1 files changed, 248 insertions, 0 deletions
diff --git a/src/Jellyfin.Database/Jellyfin.Database.Implementations/DescendantQueryHelper.cs b/src/Jellyfin.Database/Jellyfin.Database.Implementations/DescendantQueryHelper.cs
new file mode 100644
index 0000000000..43e6a8bc00
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+++ b/src/Jellyfin.Database/Jellyfin.Database.Implementations/DescendantQueryHelper.cs
@@ -0,0 +1,248 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using Jellyfin.Database.Implementations.Entities;
+using Jellyfin.Database.Implementations.MatchCriteria;
+
+namespace Jellyfin.Database.Implementations;
+
+/// <summary>
+/// Provides methods for querying item hierarchies using iterative traversal.
+/// Uses AncestorIds and LinkedChildren tables for parent-child traversal.
+/// </summary>
+public static class DescendantQueryHelper
+{
+    /// <summary>
+    /// Gets a queryable of all descendant IDs for a parent item.
+    /// Traverses AncestorIds and LinkedChildren to find all descendants.
+    /// </summary>
+    /// <param name="context">Database context.</param>
+    /// <param name="parentId">Parent item ID.</param>
+    /// <returns>Queryable of descendant item IDs.</returns>
+    public static IQueryable<Guid> GetAllDescendantIds(JellyfinDbContext context, Guid parentId)
+    {
+        ArgumentNullException.ThrowIfNull(context);
+
+        var descendants = TraverseHierarchyDown(context, [parentId]);
+
+        descendants.Remove(parentId);
+
+        return descendants.AsQueryable();
+    }
+
+    /// <summary>
+    /// Gets a queryable of all owned descendant IDs for a parent item.
+    /// Traverses only AncestorIds (hierarchical ownership), NOT LinkedChildren (associations).
+    /// Use this for deletion to avoid destroying items that are merely linked (e.g. movies in a BoxSet).
+    /// </summary>
+    /// <param name="context">Database context.</param>
+    /// <param name="parentId">Parent item ID.</param>
+    /// <returns>Queryable of owned descendant item IDs.</returns>
+    public static IQueryable<Guid> GetOwnedDescendantIds(JellyfinDbContext context, Guid parentId)
+    {
+        ArgumentNullException.ThrowIfNull(context);
+
+        var descendants = TraverseHierarchyDownOwned(context, [parentId]);
+
+        descendants.Remove(parentId);
+
+        return descendants.AsQueryable();
+    }
+
+    /// <summary>
+    /// Gets all owned descendant IDs for multiple parent items in a single traversal.
+    /// More efficient than calling <see cref="GetOwnedDescendantIds"/> per parent because
+    /// it performs one traversal for all seeds instead of N separate traversals.
+    /// </summary>
+    /// <param name="context">Database context.</param>
+    /// <param name="parentIds">Parent item IDs.</param>
+    /// <returns>Set of all owned descendant item IDs (excluding the parent IDs themselves).</returns>
+    public static HashSet<Guid> GetOwnedDescendantIdsBatch(JellyfinDbContext context, IReadOnlyList<Guid> parentIds)
+    {
+        ArgumentNullException.ThrowIfNull(context);
+        ArgumentNullException.ThrowIfNull(parentIds);
+
+        if (parentIds.Count == 0)
+        {
+            return [];
+        }
+
+        var seedSet = new HashSet<Guid>(parentIds);
+        var descendants = TraverseHierarchyDownOwned(context, seedSet);
+
+        // Remove the seed IDs — callers want only descendants
+        descendants.ExceptWith(seedSet);
+
+        return descendants;
+    }
+
+    /// <summary>
+    /// Gets a queryable of all folder IDs that have any descendant matching the specified criteria.
+    /// Can be used in LINQ .Contains() expressions.
+    /// </summary>
+    /// <param name="context">Database context.</param>
+    /// <param name="criteria">The matching criteria to apply.</param>
+    /// <returns>Queryable of folder IDs.</returns>
+    public static IQueryable<Guid> GetFolderIdsMatching(JellyfinDbContext context, FolderMatchCriteria criteria)
+    {
+        ArgumentNullException.ThrowIfNull(context);
+        ArgumentNullException.ThrowIfNull(criteria);
+        var matchingItemIds = criteria switch
+        {
+            HasSubtitles => context.MediaStreamInfos
+                .Where(ms => ms.StreamType == MediaStreamTypeEntity.Subtitle)
+                .Select(ms => ms.ItemId)
+                .Distinct()
+                .ToHashSet(),
+            HasChapterImages => context.Chapters
+                .Where(c => c.ImagePath != null)
+                .Select(c => c.ItemId)
+                .Distinct()
+                .ToHashSet(),
+            HasMediaStreamType m => GetMatchingMediaStreamItemIds(context, m),
+            _ => throw new ArgumentOutOfRangeException(nameof(criteria), $"Unknown criteria type: {criteria.GetType().Name}")
+        };
+
+        var ancestors = TraverseHierarchyUp(context, matchingItemIds);
+
+        return ancestors.AsQueryable();
+    }
+
+    private static HashSet<Guid> GetMatchingMediaStreamItemIds(JellyfinDbContext context, HasMediaStreamType criteria)
+    {
+        var query = context.MediaStreamInfos
+            .Where(ms => ms.StreamType == criteria.StreamType && ms.Language == criteria.Language);
+
+        if (criteria.IsExternal.HasValue)
+        {
+            var isExternal = criteria.IsExternal.Value;
+            query = query.Where(ms => ms.IsExternal == isExternal);
+        }
+
+        return query.Select(ms => ms.ItemId).Distinct().ToHashSet();
+    }
+
+    /// <summary>
+    /// Traverses DOWN the hierarchy from parent folders to find all descendants.
