Use DistinctBy where possible

3 files changed, 18 insertions, 29 deletions

diff --git a/Jellyfin.Server.Implementations/Item/BaseItemRepository.ByName.cs b/Jellyfin.Server.Implementations/Item/BaseItemRepository.ByName.cs
index 24892fef92..8e917f6951 100644
--- a/Jellyfin.Server.Implementations/Item/BaseItemRepository.ByName.cs
+++ b/Jellyfin.Server.Implementations/Item/BaseItemRepository.ByName.cs
@@ -208,7 +208,7 @@ public sealed partial class BaseItemRepository
         if (isMusicArtist)
         {
             // For MusicArtist, prefer the entity from a library the user can actually access.
-            // Materilaize to prevent correlated per-group first-row queries which hurt performance.
+            // Materialize to prevent correlated per-group first-row queries which hurt performance.
             var topParentIds = filter.TopParentIds;
             representativeIds = masterQuery
                 .Select(e => new { e.Id, e.PresentationUniqueKey, e.TopParentId })
diff --git a/Jellyfin.Server.Implementations/Item/BaseItemRepository.Querying.cs b/Jellyfin.Server.Implementations/Item/BaseItemRepository.Querying.cs
index 00af0bcbeb..1ff8d8f863 100644
--- a/Jellyfin.Server.Implementations/Item/BaseItemRepository.Querying.cs
+++ b/Jellyfin.Server.Implementations/Item/BaseItemRepository.Querying.cs
@@ -134,19 +134,13 @@ public sealed partial class BaseItemRepository
                 .ThenByDescending(e => e.Id)
                 .Select(e => new { e.Id, e.PresentationUniqueKey });
 
-            var seenKeys = new HashSet<string>();
-            var firstIds = new List<Guid>(limit ?? 0);
-            foreach (var row in orderedIds.AsEnumerable())
-            {
-                if (seenKeys.Add(row.PresentationUniqueKey!))
-                {
-                    firstIds.Add(row.Id);
-                    if (limit.HasValue && firstIds.Count >= limit.Value)
-                    {
-                        break;
-                    }
-                }
-            }
+            // DistinctBy and Take are lazy, so enumeration stops as soon as limit distinct keys are read.
+            var firstIds = orderedIds
+                .AsEnumerable()
+                .DistinctBy(row => row.PresentationUniqueKey)
+                .Select(row => row.Id)
+                .Take(limit ?? int.MaxValue)
+                .ToList();
 
             return LoadLatestByIds(context, firstIds, filter);
         }
diff --git a/Jellyfin.Server.Implementations/Item/NextUpService.cs b/Jellyfin.Server.Implementations/Item/NextUpService.cs
index 725b4cfaac..f478daef23 100644
--- a/Jellyfin.Server.Implementations/Item/NextUpService.cs
+++ b/Jellyfin.Server.Implementations/Item/NextUpService.cs
@@ -98,7 +98,7 @@ public class NextUpService : INextUpService
             .Where(e => e.UserData!.Any(ud => ud.UserId == userId && ud.Played));
         lastWatchedBase = _queryHelpers.ApplyAccessFiltering(context, lastWatchedBase, filter);
 
-        // Use lightweight projection + client-side grouping to avoid correlated scalar subquery
+        // Use lightweight projection + client-side dedup to avoid the correlated scalar subquery
         // per group that EF generates for GroupBy+OrderByDescending+FirstOrDefault.
         var allPlayedLite = lastWatchedBase
             .Select(e => new
@@ -110,15 +110,11 @@ public class NextUpService : INextUpService
             })
             .ToList();
 
-        var lastWatchedInfo = new Dictionary<string, Guid>();
-        foreach (var group in allPlayedLite.GroupBy(e => e.SeriesPresentationUniqueKey))
-        {
-            var lastWatched = group
-                .OrderByDescending(e => e.ParentIndexNumber)
-                .ThenByDescending(e => e.IndexNumber)
-                .First();
-            lastWatchedInfo[group.Key!] = lastWatched.Id;
-        }
+        var lastWatchedInfo = allPlayedLite
+            .OrderByDescending(e => e.ParentIndexNumber)
+            .ThenByDescending(e => e.IndexNumber)
+            .DistinctBy(e => e.SeriesPresentationUniqueKey)
+            .ToDictionary(e => e.SeriesPresentationUniqueKey!, e => e.Id);
 
         Dictionary<string, Guid> lastWatchedByDateInfo = new();
         if (includeWatchedForRewatching)
@@ -144,11 +140,10 @@ public class NextUpService : INextUpService
                     (e, ud) => new { EpisodeId = e.Id, e.SeriesPresentationUniqueKey, ud.LastPlayedDate })
                 .ToList();
 
-            foreach (var group in playedWithDates.GroupBy(x => x.SeriesPresentationUniqueKey))
-            {
-                var mostRecent = group.OrderByDescending(x => x.LastPlayedDate).First();
-                lastWatchedByDateInfo[group.Key!] = mostRecent.EpisodeId;
-            }
+            lastWatchedByDateInfo = playedWithDates
+                .OrderByDescending(x => x.LastPlayedDate)
+                .DistinctBy(x => x.SeriesPresentationUniqueKey)
+                .ToDictionary(x => x.SeriesPresentationUniqueKey!, x => x.EpisodeId);
         }
 
         var allLastWatchedIds = lastWatchedInfo.Values