From 1cbe8e8c00183e5185c1ada483cd54d456295beb Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Thu, 22 Dec 2022 17:33:52 -0500
Subject: [PATCH] fix(query): fix unexpected validation error when doing
 findOneAndReplace() with a nullish value

---
 lib/helpers/query/castUpdate.js |  8 +++++---
 test/query.test.js              | 12 +++++++++++-
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/lib/helpers/query/castUpdate.js b/lib/helpers/query/castUpdate.js
index 50d56509610..2e1dde1a6c0 100644
--- a/lib/helpers/query/castUpdate.js
+++ b/lib/helpers/query/castUpdate.js
@@ -85,9 +85,11 @@ module.exports = function castUpdate(schema, obj, options, context, filter) {
     val = ret[op];
     hasDollarKey = hasDollarKey || op.startsWith('$');
     const toUnset = {};
-    for (const key of Object.keys(val)) {
-      if (val[key] === undefined) {
-        toUnset[key] = 1;
+    if (val != null) {
+      for (const key of Object.keys(val)) {
+        if (val[key] === undefined) {
+          toUnset[key] = 1;
+        }
       }
     }
 
diff --git a/test/query.test.js b/test/query.test.js
index f291c5d226f..a02a37896d0 100644
--- a/test/query.test.js
+++ b/test/query.test.js
@@ -4314,7 +4314,7 @@ describe('Query', function() {
     assert.strictEqual(found[0].title, 'burrito bowl');
   });
 
-  it('update operation should remove fields set to undefined (gh-12794)', async() => {
+  it('update operation should remove fields set to undefined (gh-12794) (gh-12821)', async function() {
     const m = new mongoose.Mongoose();
 
     await m.connect(start.uri);
@@ -4338,5 +4338,15 @@ describe('Query', function() {
     ).lean();
 
     assert.ok('title' in updatedDoc === false);
+
+    const replacedDoc = await Test.findOneAndReplace(
+      {
+        _id: doc._id
+      },
+      { title: undefined },
+      { returnOriginal: false }
+    ).lean();
+
+    assert.ok('title' in replacedDoc === false);
   });
 });