Add collection transfer logic to beatmap import-as-update flow

osu.Game/Beatmaps/BeatmapImporter.cs

@@ -14,6 +14,7 @@ using osu.Framework.Logging;
 using osu.Framework.Platform;
 using osu.Framework.Testing;
 using osu.Game.Beatmaps.Formats;
+using osu.Game.Collections;
 using osu.Game.Database;
 using osu.Game.Extensions;
 using osu.Game.IO;
@@ -71,6 +72,8 @@ namespace osu.Game.Beatmaps
                 // Transfer local values which should be persisted across a beatmap update.
                 updated.DateAdded = original.DateAdded;
 
+                transferCollectionReferences(realm, original, updated);
+
                 foreach (var beatmap in original.Beatmaps.ToArray())
                 {
                     var updatedBeatmap = updated.Beatmaps.FirstOrDefault(b => b.Hash == beatmap.Hash);
@@ -112,6 +115,40 @@ namespace osu.Game.Beatmaps
             return first;
         }
 
+        private static void transferCollectionReferences(Realm realm, BeatmapSetInfo original, BeatmapSetInfo updated)
+        {
+            // First check if every beatmap in the original set is in any collections.
+            // In this case, we will assume they also want any newly added difficulties added to the collection.
+            foreach (var c in realm.All<BeatmapCollection>())
+            {
+                if (original.Beatmaps.Select(b => b.MD5Hash).All(c.BeatmapMD5Hashes.Contains))
+                {
+                    foreach (var b in original.Beatmaps)
+                        c.BeatmapMD5Hashes.Remove(b.MD5Hash);
+
+                    foreach (var b in updated.Beatmaps)
+                        c.BeatmapMD5Hashes.Add(b.MD5Hash);
+                }
+            }
+
+            // Handle collections using permissive difficulty name to track difficulties.
+            foreach (var originalBeatmap in original.Beatmaps)
+            {
+                var updatedBeatmap = updated.Beatmaps.FirstOrDefault(b => b.DifficultyName == originalBeatmap.DifficultyName);
+
+                if (updatedBeatmap == null)
+                    continue;
+
+                var collections = realm.All<BeatmapCollection>().AsEnumerable().Where(c => c.BeatmapMD5Hashes.Contains(originalBeatmap.MD5Hash));
+
+                foreach (var c in collections)
+                {
+                    c.BeatmapMD5Hashes.Remove(originalBeatmap.MD5Hash);
+                    c.BeatmapMD5Hashes.Add(updatedBeatmap.MD5Hash);
+                }
+            }
+        }
+
         protected override bool ShouldDeleteArchive(string path) => Path.GetExtension(path).ToLowerInvariant() == ".osz";
 
         protected override void Populate(BeatmapSetInfo beatmapSet, ArchiveReader? archive, Realm realm, CancellationToken cancellationToken = default)