Im trying to work with subcollection in the main object but when I try to loop in subcollections I get empty

Simplified I have this

struct Main: Identifiable, Decodable {
    @DocumentID var id: String?

    var title: String 
}

And the subcollection

struct SubCollection: Identifiable, Decodable {
    @DocumentID var id: String?

    var subCollectiontitle: String 
}

When I initialize the viewModel I use

@Published var mains = [Main]()

init() {
      fetchMain()
}

Now how is the proper way to get the Subs of a main ?

When I use this on a method

func fetchSubs() -> [SubCollection] {
  var subs = [SubCollection] ()
        for main: Main in self.mains {
            if let id = main.id {
                Firestore.firestore().collection("mains").document(id).collection("sub-collection").getDocuments { snapshot, _ in
                    guard let documents = snapshot?.documents else { return }
                    subs = documents.compactMap({ try! $0.data(as: SubCollection.self)})
                }
            }
        }

        return subs
    }

Always is empty, but when I do it in the init gets the data