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fix(671): review round 2 -- guard Song.Artists, cover the second consumer

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Cold independent review of 017ef988d. Three findings taken, one filed out.

The important one is a regression this branch INTRODUCED. `SongMetadata.Artists`
is a nullable EF primitive collection (JSON in one column, not a navigation)
that FallbackMetadataProvider leaves unassigned when a song's tags fail to read,
and `string.Join` throws ArgumentNullException on a null sequence. The rerun
list previously did not load SongMetadata at all, so the throw was unreachable
there; adding the include promoted it to a live 500 that would have failed the
whole page. Confirmed by reverting the guard: ArgumentNullException, parameter
'values'. The file header claiming every member was guarded was false.
The empty case is filtered too, so an artist-less song loses its bare " - ".

Second: `GetPlaylistItemsHandler` had no handler-level test at all (its
controller tests stub the mediator), so the RemoteStream include added last
round was discharged by inspection -- the same method that produced #671. It
now runs the same 13-type matrix via a shared SelectionSeedData; removing the
include fails that matrix.

Third: dropped the dead `(i as Season).SeasonMetadata` include leg -- the Season
projection reads Show.ShowMetadata and the scalar SeasonNumber, never
SeasonMetadata.

Filed #690 for the pre-existing, out-of-scope finding: the paged TotalCount
ignores the search query, so the SPA renders empty pages.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This commit is contained in:
timothy
2026-07-28 19:33:52 +02:00
co-authored by Claude Opus 5
parent 017ef988d0
commit 572737a29e
7 changed files with 374 additions and 178 deletions
Download Patch File Download Diff File Expand all files Collapse all files
ErsatzTV.Application/MediaCollections/RerunCollectionQueryExtensions.cs
+2 -2
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@@ -28,8 +28,8 @@ internal static class RerunCollectionQueryExtensions
.ThenInclude(i => (i as Movie).MovieMetadata)
.Include(c => c.MediaItem)
.ThenInclude(i => (i as Show).ShowMetadata)
.Include(c => c.MediaItem)
.ThenInclude(i => (i as Season).SeasonMetadata)
// No (i as Season).SeasonMetadata leg on purpose: ProjectToViewModel(Season) builds its name
// from Show.ShowMetadata and the scalar SeasonNumber, and never reads SeasonMetadata.
.Include(c => c.MediaItem)
.ThenInclude(i => (i as Season).Show)
.ThenInclude(s => s.ShowMetadata)
ErsatzTV.Application/MediaItems/Mapper.cs
+7 -1
View File
@@ -127,8 +127,14 @@ internal static class Mapper
private static string SongTitle(Song s)
{
// Artists is a NULLABLE primitive collection, not a navigation: a song whose tags failed to read
// is persisted by FallbackMetadataProvider with Artists never assigned, and string.Join throws
// ArgumentNullException on a null sequence. Filtering the empty case too avoids prefixing an
// artist-less song with a bare " - ".
string songArtist = Optional(s.SongMetadata).Flatten().HeadOrNone()
.Map(sm => $"{string.Join(", ", sm.Artists)} - ")
.Map(sm => Optional(sm.Artists).Flatten().ToList())
.Filter(artists => artists.Count > 0)
.Map(artists => $"{string.Join(", ", artists)} - ")
.IfNone(string.Empty);
return Optional(s.SongMetadata).Flatten().HeadOrNone()
.Map(sm => $"{songArtist}{sm.Title ?? string.Empty}")
ErsatzTV.Tests/Application/MediaCollections/GetPlaylistItemsHandlerTests.cs
+89
View File
@@ -0,0 +1,89 @@
using ErsatzTV.Application.MediaCollections;
using ErsatzTV.Core.Domain;
using ErsatzTV.Infrastructure.Data;
using ErsatzTV.Tests.Support;
using NUnit.Framework;
using Shouldly;
namespace ErsatzTV.Tests.Application.MediaCollections;
/// <summary>
/// The second consumer of the shared <c>ProjectMediaItemToViewModel</c> switch (issue #671).
/// <c>GetPlaylistItemsHandler</c> had no handler-level test — the controller tests stub the mediator
/// and never execute the query — so the only symptom of a missing include here was a silent "???"
/// name that nothing in the suite could see. Widening the shared switch with a RemoteStream arm
/// obliged this handler to gain a matching include; proving that by inspection would have repeated
/// the very method that produced #671, so it gets the same full matrix the rerun handlers get.
