How to convert Seq[Object] to Map[User, Set[String] in Scala - scala

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 UserCustomFeatures 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