val likesByKey = likesStream
  .groupByKey
  .aggregate(
    () => Set.empty[Like],
    (_, like: Like, likes: Set[Like]) => if (like.unliked) likes - like else likes + like,
    Materialized.`with`[Id[Post], Set[Like], KeyValueStore[Bytes, Array[Byte]]](
      CirceSerdes.serde[Id[Post]],
      CirceSerdes.serde[Set[Like]]
    )
  )

val commentCountByKey = commentsStream
  .groupByKey
  .aggregate(
    () => 0,
    (_, comment: Comment, count: Int) => if (comment.deleted) count - 1 else count + 1,
    Materialized.`with`[Id[Post], Int, KeyValueStore[Bytes, Array[Byte]]](
      CirceSerdes.serde[Id[Post]],
      CirceSerdes.serde[Int]
    )
  )