Guard completed-download book override to local DB books only

src/NzbDrone.Core.Test/Download/CompletedDownloadServiceTests/ImportFixture.cs

@@ -365,11 +365,15 @@ public void should_mark_as_imported_if_the_download_can_be_tracked_using_the_sou
         }
 
         [Test]
-        public void should_pass_book_override_when_remotebook_has_single_book()
+        public void should_pass_book_override_when_remotebook_has_single_valid_local_book()
         {
             var book = CreateBook(42);
             _trackedDownload.RemoteBook.Books = new List<Book> { book };
 
+            Mocker.GetMock<IBookService>()
+                  .Setup(s => s.GetBook(42))
+                  .Returns(book);
+
             Mocker.GetMock<IDownloadedBooksImportService>()
                   .Setup(v => v.ProcessPath(It.IsAny<string>(), It.IsAny<ImportMode>(), It.IsAny<IdentificationOverrides>(), It.IsAny<DownloadClientItem>()))
                   .Returns(new List<ImportResult>
@@ -388,6 +392,55 @@ public void should_pass_book_override_when_remotebook_has_single_book()
                   Times.Once());
         }
 
+        [Test]
+        public void should_not_pass_book_override_when_single_book_has_zero_id()
+        {
+            var book = CreateBook(0);
+            _trackedDownload.RemoteBook.Books = new List<Book> { book };
+
+            Mocker.GetMock<IDownloadedBooksImportService>()
+                  .Setup(v => v.ProcessPath(It.IsAny<string>(), It.IsAny<ImportMode>(), It.IsAny<IdentificationOverrides>(), It.IsAny<DownloadClientItem>()))
+                  .Returns(new List<ImportResult>());
+
+            Subject.Import(_trackedDownload);
+
+            Mocker.GetMock<IDownloadedBooksImportService>()
+                  .Verify(v => v.ProcessPath(
+                      It.IsAny<string>(),
+                      ImportMode.Auto,
+                      It.Is<IdentificationOverrides>(o => o.Author == _trackedDownload.RemoteBook.Author && o.Book == null),
+                      It.IsAny<DownloadClientItem>()),
+                  Times.Once());
+
+            Mocker.GetMock<IBookService>()
+                  .Verify(s => s.GetBook(It.IsAny<int>()), Times.Never());
+        }
+
+        [Test]
+        public void should_not_pass_book_override_when_single_book_not_found_in_db()
+        {
+            var book = CreateBook(99);
+            _trackedDownload.RemoteBook.Books = new List<Book> { book };
+
+            Mocker.GetMock<IBookService>()
+                  .Setup(s => s.GetBook(99))
+                  .Returns((Book)null);
+
+            Mocker.GetMock<IDownloadedBooksImportService>()
+                  .Setup(v => v.ProcessPath(It.IsAny<string>(), It.IsAny<ImportMode>(), It.IsAny<IdentificationOverrides>(), It.IsAny<DownloadClientItem>()))
+                  .Returns(new List<ImportResult>());
+
+            Subject.Import(_trackedDownload);
+
+            Mocker.GetMock<IDownloadedBooksImportService>()
+                  .Verify(v => v.ProcessPath(
+                      It.IsAny<string>(),
+                      ImportMode.Auto,
+                      It.Is<IdentificationOverrides>(o => o.Author == _trackedDownload.RemoteBook.Author && o.Book == null),
+                      It.IsAny<DownloadClientItem>()),
+                  Times.Once());
+        }
+
         [Test]
         public void should_not_pass_book_override_when_remotebook_has_multiple_books()
         {

src/NzbDrone.Core.Test/MediaFiles/TrackImport/Identification/CandidateServiceFixture.cs