/// </summary>
[TestFixture]
public class GetPlaylistItemsHandlerTests : MediaCollectionHandlerTestBase
{
private static IEnumerable<CollectionType> SupportedSelectionTypes => SelectionSeedData.SupportedSelectionTypes;
[TestCaseSource(nameof(SupportedSelectionTypes))]
public async Task GetPlaylistItems_Should_Resolve_The_Selection(CollectionType collectionType)
{
await SeedSelection(collectionType);
await SeedPlaylistItem(collectionType);
var handler = new GetPlaylistItemsHandler(Db.Factory);
List<PlaylistItemViewModel> items =
await handler.Handle(new GetPlaylistItems(1), CancellationToken.None);
items.Count.ShouldBe(1);
PlaylistItemViewModel item = items[0];
int? selectedId = item.Collection?.Id
?? item.MultiCollection?.Id
?? item.SmartCollection?.Id
?? item.MediaItem?.MediaItemId;
string selectedName = item.Collection?.Name
?? item.MultiCollection?.Name
?? item.SmartCollection?.Name
?? item.MediaItem?.Name;
selectedId.ShouldBe(SelectionSeedData.SelectedId, $"{collectionType} lost its selected id");
selectedName.ShouldBe(
SelectionSeedData.ExpectedName(collectionType),
$"{collectionType} projected the wrong name");
}
private async Task SeedSelection(CollectionType collectionType)
{
await using TvContext context = Db.CreateContext();
await SelectionSeedData.SeedSelection(context, collectionType);
}
private async Task SeedPlaylistItem(CollectionType collectionType)
{
await using TvContext context = Db.CreateContext();
var item = new PlaylistItem
{
Id = 1,
Index = 0,
PlaylistId = 1,
CollectionType = collectionType,
PlaybackOrder = PlaybackOrder.Chronological
};
SelectionSeedData.ApplySelection(
collectionType,
v => item.CollectionId = v,
v => item.MultiCollectionId = v,
v => item.SmartCollectionId = v,
v => item.MediaItemId = v);
context.Playlists.Add(new Playlist
{
Id = 1,
Name = "Playlist",
Items = [item]
});
await context.SaveChangesAsync();
}
}
ErsatzTV.Tests/Application/MediaCollections/RerunCollectionQueryHandlerTests.cs
+44 -174
View File
@@ -1,5 +1,4 @@
using ErsatzTV.Application.MediaCollections;
using ErsatzTV.Controllers.Api.Requests;
using ErsatzTV.Core.Domain;
using ErsatzTV.Infrastructure.Data;
using ErsatzTV.Tests.Support;
@@ -12,23 +11,18 @@ namespace ErsatzTV.Tests.Application.MediaCollections;
/// <summary>
/// Read-path coverage for the two rerun-collection query handlers (issue #671). The defect was
/// precisely that nobody enumerated the selection types: the list handler eager-loaded nothing, and
/// the by-id handler loaded metadata for only four of the ten media types. So the matrix here is
/// derived from the production predicate rather than hand-listed — a newly-supported
/// <see cref="CollectionType" /> joins it automatically and fails loudly in <c>SeedSelection</c>
/// until someone teaches the suite how to seed it.
/// the by-id handler loaded metadata for only four of the ten media types. So the matrix is derived
/// from the production predicate (see <see cref="SelectionSeedData" />) rather than hand-listed.
/// </summary>
[TestFixture]
public class RerunCollectionQueryHandlerTests : MediaCollectionHandlerTestBase
{
private const int SelectedId = 42;
private static IEnumerable<CollectionType> SupportedSelectionTypes =>
Enum.GetValues<CollectionType>().Where(RerunCollectionRequestMapping.IsSupportedSelectionType);
private static IEnumerable<CollectionType> SupportedSelectionTypes => SelectionSeedData.SupportedSelectionTypes;
/// <summary>
/// Completeness guard. Without it, a change that narrowed <c>IsSupportedSelectionType</c> would
/// shrink the matrix silently and every remaining case would still pass — the "filters on the
/// property it asserts" failure mode.
/// property it asserts" failure mode. Set equality, so it fails on widening too.
