improve multi-episode grouping logic (#2744)

CHANGELOG.md

@@ -63,6 +63,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Fix startup error caused by duplicate smart collection names (and no longer allow duplicate smart collection names)
 - Fix erroneous downgrade health check failure with some installations that use MariaDB
 - Sequential schedules: fix `count` instruction validation to accept integer (constant) or string (expression)
+- Fix multi-part episode grouping logic so that it does NOT require release date metadata for episodes within a single show
+  - When **Treat Collections As Shows** is enabled (i.e. for crossover episodes) release date metadata is required for proper grouping
 
 ### Changed
 - No longer round framerate to nearest integer when normalizing framerate

ErsatzTV.Core/Scheduling/MultiPartEpisodeGrouper.cs

@@ -40,9 +40,17 @@ public static partial class MultiPartEpisodeGrouper
                 groups.Add(new GroupedMediaItem(item, null));
             }
 
+            // when treating multiple shows as a single "show" - we need to
+            // first sort by release date before looking for part 1, part 2, etc. to group
+            // otherwise, we can simply sort by season, episode number
+            // which does not require release date metadata
+            IComparer<MediaItem> comparer = treatCollectionsAsShows
+                ? new ChronologicalMediaComparer()
+                : new SeasonEpisodeMediaComparer();
+
             IEnumerable<Episode> sortedEpisodes = showId.Match(
                 id => episodes.Filter(e => e.Season.ShowId == id),
-                () => episodes).OrderBy(identity, new ChronologicalMediaComparer());
+                () => episodes).OrderBy(identity, comparer);
 
             foreach (Episode episode in sortedEpisodes)
             {