From 78bca702fa3dc6b59086fef8d9aed2ca570ee448 Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Fri, 8 Sep 2023 09:56:40 -0400
Subject: [PATCH] test: add test case for #13839

---
 .../populate/markArraySubdocsPopulated.js     |  2 +-
 test/model.populate.test.js                   | 22 +++++++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/lib/helpers/populate/markArraySubdocsPopulated.js b/lib/helpers/populate/markArraySubdocsPopulated.js
index a22605a1d78..f9cc83cb6bf 100644
--- a/lib/helpers/populate/markArraySubdocsPopulated.js
+++ b/lib/helpers/populate/markArraySubdocsPopulated.js
@@ -38,7 +38,7 @@ module.exports = function markArraySubdocsPopulated(doc, populated) {
 
       if (utils.isMongooseDocumentArray(val)) {
         for (let j = 0; j < val.length; ++j) {
-          if(val[j]){
+          if (val[j]) {
             val[j].populated(rest, item._docs[id] == null ? void 0 : item._docs[id][j], item);
           }
         }
diff --git a/test/model.populate.test.js b/test/model.populate.test.js
index 690e7783872..bd79133c9a1 100644
--- a/test/model.populate.test.js
+++ b/test/model.populate.test.js
@@ -10285,6 +10285,28 @@ describe('model: populate:', function() {
 
   });
 
+  it('handles populating underneath document arrays that have null (gh-13839)', async function() {
+    const schema = new Schema({
+      children: [
+        {
+          doc: { type: mongoose.Schema.Types.ObjectId, ref: 'Child' },
+          name: String
+        },
+    ]
+    });
+    const Parent = db.model('Parent', schema);
+    const Child = db.model('Child', new Schema({ name: String }));
+
+    const child = new Child({ name: 'gh-13839-test' });
+    await child.save();
+    let parent = new Parent({ children: [{ doc: child._id, name: 'foo' }, null] });
+    await parent.save();
+
+    parent = await Parent.findById(parent._id).populate('children.doc');
+    assert.equal(parent.children.length, 2);
+    assert.equal(parent.children[0].doc.name, 'gh-13839-test');
+    assert.equal(parent.children[1], null);
+  });
 
   describe('strictPopulate', function() {
     it('reports full path when throwing `strictPopulate` error with deep populate (gh-10923)', async function() {