fix(author): ApplyChanges wipes stored paths when an update omits them

src/Chaptarr.Core.Test/Books/AuthorApplyChangesPathPreservationFixture.cs

@@ -0,0 +1,72 @@
+using NUnit.Framework;
+using NzbDrone.Core.Books;
+
+namespace Chaptarr.Core.Test.Books
+{
+    [TestFixture]
+    public class AuthorApplyChangesPathPreservationFixture
+    {
+        private static Author StoredAuthor()
+        {
+            return new Author
+            {
+                Path = "/library/authors/Stored Author",
+                AudiobookRootFolderPath = "/library/audiobooks",
+                EbookRootFolderPath = "/library/ebooks"
+            };
+        }
+
+        [Test]
+        public void should_preserve_stored_paths_when_incoming_author_omits_them()
+        {
+            var stored = StoredAuthor();
+
+            // An update that does not mention paths - e.g. a PUT that only changes
+            // monitoring, or an Author built from remote metadata during a refresh.
+            stored.ApplyChanges(new Author());
+
+            Assert.Multiple(() =>
+            {
+                Assert.That(stored.Path, Is.EqualTo("/library/authors/Stored Author"));
+                Assert.That(stored.AudiobookRootFolderPath, Is.EqualTo("/library/audiobooks"));
+                Assert.That(stored.EbookRootFolderPath, Is.EqualTo("/library/ebooks"));
+            });
+        }
+
+        [Test]
+        public void should_apply_paths_when_incoming_author_supplies_them()
+        {
+            var stored = StoredAuthor();
+
+            stored.ApplyChanges(new Author
+            {
+                Path = "/library/authors/Moved Author",
+                AudiobookRootFolderPath = "/library/audiobooks-2",
+                EbookRootFolderPath = "/library/ebooks-2"
+            });
+
+            Assert.Multiple(() =>
+            {
+                Assert.That(stored.Path, Is.EqualTo("/library/authors/Moved Author"));
+                Assert.That(stored.AudiobookRootFolderPath, Is.EqualTo("/library/audiobooks-2"));
+                Assert.That(stored.EbookRootFolderPath, Is.EqualTo("/library/ebooks-2"));
+            });
+        }
+
+        [Test]
+        public void should_preserve_each_path_independently()
+        {
+            var stored = StoredAuthor();
+
+            // Only the audiobook root is being changed; the other two must survive.
+            stored.ApplyChanges(new Author { AudiobookRootFolderPath = "/library/audiobooks-2" });
+
+            Assert.Multiple(() =>
+            {
+                Assert.That(stored.AudiobookRootFolderPath, Is.EqualTo("/library/audiobooks-2"));
+                Assert.That(stored.Path, Is.EqualTo("/library/authors/Stored Author"));
+                Assert.That(stored.EbookRootFolderPath, Is.EqualTo("/library/ebooks"));
+            });
+        }
+    }
+}

src/NzbDrone.Core/Books/Model/Author.cs

@@ -271,7 +271,16 @@ public override void UseDbFieldsFrom(Author other)
 
         public override void ApplyChanges(Author other)
         {
-            Path = other.Path;
+            // Only overwrite when a value is supplied. ApplyChanges is called both from
+            // the API -- where a PUT may legitimately omit fields it is not changing --
+            // and from metadata refresh, where the incoming Author is built from remote
+            // metadata and carries no local paths at all. That is the same reason the
+            // quality and metadata profiles below are guarded; these three were not, so
+            // an update that omitted them wiped the stored paths.
+            if (other.Path != null)
+            {
+                Path = other.Path;
+            }
 
             // Don't overwrite quality profiles if they're already set
             // Quality profiles from metadata sources are always null/0
@@ -326,8 +335,15 @@ public override void ApplyChanges(Author other)
 
             Tags = (AudiobookTags ?? new HashSet<int>()).Concat(EbookTags ?? new HashSet<int>()).ToHashSet();
             AddOptions = other.AddOptions;
-            AudiobookRootFolderPath = other.AudiobookRootFolderPath;
-            EbookRootFolderPath = other.EbookRootFolderPath;
+            if (other.AudiobookRootFolderPath != null)
+            {
+                AudiobookRootFolderPath = other.AudiobookRootFolderPath;
+            }
+
+            if (other.EbookRootFolderPath != null)
+            {
+                EbookRootFolderPath = other.EbookRootFolderPath;
+            }
             Monitored = other.Monitored;
             // TRI-STATE MONITORING SYSTEM - Copy only when explicitly provided (not null).
             // This prevents partial updates from wiping existing monitoring values.