@@ -1,7 +1,12 @@
 using System.Collections.Generic;
+using System.Linq;
 using FluentAssertions;
 using Moq;
 using NUnit.Framework;
+using NzbDrone.Core.Books;
+using NzbDrone.Core.Datastore;
+using NzbDrone.Core.MediaFiles;
+using NzbDrone.Core.MediaFiles.BookImport;
 using NzbDrone.Core.MediaFiles.BookImport.Identification;
 using NzbDrone.Core.MetadataSource;
 using NzbDrone.Core.MetadataSource.Goodreads;
@@ -13,6 +18,70 @@ namespace NzbDrone.Core.Test.MediaFiles.BookImport.Identification
     [TestFixture]
     public class CandidateServiceFixture : CoreTest<CandidateService>
     {
+        private LocalEdition BuildLocalEdition(string bookTitle = "Book", string author = "Author")
+        {
+            return new LocalEdition
+            {
+                LocalBooks = new List<LocalBook>
+                {
+                    new LocalBook
+                    {
+                        FileTrackInfo = new ParsedTrackInfo
+                        {
+                            Authors = new List<string> { author },
+                            BookTitle = bookTitle
+                        }
+                    }
+                }
+            };
+        }
+
+        [Test]
+        public void should_fall_back_to_author_search_when_book_override_returns_no_candidates()
+        {
+            var author = new Author { Id = 1, AuthorMetadataId = 10 };
+            var book = new Book { Id = 99 };
+            var idOverrides = new IdentificationOverrides { Author = author, Book = book };
+
+            // Book override path returns nothing (stale book)
+            Mocker.GetMock<IEditionService>()
+                  .Setup(s => s.GetEditionsByBook(99))
+                  .Returns(new List<Edition>());
+
+            // Author fallback should find candidates
+            var edition = new Edition
+            {
+                Id = 5,
+                BookId = 50,
+                ForeignEditionId = "edition-1",
+                Ratings = new Ratings { Votes = 1, Value = 1.0m },
+                Book = new LazyLoaded<Book>(new Book
+                {
+                    Id = 50,
+                    ForeignBookId = "foreign-50",
+                    Author = new LazyLoaded<Author>(author)
+                })
+            };
+
+            Mocker.GetMock<IBookService>()
+                  .Setup(s => s.GetCandidates(10, "Book"))
+                  .Returns(new List<Book> { new Book { Id = 50 } });
+
+            Mocker.GetMock<IEditionService>()
+                  .Setup(s => s.GetEditionsByBook(50))
+                  .Returns(new List<Edition> { edition });
+
+            Mocker.GetMock<IMediaFileService>()
+                  .Setup(s => s.GetFilesByBook(It.IsAny<int>()))
+                  .Returns(new List<BookFile>());
+
+            var localEdition = BuildLocalEdition();
+            var result = Subject.GetDbCandidatesFromTags(localEdition, idOverrides, false);
+
+            result.Should().HaveCount(1);
+            result.First().Edition.Id.Should().Be(5);
+        }
+
         [Test]
         public void should_not_throw_on_goodreads_exception()
         {

src/NzbDrone.Core/Download/CompletedDownloadService.cs

@@ -7,6 +7,7 @@
 using NzbDrone.Common.Extensions;
 using NzbDrone.Core.Download.TrackedDownloads;
 using NzbDrone.Core.History;
+using NzbDrone.Core.Books;
 using NzbDrone.Core.MediaFiles;
 using NzbDrone.Core.MediaFiles.BookImport;
 using NzbDrone.Core.MediaFiles.Events;
@@ -28,20 +29,23 @@ public class CompletedDownloadService : ICompletedDownloadService
         private readonly IProvideImportItemService _provideImportItemService;
         private readonly IDownloadedBooksImportService _downloadedTracksImportService;
         private readonly ITrackedDownloadAlreadyImported _trackedDownloadAlreadyImported;
+        private readonly IBookService _bookService;
         private readonly Logger _logger;
 
         public CompletedDownloadService(IEventAggregator eventAggregator,
                                         IHistoryService historyService,
                                         IProvideImportItemService provideImportItemService,
                                         IDownloadedBooksImportService downloadedTracksImportService,
                                         ITrackedDownloadAlreadyImported trackedDownloadAlreadyImported,
+                                        IBookService bookService,
                                         Logger logger)
         {
             _eventAggregator = eventAggregator;
             _historyService = historyService;
             _provideImportItemService = provideImportItemService;
             _downloadedTracksImportService = downloadedTracksImportService;
             _trackedDownloadAlreadyImported = trackedDownloadAlreadyImported;
+            _bookService = bookService;
             _logger = logger;
         }
 
