Complete LinkedChildren integration and batch DTO optimizations

This commit integrates remaining performance changes:

- Add batch user data fetching in DtoService to reduce N+1 queries
- Add GetNextUpEpisodesBatch in TVSeriesManager for efficient batch retrieval
- Update Video/Movie/BoxSet to use LibraryManager for alternate versions
- Transition LinkedChild to use ItemId instead of Path (obsolete Path/LibraryItemId)
- Update providers and controllers for LinkedChildren-based references
- Add NextUpEpisodeBatchResult for batched episode queries
- Integrate IDescendantQueryProvider in SqliteDatabaseProvider

2 files changed, 19 insertions, 17 deletions

diff --git a/MediaBrowser.LocalMetadata/Parsers/BaseItemXmlParser.cs b/MediaBrowser.LocalMetadata/Parsers/BaseItemXmlParser.cs
index 119effe791..cf1423d02d 100644
--- a/MediaBrowser.LocalMetadata/Parsers/BaseItemXmlParser.cs
+++ b/MediaBrowser.LocalMetadata/Parsers/BaseItemXmlParser.cs
@@ -780,7 +780,8 @@ namespace MediaBrowser.LocalMetadata.Parsers
         }
 
         /// <summary>
-        /// Get linked child.
+        /// Get linked child from XML. Uses deprecated Path/LibraryItemId properties for backward compatibility
+        /// with existing XML files. These will be resolved to ItemId when the linked child is accessed.
         /// </summary>
         /// <param name="reader">The xml reader.</param>
         /// <returns>The linked child.</returns>
@@ -791,6 +792,7 @@ namespace MediaBrowser.LocalMetadata.Parsers
             reader.MoveToContent();
             reader.Read();
 
+#pragma warning disable CS0618 // Type or member is obsolete - reading legacy XML format for backward compatibility
             // Loop through each element
             while (!reader.EOF && reader.ReadState == ReadState.Interactive)
             {
@@ -820,6 +822,7 @@ namespace MediaBrowser.LocalMetadata.Parsers
             {
                 return linkedItem;
             }
+#pragma warning restore CS0618
 
             return null;
         }
diff --git a/MediaBrowser.LocalMetadata/Savers/BaseXmlSaver.cs b/MediaBrowser.LocalMetadata/Savers/BaseXmlSaver.cs
index 025a815247..a065b68321 100644
--- a/MediaBrowser.LocalMetadata/Savers/BaseXmlSaver.cs
+++ b/MediaBrowser.LocalMetadata/Savers/BaseXmlSaver.cs
@@ -467,41 +467,40 @@ namespace MediaBrowser.LocalMetadata.Savers
         }
 
         /// <summary>
-        /// ADd linked children.
+        /// Add linked children.
         /// </summary>
         /// <param name="item">The item.</param>
         /// <param name="writer">The xml writer.</param>
         /// <param name="pluralNodeName">The plural node name.</param>
         /// <param name="singularNodeName">The singular node name.</param>
         /// <returns>The task object representing the asynchronous operation.</returns>
-        private static async Task AddLinkedChildren(Folder item, XmlWriter writer, string pluralNodeName, string singularNodeName)
+        private async Task AddLinkedChildren(Folder item, XmlWriter writer, string pluralNodeName, string singularNodeName)
         {
-            var items = item.LinkedChildren
+            var linkedChildren = item.LinkedChildren
                 .Where(i => i.Type == LinkedChildType.Manual)
                 .ToList();
 
-            if (items.Count == 0)
+            if (linkedChildren.Count == 0)
             {
                 return;
             }
 
             await writer.WriteStartElementAsync(null, pluralNodeName, null).ConfigureAwait(false);
 
-            foreach (var link in items)
+            foreach (var link in linkedChildren)
             {
-                if (!string.IsNullOrWhiteSpace(link.Path) || !string.IsNullOrWhiteSpace(link.LibraryItemId))
+                // Resolve ItemId to get the item's path for XML portability
+                string? path = null;
+                if (link.ItemId.HasValue && !link.ItemId.Value.Equals(Guid.Empty))
                 {
-                    await writer.WriteStartElementAsync(null, singularNodeName, null).ConfigureAwait(false);
-                    if (!string.IsNullOrWhiteSpace(link.Path))
-                    {
-                        await writer.WriteElementStringAsync(null, "Path", null, link.Path).ConfigureAwait(false);
-                    }
-
-                    if (!string.IsNullOrWhiteSpace(link.LibraryItemId))
-                    {
-                        await writer.WriteElementStringAsync(null, "ItemId", null, link.LibraryItemId).ConfigureAwait(false);
-                    }
+                    var linkedItem = LibraryManager.GetItemById(link.ItemId.Value);
+                    path = linkedItem?.Path;
+                }
 
+                if (!string.IsNullOrWhiteSpace(path))
+                {
+                    await writer.WriteStartElementAsync(null, singularNodeName, null).ConfigureAwait(false);
+                    await writer.WriteElementStringAsync(null, "Path", null, path).ConfigureAwait(false);
                     await writer.WriteEndElementAsync().ConfigureAwait(false);
                 }
             }