Fix recursive handling for LinkedChildren

MediaBrowser.Controller/Entities/InternalItemsQuery.cs

@@ -103,6 +103,7 @@ namespace MediaBrowser.Controller.Entities
             || SubtitleLanguages.Count > 0
             || LinkedChildAncestorIds.Length > 0
             || AncestorIds.Length > 0
+            || DescendantOfId.HasValue
             || IsFavorite.HasValue
             || IsFavoriteOrLiked.HasValue
             || IsLiked.HasValue
@@ -368,6 +369,13 @@ namespace MediaBrowser.Controller.Entities
         /// </summary>
         public Guid[] LinkedChildAncestorIds { get; set; }
 
+        /// <summary>
+        /// Gets or sets the id of a folder whose descendants the items must be part of.
+        /// Unlike <see cref="AncestorIds"/> this also follows the linked children of BoxSets and
+        /// Playlists, so it reaches the items below a linked folder (a Series' episodes, for example).
+        /// </summary>
+        public Guid? DescendantOfId { get; set; }
+
         public Guid[] TopParentIds { get; set; }
 
         public CollectionType?[] PresetViews { get; set; }