/// </summary>
[Test]
public void Supported_Selection_Types_Should_Be_The_Full_Documented_Set()
@@ -84,30 +78,37 @@ public class RerunCollectionQueryHandlerTests : MediaCollectionHandlerTestBase
}
/// <summary>
/// The exact projected name per type. Pinning the whole string — rather than merely asserting
/// "not a placeholder" — is what makes a missing NESTED include leg visible: dropping
/// Episode → Season → Show still yields the non-placeholder "s00e04 - Selected episode", and
/// dropping MusicVideo → Artist still yields "Selected music video". Both would sail past a
/// looser assertion while having lost real data.
/// <c>SongMetadata.Artists</c> is a NULLABLE primitive collection, and a song whose tags failed to
/// read is persisted with it never assigned. Before #671 the rerun list did not load SongMetadata
/// at all, so this was unreachable there; eager-loading it made a latent `string.Join` throw into a
/// live 500 that would take down the whole page.
/// </summary>
private static string ExpectedName(CollectionType collectionType) =>
collectionType switch
[TestCase(null, "Selected song", TestName = "GetById_Song_With_Null_Artists_Should_Not_Throw")]
[TestCase(new string[] { }, "Selected song", TestName = "GetById_Song_With_No_Artists_Should_Not_Prefix")]
public async Task GetById_Should_Tolerate_Song_Artists(string[] artists, string expectedName)
{
await using (TvContext context = Db.CreateContext())
{
CollectionType.Collection => "Selected collection",
CollectionType.MultiCollection => "Selected multi collection",
CollectionType.SmartCollection => "Selected smart collection",
CollectionType.TelevisionShow => "Selected show (2020)",
CollectionType.TelevisionSeason => "Parent show (2020) - Season 3",
CollectionType.Artist => "Selected artist",
CollectionType.Movie => "Selected movie (2019)",
CollectionType.Episode => "Episode's show - s02e04 - Selected episode",
CollectionType.MusicVideo => "Video's artist - Selected music video",
CollectionType.OtherVideo => "Selected other video",
CollectionType.Song => "Song artist - Selected song",
CollectionType.Image => "Selected image",
CollectionType.RemoteStream => "Selected remote stream",
_ => throw new AssertionException($"No expected name pinned for {collectionType}")
};
context.Songs.Add(new Song
{
Id = SelectionSeedData.SelectedId,
SongMetadata = [new SongMetadata { Title = "Selected song", Artists = artists?.ToList() }]
});
await context.SaveChangesAsync();
}
await SeedRerunCollection(1, CollectionType.Song);
var handler = new GetRerunCollectionByIdHandler(Db.Factory);
Option<RerunCollectionViewModel> result =
await handler.Handle(new GetRerunCollectionById(1), CancellationToken.None);
RerunCollectionViewModel vm = result.IfNone(() => throw new AssertionException("Expected a result"));
vm.MediaItem.ShouldNotBeNull();
vm.MediaItem.MediaItemId.ShouldBe(SelectionSeedData.SelectedId);
vm.MediaItem.Name.ShouldBe(expectedName);
}
/// <summary>
/// Mirrors <c>RerunCollectionController.ProjectToResponseModel</c>, which flattens the tagged
@@ -127,138 +128,16 @@ public class RerunCollectionQueryHandlerTests : MediaCollectionHandlerTestBase
?? vm.SmartCollection?.Name
?? vm.MediaItem?.Name;
selectedId.ShouldBe(SelectedId, $"{collectionType} lost its selected id");
selectedName.ShouldBe(ExpectedName(collectionType), $"{collectionType} projected the wrong name");
selectedId.ShouldBe(SelectionSeedData.SelectedId, $"{collectionType} lost its selected id");
selectedName.ShouldBe(
SelectionSeedData.ExpectedName(collectionType),
$"{collectionType} projected the wrong name");
}
private async Task SeedSelection(CollectionType collectionType)
{
await using TvContext context = Db.CreateContext();
switch (collectionType)
{
case CollectionType.Collection:
context.Collections.Add(new Collection
{
Id = SelectedId,
Name = "Selected collection",
MediaItems = []
});
break;
case CollectionType.MultiCollection:
context.MultiCollections.Add(new MultiCollection
{
Id = SelectedId,
Name = "Selected multi collection"
});
break;
case CollectionType.SmartCollection:
