RealmSwift - Getting old value in a RealmCollection's observe?

[mefuzar]

Hello, I have a: "Group" model which contains: ObjectId List of children:

class Group: Object {
    @Persisted var childrenIds: List<ObjectId>
}

I have defined a RealmCollection observer:

var groupsObserver = realm.objects(Group.self).observe { (changes: RealmCollectionChange) in
    switch changes {
    case .update(let groups, let deletions, let insertions, let modifications):
        let modifiedGroups = modifications.map { groups[$0] }

        for modifiedGroup in modifiedGroups {
                 // Here I would like to get the childrenIds value BEFORE the change
                 // modifiedGroup.childrenIds.oldValue   
         }
     // ...
     }
}

I would like to access the old: "childrenIds" value in order to compare with the newValue on a value changed.
I know that the RealmCollection observer doesn't have a: oldValue property, so I tried to define a: a Key Path Change Listener but since I need it for every Group object that is in memory - i tried to setup the Listener on the Group's init method but then I get the following crash exception:
*** Terminating app due to uncaught exception 'RLMException', reason: 'Only objects which are managed by a Realm support change notifications'

Any ideas how to approach this problem?