From 5dd256f76c9f5cee1644bdf2dcc2b136db81e513 Mon Sep 17 00:00:00 2001
From: Bart Koelman <10324372+bkoelman@users.noreply.github.com>
Date: Fri, 7 Apr 2023 18:08:49 +0200
Subject: [PATCH] Improve performance of response serialization for large
 number of included resources

---
 .../Serialization/Response/ResourceObjectTreeNode.cs     | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/src/JsonApiDotNetCore/Serialization/Response/ResourceObjectTreeNode.cs b/src/JsonApiDotNetCore/Serialization/Response/ResourceObjectTreeNode.cs
index 4c3a44fe38..6226d6e597 100644
--- a/src/JsonApiDotNetCore/Serialization/Response/ResourceObjectTreeNode.cs
+++ b/src/JsonApiDotNetCore/Serialization/Response/ResourceObjectTreeNode.cs
@@ -181,13 +181,14 @@ public IList<ResourceObject> GetResponseIncluded()
             VisitRelationshipChildrenInSubtree(child, visited);
         }
 
-        List<ResourceObject> includes = visited.Select(node => node.ResourceObject).ToList();
+        ISet<ResourceObject> primaryResourceObjectSet = GetDirectChildren().Select(node => node.ResourceObject).ToHashSet(ResourceObjectComparer.Instance);
+        List<ResourceObject> includes = new();
 
-        foreach (ResourceObject primaryResourceObject in GetDirectChildren().Select(node => node.ResourceObject))
+        foreach (ResourceObject include in visited.Select(node => node.ResourceObject))
         {
-            if (includes.Contains(primaryResourceObject, ResourceObjectComparer.Instance))
+            if (!primaryResourceObjectSet.Contains(include))
             {
-                includes.Remove(primaryResourceObject);
+                includes.Add(include);
             }
         }