Address review on MediaSourceCount batching

Rename GetItemsWithAlternateVersions to GetItemIdsWithAlternateVersions
across the interfaces and implementations since it returns ids. Return
the hashset straight from the query instead of materializing an array
first. Rename the DtoService guard to mayHaveAlternateVersions and
invert it so the computed path is the explicit case. Assert the media
source count value in the batch skip test and add a test covering an
item that is in the returned set still resolving to the correct count.

1 files changed, 44 insertions, 3 deletions

diff --git a/tests/Jellyfin.Server.Implementations.Tests/Dto/DtoServiceImageInheritanceTests.cs b/tests/Jellyfin.Server.Implementations.Tests/Dto/DtoServiceImageInheritanceTests.cs
index fa94250287..6a3dcab57a 100644
--- a/tests/Jellyfin.Server.Implementations.Tests/Dto/DtoServiceImageInheritanceTests.cs
+++ b/tests/Jellyfin.Server.Implementations.Tests/Dto/DtoServiceImageInheritanceTests.cs
@@ -222,11 +222,11 @@ public class DtoServiceImageInheritanceTests
         var libraryManager = new Mock<ILibraryManager>();
 
         // DtoService detects which videos own alternate versions in ONE batch
-        // (GetItemsWithAlternateVersions) before the per-item loop. Videos absent from that set have a
+        // (GetItemIdsWithAlternateVersions) before the per-item loop. Videos absent from that set have a
         // single media source, so the per-item GetLinkedAlternateVersions/GetLocalAlternateVersionIds
         // queries (the N+1) must be skipped entirely. Here neither movie has alternate versions.
         libraryManager
-            .Setup(x => x.GetItemsWithAlternateVersions(It.IsAny<IReadOnlyList<Guid>>()))
+            .Setup(x => x.GetItemIdsWithAlternateVersions(It.IsAny<IReadOnlyList<Guid>>()))
             .Returns(new HashSet<Guid>());
 
         var dtoService = BuildDtoService(libraryManager);
@@ -236,13 +236,54 @@ public class DtoServiceImageInheritanceTests
 
         Assert.Equal(2, dtos.Count);
 
+        // A single media source is the default, so the count is left unset (the client treats null as one).
+        foreach (var dto in dtos)
+        {
+            Assert.Null(dto.MediaSourceCount);
+        }
+
         // The alternate-version check is batched once for the whole set, and the per-item lookups are
         // never reached because the batch already ruled out alternate versions.
-        libraryManager.Verify(x => x.GetItemsWithAlternateVersions(It.IsAny<IReadOnlyList<Guid>>()), Times.Once);
+        libraryManager.Verify(x => x.GetItemIdsWithAlternateVersions(It.IsAny<IReadOnlyList<Guid>>()), Times.Once);
         libraryManager.Verify(x => x.GetLinkedAlternateVersions(It.IsAny<Video>()), Times.Never);
         libraryManager.Verify(x => x.GetLocalAlternateVersionIds(It.IsAny<Video>()), Times.Never);
     }
 
+    [Fact]
+    public void GetBaseItemDtos_VideoInAlternateVersionBatch_ResolvesRealCount()
+    {
+        var movie = new Movie
+        {
+            Id = Guid.NewGuid(),
+            Name = "Movie",
+            ImageInfos = []
+        };
+
+        var libraryManager = new Mock<ILibraryManager>();
+
+        // This movie IS in the batch set, so the fast path must not short-circuit it: the per-item
+        // lookups still run and the count is computed exactly as it was before batching. Two linked
+        // alternate versions plus the movie itself is a count of three.
+        libraryManager
+            .Setup(x => x.GetItemIdsWithAlternateVersions(It.IsAny<IReadOnlyList<Guid>>()))
+            .Returns(new HashSet<Guid> { movie.Id });
+        libraryManager
+            .Setup(x => x.GetLinkedAlternateVersions(It.IsAny<Video>()))
+            .Returns([new Movie { Id = Guid.NewGuid() }, new Movie { Id = Guid.NewGuid() }]);
+        libraryManager
+            .Setup(x => x.GetLocalAlternateVersionIds(It.IsAny<Video>()))
+            .Returns([]);
+
+        var dtoService = BuildDtoService(libraryManager);
+
+        var options = new DtoOptions(false) { Fields = [ItemFields.MediaSourceCount] };
+        var dtos = dtoService.GetBaseItemDtos([movie], options);
+
+        Assert.Single(dtos);
+        Assert.Equal(3, dtos[0].MediaSourceCount);
+        libraryManager.Verify(x => x.GetItemIdsWithAlternateVersions(It.IsAny<IReadOnlyList<Guid>>()), Times.Once);
+    }
+
     private static DtoService BuildDtoService(BaseItem displayParent)
     {
         var libraryManager = new Mock<ILibraryManager>();