tech_blog_20220405_5.scala

val result: Seq[UserView] = userSeq.map(user => { // ①ユーザー人数分回る
  val groupIdSeq: Seq[GroupId] =
    groupMemberSeq
      .filter(_.userId == user.id) // ②マッピング数分回る
      .map(_.groupId)
val userGroups: Seq[Group] =
    groupSeq.filter(group => // ③グループ数分回る
      groupIdSeq.contains(group.id)
    )
UserView(
    user.id,
    user.name,
    userGroups
  )
})