broken_complex_batch_prefetch.scala

sealed trait Event
case class NewPost(userId: Id[User], groupId: Id[Group], postId: Id[Post]) extends Event
case class NewMessage(from: Id[User], to: Id[User], msgId: Id[Message]) extends Event

def generateNotifications(activity: Seq[Event]): Seq[String] = {
  val (allUsers, allGroups, allPosts, allMsgs) = {
    val userIds: Set[Id[User]] = activity.collect {
      case NewPost(userId, _, _) => userId
      case NewMessage(userId, _, _) => userId
    }.toSet
    val groupIds: Set[Id[Group]] = activity.collect {
      case NewPost(_, groupId, _) => groupId
    }.toSet
    val postIds: Set[Id[Post]] = activity.collect {
      case NewPost(_, _, postId) => postId
    }.toSet
    val msgIds: Set[Id[Message]] = activity.collect {
      case NewMessage(_, _, msgId) => msgId
    }.toSet
    (getManyUsersFromDb(userIds),
     getManyGroupsFromDb(groupIds),
     getManyPostsFromDb(postIds),
     getManyMsgsFromDb(msgIds))
  }
  activity.map {
    case NewPost(userId, groupId, postId) =>
      val user = allUsers(userId)
      val group = allGroups(groupId)
      val post = allPosts(postId)
      s"${user.fullName} shared ${post.title} with ${group.name}"
    case NewMessage(fromId, toId, msgId) =>
      val (from, to) = (allUsers(fromId), allUsers(toId))
      val msg = allMsgs(msgId)
      s"${from.fullName} said {msg.text} to ${to.fullName}"
  }
}