tech_blog_20220405_6.scala

val startTime = System.currentTimeMillis
val groupMap: Map[UserId, Seq[Group]] =
  groupMemberSeq.groupBy(_.userId) map { case (userId, members) =>
    (
      userId,
      members.flatMap(member =>
        groupSeq.find(_.id == member.groupId)
      )
    )
}
val result: Seq[UserView] = userSeq.map(user =>
  UserView(
    user.id,
    user.name,
    groupMap(user.id)
  )
)
println((System.currentTimeMillis - startTime).toString + "ミリ秒")