+    /// </summary>
+    private static HashSet<Guid> TraverseHierarchyDown(JellyfinDbContext context, ICollection<Guid> startIds)
+    {
+        var visited = new HashSet<Guid>(startIds);
+        var folderStack = new HashSet<Guid>(startIds);
+
+        while (folderStack.Count != 0)
+        {
+            var currentFolders = folderStack.ToArray();
+            folderStack.Clear();
+
+            var directChildren = context.AncestorIds
+                .WhereOneOrMany(currentFolders, e => e.ParentItemId)
+                .Select(e => e.ItemId)
+                .ToArray();
+
+            var linkedChildren = context.LinkedChildren
+                .WhereOneOrMany(currentFolders, e => e.ParentId)
+                .Select(e => e.ChildId)
+                .ToArray();
+
+            var allChildren = directChildren.Concat(linkedChildren).Distinct().ToArray();
+
+            if (allChildren.Length == 0)
+            {
+                break;
+            }
+
+            var childFolders = context.BaseItems
+                .WhereOneOrMany(allChildren, e => e.Id)
+                .Where(e => e.IsFolder)
+                .Select(e => e.Id)
+                .ToHashSet();
+
+            foreach (var childId in allChildren)
+            {
+                if (visited.Add(childId) && childFolders.Contains(childId))
+                {
+                    folderStack.Add(childId);
+                }
+            }
+        }
+
+        return visited;
+    }
+
+    /// <summary>
+    /// Traverses DOWN the hierarchy using only AncestorIds (ownership), not LinkedChildren.
+    /// </summary>
+    private static HashSet<Guid> TraverseHierarchyDownOwned(JellyfinDbContext context, ICollection<Guid> startIds)
+    {
+        var visited = new HashSet<Guid>(startIds);
+        var folderStack = new HashSet<Guid>(startIds);
+
+        while (folderStack.Count != 0)
+        {
+            var currentFolders = folderStack.ToArray();
+            folderStack.Clear();
+
+            var directChildren = context.AncestorIds
+                .WhereOneOrMany(currentFolders, e => e.ParentItemId)
+                .Select(e => e.ItemId)
+                .ToArray();
+
+            if (directChildren.Length == 0)
+            {
+                break;
+            }
+
+            var childFolders = context.BaseItems
+                .WhereOneOrMany(directChildren, e => e.Id)
+                .Where(e => e.IsFolder)
+                .Select(e => e.Id)
+                .ToHashSet();
+
+            foreach (var childId in directChildren)
+            {
+                if (visited.Add(childId) && childFolders.Contains(childId))
+                {
+                    folderStack.Add(childId);
+                }
+            }
+        }
+
+        return visited;
+    }
+
+    /// <summary>
+    /// Traverses UP the hierarchy from items to find all ancestor folders.
+    /// </summary>
+    private static HashSet<Guid> TraverseHierarchyUp(JellyfinDbContext context, ICollection<Guid> startIds)
+    {
+        var ancestors = new HashSet<Guid>();
+        var itemStack = new HashSet<Guid>(startIds);
+
+        while (itemStack.Count != 0)
+        {
+            var currentItems = itemStack.ToArray();
+            itemStack.Clear();
+
+            var ancestorParents = context.AncestorIds
+                .WhereOneOrMany(currentItems, e => e.ItemId)
+                .Select(e => e.ParentItemId)
+                .ToArray();
+
+            var linkedParents = context.LinkedChildren
+                .WhereOneOrMany(currentItems, e => e.ChildId)
+                .Select(e => e.ParentId)
+                .ToArray();
+
+            foreach (var parentId in ancestorParents.Concat(linkedParents))
+            {
+                if (ancestors.Add(parentId))
+                {
+                    itemStack.Add(parentId);
+                }
+            }
+        }
+
+        return ancestors;
+    }
+}