context.SmartCollections.Add(new SmartCollection
{
Id = SelectedId,
Name = "Selected smart collection",
Query = "tag:family"
});
break;
case CollectionType.TelevisionShow:
context.Shows.Add(new Show
{
Id = SelectedId,
ShowMetadata = [new ShowMetadata { Title = "Selected show", Year = 2020 }]
});
break;
case CollectionType.TelevisionSeason:
context.Seasons.Add(new Season
{
Id = SelectedId,
SeasonNumber = 3,
Show = new Show
{
Id = 900,
ShowMetadata = [new ShowMetadata { Title = "Parent show", Year = 2020 }]
}
});
break;
case CollectionType.Artist:
context.Artists.Add(new Artist
{
Id = SelectedId,
ArtistMetadata = [new ArtistMetadata { Title = "Selected artist" }]
});
break;
case CollectionType.Movie:
context.Movies.Add(new Movie
{
Id = SelectedId,
MovieMetadata = [new MovieMetadata { Title = "Selected movie", Year = 2019 }]
});
break;
case CollectionType.Episode:
context.Episodes.Add(new Episode
{
Id = SelectedId,
EpisodeMetadata = [new EpisodeMetadata { Title = "Selected episode", EpisodeNumber = 4 }],
Season = new Season
{
Id = 901,
SeasonNumber = 2,
Show = new Show
{
Id = 902,
ShowMetadata = [new ShowMetadata { Title = "Episode's show", Year = 2018 }]
}
}
});
break;
case CollectionType.MusicVideo:
context.MusicVideos.Add(new MusicVideo
{
Id = SelectedId,
MusicVideoMetadata = [new MusicVideoMetadata { Title = "Selected music video" }],
Artist = new Artist
{
Id = 903,
ArtistMetadata = [new ArtistMetadata { Title = "Video's artist" }]
}
});
break;
case CollectionType.OtherVideo:
context.OtherVideos.Add(new OtherVideo
{
Id = SelectedId,
OtherVideoMetadata = [new OtherVideoMetadata { Title = "Selected other video" }]
});
break;
case CollectionType.Song:
context.Songs.Add(new Song
{
Id = SelectedId,
SongMetadata =
[new SongMetadata { Title = "Selected song", Artists = ["Song artist"] }]
});
break;
case CollectionType.Image:
context.Images.Add(new Image
{
Id = SelectedId,
ImageMetadata = [new ImageMetadata { Title = "Selected image" }]
});
break;
case CollectionType.RemoteStream:
context.RemoteStreams.Add(new RemoteStream
{
Id = SelectedId,
Url = "http://example.invalid/stream",
RemoteStreamMetadata = [new RemoteStreamMetadata { Title = "Selected remote stream" }]
});
break;
default:
throw new AssertionException(
$"{collectionType} is a supported rerun-collection selection type but this suite does " +
"not know how to seed it — teach SeedSelection about it rather than narrowing the matrix.");
}
await context.SaveChangesAsync();
await SelectionSeedData.SeedSelection(context, collectionType);
}
private async Task SeedRerunCollection(int id, CollectionType collectionType)
@@ -274,21 +153,12 @@ public class RerunCollectionQueryHandlerTests : MediaCollectionHandlerTestBase
RerunPlaybackOrder = PlaybackOrder.Chronological
};
switch (collectionType)
{
case CollectionType.Collection:
rerunCollection.CollectionId = SelectedId;
break;
case CollectionType.MultiCollection:
rerunCollection.MultiCollectionId = SelectedId;
break;
case CollectionType.SmartCollection:
rerunCollection.SmartCollectionId = SelectedId;
break;
default:
rerunCollection.MediaItemId = SelectedId;
break;
}
SelectionSeedData.ApplySelection(
collectionType,
v => rerunCollection.CollectionId = v,
v => rerunCollection.MultiCollectionId = v,
v => rerunCollection.SmartCollectionId = v,
v => rerunCollection.MediaItemId = v);
context.RerunCollections.Add(rerunCollection);
await context.SaveChangesAsync();
ErsatzTV.Tests/Support/SelectionSeedData.cs
+207
View File
@@ -0,0 +1,207 @@
using ErsatzTV.Controllers.Api.Requests;
using ErsatzTV.Core.Domain;
using ErsatzTV.Infrastructure.Data;
using NUnit.Framework;
namespace ErsatzTV.Tests.Support;
/// <summary>
/// One selection-type matrix, shared by every fixture that exercises a tagged-union selection
/// (rerun collections and playlist items). Both consumers of
/// <c>MediaCollections.Mapper.ProjectMediaItemToViewModel</c> are proved against the SAME data, so
/// widening the shared switch cannot be discharged for the second consumer by inspection alone —
/// which is the method that produced #671 in the first place.
