From fbc01b8a53a039e08142660379819a86101ca560 Mon Sep 17 00:00:00 2001
From: Zurab Sabakhtarishvili <zsabakhtarishvili@atu.edu>
Date: Fri, 8 Sep 2023 00:04:26 -0500
Subject: [PATCH 1/3] Fixed exception where a null element within an array
 would be attempted to be populated.

Prior to this change, if there was a wild NULL object in an array within a document, mongoose would attempt to populate it, and would crash.

For example, if we had a schema containing founders array in such a way -
  founders: [
    {
      legal_form: { type: mongoose.Schema.Types.ObjectId, ref: "Legal_Form" },
      name: String,
      identification_code: String,
      ownership_percentage: Number,
    },
  ],


and our database contained a document with founders looking like following -

[
  {
    name: ''name",
    identification_code: ''id_code",
    ownership_percentage: 51.5,
    _id: new ObjectId("63e1fc49d4ff7ce99a3a3faf")
  },
  null
]

Then following error would be thrown -
TypeError: Cannot read properties of null (reading 'populated')
---
 lib/helpers/populate/markArraySubdocsPopulated.js | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/helpers/populate/markArraySubdocsPopulated.js b/lib/helpers/populate/markArraySubdocsPopulated.js
index 28b19a1989a..a22605a1d78 100644
--- a/lib/helpers/populate/markArraySubdocsPopulated.js
+++ b/lib/helpers/populate/markArraySubdocsPopulated.js
@@ -38,7 +38,9 @@ module.exports = function markArraySubdocsPopulated(doc, populated) {
 
       if (utils.isMongooseDocumentArray(val)) {
         for (let j = 0; j < val.length; ++j) {
-          val[j].populated(rest, item._docs[id] == null ? void 0 : item._docs[id][j], item);
+          if(val[j]){
+            val[j].populated(rest, item._docs[id] == null ? void 0 : item._docs[id][j], item);
+          }
         }
         break;
       }

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 2/3] 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() {

From e42aaafc4d2f00861434219e152478512a2b006a Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Fri, 8 Sep 2023 10:36:04 -0400
Subject: [PATCH 3/3] style: fix lint

---
 test/model.populate.test.js | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/test/model.populate.test.js b/test/model.populate.test.js
index bd79133c9a1..c1007f69315 100644
--- a/test/model.populate.test.js
+++ b/test/model.populate.test.js
@@ -10291,8 +10291,8 @@ describe('model: populate:', function() {
         {
           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 }));