如何在Scala中将Seq [Object]转换为Map [User，Set [String]

Question

It's really hard to explain in the title but here's what I want to do.

I'm pretty new to Scala. I have an object User which is just a user that two users can be equal given the same user id

case class UserCustomFeature(
  hobby: String,
  users: Set[User]
) {}

My input is Seq[UserCustomFeature] So basically a list of objects of a hobby -> users. For example,

[('tv' -> Set('user1', 'user2')),
('swimming' -> Set('user2', 'user3'))]

And I want the result to be

('user1' -> Set('tv')),
('user2' -> Set('tv', 'swimming')),
('user3' -> Set('swimming'))

I have something like this so far but I'm not sure how to group them later

userHobbyMap
      .map({
        case (hobby, users) => {
          users.map(user => {
            (user, hobby)
          })
        }
      })

Answer 1

case class User(id: String)
case class UserCustomFeature(
  hobby: String,
  users: Set[User]
) {}

val input = Seq(
  UserCustomFeature("tv", Set(User("1"), User("2"))),
  UserCustomFeature("swimming", Set(User("2"), User("3")))
)

val output = (for (UserCustomFeature(h, us) <- input; u <- us) yield (u, h))
  .groupBy(_._1)
  .mapValues(_.map(_._2).toSet)

output foreach println

Generates output:

(User(1),Set(tv))
(User(3),Set(swimming))
(User(2),Set(tv, swimming))

Brief explanation:

• for -comprehension transposes the sequence of UserCustomFeature s into a list of (user, hobby) pairs.
• groupBy groups hobbies by user (first component)
• The map(_._2) drops the redundant user id from grouped pairs
• toSet converts the resulting list of hobbies to a set of hobbies