Realm - How to delete objects without any associated objects?

Question

In my app I load stored data from Realm and then make a call to the API to fetch the most current state of user's objects. After that I filter which objects are new and which should be deleted, like that:

// tracks is an array of objects downloaded from API and passed to the method
    let result = realm.objects(RealmTrack.self)

    print("Discovering new tracks")
    // Discover new tracks
    let new = tracks.filter({ track in
        !result.contains(where: { $0.trackUid == track.trackUid })
    })

    print("Discovering old tracks")
    // Discover old tracks in database
    let old = result.filter({ track in
        !tracks.contains(where: { $0.trackUid == track.trackUid })
    })

Then I'm performing a write transaction to realm, it goes well without any errors.

    print("Modifying database")
        try realm.write {
            //realm.create(RealmTrack.self, value: realms, update: .modified)
            realm.add(realms, update: Realm.UpdatePolicy.modified)
            //realm.add(realms)
            realm.delete(old)
        }

My problem is that each RealmTrack can have other associated Realm objects:

dynamic var _primaryAlbum: RealmAlbum?

dynamic var _primaryArtist: RealmArtist?

And both RealmAlbum and RealmArtist have a list of RealmTrack objects:

let tracks = List<RealmTrack>()

After deleting old tracks, how can I get remaining RealmAlbum and RealmArtist which don't have any associated RealmTrack's with them? I have to delete them as well

What I tried is just after the write transaction block, check which objects have empty list, but surprisingly it's always 0:

//realm.objects(RealmAlbum.self).count always returns 0 here
        let invalidatedAlbums = realm.objects(RealmAlbum.self).filter {
            $0.tracks.count == 0
            //$0.isInvalidated == true
        }

        let invalidatedArtists = realm.objects(RealmArtist.self).filter {
            $0.tracks.count == 0
            //$0.isInvalidated == true
        }

        try realm.write {
            realm.delete(invalidatedAlbums)
            realm.delete(invalidatedArtists)
        }

EDIT: Added class declarations

RealmTrack

@objcMembers class RealmTrack: Object, AWTrack {

        dynamic var _source: String = ""
        var source: AWMediaSource {
            return AWMediaSource(rawValue: _source)!
        }

        dynamic var _trackName: String = ""
        var trackName: String {
            return _trackName
        }

        dynamic var _trackUid: String = ""
        var trackUid: String {
            return _trackUid
        }

        dynamic var _playableUrl: String? = ""
        var playableUrl: String? {
            return _playableUrl
        }

        dynamic var _duration: TimeInterval = 0
        var duration: TimeInterval {
            return _duration
        }

        dynamic var _albumTrackNumber: Int = 0
        var albumTrackNumber: Int {
            return _albumTrackNumber
        }

        dynamic var _localizedTitle: String = ""
        var localizedTitle: String {
            return _localizedTitle
        }

        dynamic var _localizedAlbumTitle: String = ""
        var localizedAlbumTitle: String {
            return _localizedAlbumTitle
        }

        dynamic var _localizedAlbumArtist: String = ""
        var localizedAlbumArtist: String {
            return _localizedAlbumArtist
        }

        dynamic var _localizedArtist: String = ""
        var localizedArtist: String {
            return _localizedArtist
        }

        dynamic var _albumUid: String = ""
        var albumUid: String {
            return _albumUid
        }

        dynamic var _artistUid: String = ""
        var artistUid: String {
            return _artistUid
        }

        dynamic var _primaryAlbum: RealmAlbum?
        var primaryAlbum: AWAlbum? {
            return _primaryAlbum
        }

        dynamic var _primaryArtist: RealmArtist?
        var primaryArtist: AWArtist? {
            return _primaryArtist
        }

        override class func primaryKey() -> String? {
            return "_trackUid"
        }

RealmAlbum

@objcMembers class RealmAlbum: Object, AWAlbum {

dynamic var _source: String = ""
var source: AWMediaSource {
    return AWMediaSource(rawValue: _source)!
}

dynamic var _albumName: String = ""
var albumName: String {
    return _albumName
}

dynamic var _albumUid: String = ""
var albumUid: String {
    return _albumUid
}

dynamic var _artistName: String = ""
var artistName: String {
    return _artistName
}

let tracks = List<RealmTrack>()

dynamic var _primaryArtist: RealmArtist?
var primaryArtist: AWArtist? {
    return _primaryArtist
}

override class func primaryKey() -> String? {
    return "_albumUid"
}

RealmArtist

@objcMembers class RealmArtist: Object, AWArtist {

dynamic var _source: String = ""
var source: AWMediaSource {
    return AWMediaSource(rawValue: _source)!
}

dynamic var _artistUid: String = ""
var artistUid: String {
    return _artistUid
}

dynamic var _artistName: String = ""
var artistName: String {
    return _artistName
}


let tracks = List<RealmTrack>()

override class func primaryKey() -> String? {
    return "_artistUid"
}

Answer 1

The full scope of the question is a bit unclear to me so I may not be fully understanding what is being asked so let me attempt to answer this question:

How can I query for objects that have nothing in a list property.

If you have an object that has a list property of other objects like this:

class PersonClass: Object {
   @objc dynamic var person_name = ""
   let dogs = List<DogClass>()
}

class DogClass: Object {
   @objc dynamic var dog_name = ""
}

and suppose you have a person with two dogs in their list and then a person with no dogs in their list. You goal is to query for all people that have no dogs in their list.

let personResults = realm.objects(PersonClass.self).filter("dogs.@count == 0")

@count is an aggregate expression supported on List and Results properties.