@@ -203,11 +207,17 @@ private IdentificationOverrides BuildIdentificationOverrides(Parser.Model.Remote
                 Author = remoteBook.Author
             };
 
-            // Only set Book override when the grab targeted exactly one book,
-            // so we don't incorrectly force multi-book/omnibus downloads to a single book.
+            // Only set Book override when the grab targeted exactly one book
+            // AND we can confirm the book exists in the local database.
+            // RemoteBook.Books may contain stale or unmapped objects that would
+            // cause CandidateService.GetDbCandidatesByBook to return zero candidates.
             if (remoteBook.Books != null && remoteBook.Books.Count == 1)
             {
-                overrides.Book = remoteBook.Books.First();
+                var candidate = remoteBook.Books.First();
+                if (candidate.Id > 0 && _bookService.GetBook(candidate.Id) != null)
+                {
+                    overrides.Book = candidate;
+                }
             }
 
             return overrides;

src/NzbDrone.Core/MediaFiles/BookImport/Identification/CandidateService.cs

@@ -80,6 +80,14 @@ public List<CandidateEdition> GetDbCandidatesFromTags(LocalEdition localEdition,
                 {
                     candidateReleases = GetDbCandidatesByBook(idOverrides.Book, includeExisting);
                 }
+
+                // If the book override produced no candidates (e.g. stale/invalid book),
+                // fall back to author-scoped search so import can still proceed.
+                if (!candidateReleases.Any() && idOverrides.Author != null)
+                {
+                    _logger.Debug("Book override returned no candidates, falling back to author search");
+                    candidateReleases = GetDbCandidatesByAuthor(localEdition, idOverrides.Author, includeExisting);
+                }
             }
             else if (idOverrides?.Author != null)
             {

src/NzbDrone.Core/MediaFiles/DownloadedBooksCommandService.cs

@@ -5,6 +5,7 @@
 using NzbDrone.Common.Disk;
 using NzbDrone.Common.Extensions;
 using NzbDrone.Common.Instrumentation.Extensions;
+using NzbDrone.Core.Books;
 using NzbDrone.Core.Download;
 using NzbDrone.Core.Download.TrackedDownloads;
 using NzbDrone.Core.MediaFiles.BookImport;
@@ -20,20 +21,23 @@ public class DownloadedBooksCommandService : IExecute<DownloadedBooksScanCommand
         private readonly ITrackedDownloadService _trackedDownloadService;
         private readonly IDiskProvider _diskProvider;
         private readonly ICompletedDownloadService _completedDownloadService;
+        private readonly IBookService _bookService;
         private readonly ICommandResultReporter _commandResultReporter;
         private readonly Logger _logger;
 
         public DownloadedBooksCommandService(IDownloadedBooksImportService downloadedTracksImportService,
                                                 ITrackedDownloadService trackedDownloadService,
                                                 IDiskProvider diskProvider,
                                                 ICompletedDownloadService completedDownloadService,
+                                                IBookService bookService,
                                                 ICommandResultReporter commandResultReporter,
                                                 Logger logger)
         {
             _downloadedTracksImportService = downloadedTracksImportService;
             _trackedDownloadService = trackedDownloadService;
             _diskProvider = diskProvider;
             _completedDownloadService = completedDownloadService;
+            _bookService = bookService;
             _commandResultReporter = commandResultReporter;
             _logger = logger;
         }
@@ -82,7 +86,11 @@ private IdentificationOverrides BuildIdentificationOverrides(RemoteBook remoteBo
 
             if (remoteBook.Books != null && remoteBook.Books.Count == 1)
             {
-                overrides.Book = remoteBook.Books.First();
+                var candidate = remoteBook.Books.First();
+                if (candidate.Id > 0 && _bookService.GetBook(candidate.Id) != null)
+                {
+                    overrides.Book = candidate;
+                }
             }
 
             return overrides;