/// </summary>
internal static class SelectionSeedData
{
public const int SelectedId = 42;
/// <summary>
/// Derived from production rather than hand-listed, so a newly-supported type joins the matrix
/// automatically and trips the <c>default:</c> arms below until someone teaches them about it.
/// </summary>
public static IEnumerable<CollectionType> SupportedSelectionTypes =>
Enum.GetValues<CollectionType>().Where(RerunCollectionRequestMapping.IsSupportedSelectionType);
/// <summary>
/// The exact projected name per type. Pinning the whole string — rather than merely asserting
/// "not a placeholder" — is what makes a missing NESTED include leg visible: dropping
/// Episode → Season → Show still yields the placeholder-free "s??e04 - Selected episode", and
/// dropping MusicVideo → Artist still yields "Selected music video". Both would sail past a
/// looser assertion while having lost real data.
/// </summary>
public static string ExpectedName(CollectionType collectionType) =>
collectionType switch
{
CollectionType.Collection => "Selected collection",
CollectionType.MultiCollection => "Selected multi collection",
CollectionType.SmartCollection => "Selected smart collection",
CollectionType.TelevisionShow => "Selected show (2020)",
CollectionType.TelevisionSeason => "Parent show (2020) - Season 3",
CollectionType.Artist => "Selected artist",
CollectionType.Movie => "Selected movie (2019)",
CollectionType.Episode => "Episode's show - s02e04 - Selected episode",
CollectionType.MusicVideo => "Video's artist - Selected music video",
CollectionType.OtherVideo => "Selected other video",
CollectionType.Song => "Song artist - Selected song",
CollectionType.Image => "Selected image",
CollectionType.RemoteStream => "Selected remote stream",
_ => throw new AssertionException($"No expected name pinned for {collectionType}")
};
public static async Task SeedSelection(TvContext context, CollectionType collectionType)
{
switch (collectionType)
{
case CollectionType.Collection:
context.Collections.Add(new Collection
{
Id = SelectedId,
Name = "Selected collection",
MediaItems = []
});
break;
case CollectionType.MultiCollection:
context.MultiCollections.Add(new MultiCollection
{
Id = SelectedId,
Name = "Selected multi collection"
});
break;
case CollectionType.SmartCollection:
context.SmartCollections.Add(new SmartCollection
{
Id = SelectedId,
Name = "Selected smart collection",
Query = "tag:family"
});
break;
case CollectionType.TelevisionShow:
context.Shows.Add(new Show
{
Id = SelectedId,
ShowMetadata = [new ShowMetadata { Title = "Selected show", Year = 2020 }]
});
break;
case CollectionType.TelevisionSeason:
context.Seasons.Add(new Season
{
Id = SelectedId,
SeasonNumber = 3,
Show = new Show
{
Id = 900,
ShowMetadata = [new ShowMetadata { Title = "Parent show", Year = 2020 }]
}
});
break;
case CollectionType.Artist:
context.Artists.Add(new Artist
{
Id = SelectedId,
ArtistMetadata = [new ArtistMetadata { Title = "Selected artist" }]
});
break;
case CollectionType.Movie:
context.Movies.Add(new Movie
{
Id = SelectedId,
MovieMetadata = [new MovieMetadata { Title = "Selected movie", Year = 2019 }]
});
break;
case CollectionType.Episode:
context.Episodes.Add(new Episode
{
Id = SelectedId,
EpisodeMetadata = [new EpisodeMetadata { Title = "Selected episode", EpisodeNumber = 4 }],
Season = new Season
{
Id = 901,
SeasonNumber = 2,
Show = new Show
{
Id = 902,
ShowMetadata = [new ShowMetadata { Title = "Episode's show", Year = 2018 }]
}
}
});
break;
case CollectionType.MusicVideo:
context.MusicVideos.Add(new MusicVideo
{
Id = SelectedId,
MusicVideoMetadata = [new MusicVideoMetadata { Title = "Selected music video" }],
Artist = new Artist
{
Id = 903,
ArtistMetadata = [new ArtistMetadata { Title = "Video's artist" }]
}
});
break;
case CollectionType.OtherVideo:
context.OtherVideos.Add(new OtherVideo
{
Id = SelectedId,
OtherVideoMetadata = [new OtherVideoMetadata { Title = "Selected other video" }]
});
break;
case CollectionType.Song:
context.Songs.Add(new Song
{
Id = SelectedId,
SongMetadata =
[new SongMetadata { Title = "Selected song", Artists = ["Song artist"] }]
});
break;
case CollectionType.Image:
context.Images.Add(new Image
{
Id = SelectedId,
ImageMetadata = [new ImageMetadata { Title = "Selected image" }]
});
break;
case CollectionType.RemoteStream:
context.RemoteStreams.Add(new RemoteStream
{
Id = SelectedId,
Url = "http://example.invalid/stream",
RemoteStreamMetadata = [new RemoteStreamMetadata { Title = "Selected remote stream" }]
});
break;
default:
throw new AssertionException(
$"{collectionType} is a supported selection type but this suite does not know how " +
"to seed it — teach SeedSelection about it rather than narrowing the matrix.");
}
await context.SaveChangesAsync();
}
/// <summary>
/// Assigns the one foreign key the tagged union uses for this type. Shared so the rerun and
/// playlist fixtures cannot disagree about which slot a type occupies.
