManyGroupsFromDb(groupIds), getManyPostsFromDb(postIds))
  }
  activity.map {
    case NewPost(userId, groupId, postId) =>
      val user = allUsers(userId)
      val group = allGroups(libraryId)
      val post = allPosts(postId)
      s"${user.fullName} shared ${post.title} with ${group.name}"
  }
}