/// </summary>
public static void ApplySelection(
CollectionType collectionType,
Action<int> setCollectionId,
Action<int> setMultiCollectionId,
Action<int> setSmartCollectionId,
Action<int> setMediaItemId)
{
switch (collectionType)
{
case CollectionType.Collection:
setCollectionId(SelectedId);
break;
case CollectionType.MultiCollection:
setMultiCollectionId(SelectedId);
break;
case CollectionType.SmartCollection:
setSmartCollectionId(SelectedId);
break;
default:
setMediaItemId(SelectedId);
break;
}
}
}
docs/api-conventions.md
+7
View File
@@ -174,6 +174,13 @@ whose include chains differ, so every metadata navigation is read through `Optio
and degrades to the `"???"` placeholder rather than throwing. A bare `x.Season.Show.ShowMetadata`
inside a projection is a latent 500 on some other caller's GET.
**And it is not only navigations.** `SongMetadata.Artists` is a nullable EF *primitive collection*
(a JSON array in one column), which `FallbackMetadataProvider` leaves unassigned for a song whose
tags failed to read — and `string.Join` throws `ArgumentNullException` on a null sequence, not a
`NullReferenceException`. Adding an include is therefore not automatically safe: it can promote a
latent throw on a previously-unloaded member into a live 500 that fails the whole page. When you
widen an include chain, audit what the newly-reachable projection dereferences.
## 3. Error mapping
Central helper: `ErsatzTV/Extensions/ApiResults.cs`. Use these extension methods instead of
docs/decisions/records/api/selection-projection-include-chain.md
+18 -1
View File
@@ -46,7 +46,24 @@ was never broken in production; adding the shared `RemoteStream` arm did oblige
matching `RemoteStreamMetadata` include, or that name alone would have degraded to `"???"`.
**Widening a shared switch incurs a debt in every caller loading for it.** Adding an arm is not
free: each consumer must be re-checked against the navigations the new arm dereferences.
free: each consumer must be re-checked against the navigations the new arm dereferences — and
discharged by a TEST, not by inspection, since inspection is the method that produced this bug.
`GetPlaylistItemsHandler` had no handler-level test at all (its controller tests stub the mediator),
so it gained the same 13-type matrix via the shared `SelectionSeedData`.
## Adding an include can CREATE a 500
Eager-loading is not automatically safe. `SongMetadata.Artists` is a nullable EF **primitive
collection** — a JSON array in one column, not a navigation — that `FallbackMetadataProvider` leaves
unassigned whenever a song's tags fail to read, and `string.Join` throws `ArgumentNullException` (not
`NullReferenceException`) on a null sequence. The rerun list previously did not load `SongMetadata`
at all, so the throw was unreachable there; adding the include promoted it to a live 500 that would
have failed the entire page. The guard is `Optional(sm.Artists).Flatten()`, and the empty case is
filtered too so an artist-less song is not prefixed with a bare `" - "`.
The general rule: when you widen an include chain, audit what the newly-reachable projection
dereferences. A "null navigation" audit is not enough — nullable primitive collections throw a
different exception type and will not turn up in a grep for `NullReferenceException`.
## Verification worth repeating