From 965b7d596d3c62e1a6917dcc29bbd748e404dfed Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Fri, 1 Dec 2023 14:53:22 -0500
Subject: [PATCH 1/2] fix(populate): call `transform` object with single id
 instead of array when populating a justOne path under an array

Fix #14073
---
 lib/helpers/populate/assignVals.js |  2 ++
 test/model.populate.test.js        | 55 ++++++++++++++++++++++++++++++
 2 files changed, 57 insertions(+)

diff --git a/lib/helpers/populate/assignVals.js b/lib/helpers/populate/assignVals.js
index 017066add17..858846b42c0 100644
--- a/lib/helpers/populate/assignVals.js
+++ b/lib/helpers/populate/assignVals.js
@@ -80,6 +80,8 @@ module.exports = function assignVals(o) {
       return valueFilter(val[0], options, populateOptions, _allIds);
     } else if (o.justOne === false && !Array.isArray(val)) {
       return valueFilter([val], options, populateOptions, _allIds);
+    } else if (o.justOne === true && !Array.isArray(val) && Array.isArray(_allIds)) {
+      return valueFilter(val, options, populateOptions, val == null ? val : _allIds[o.rawOrder[val._id]]);
     }
     return valueFilter(val, options, populateOptions, _allIds);
   }
diff --git a/test/model.populate.test.js b/test/model.populate.test.js
index 4e638a90178..4072a2a081a 100644
--- a/test/model.populate.test.js
+++ b/test/model.populate.test.js
@@ -10776,4 +10776,59 @@ describe('model: populate:', function() {
       new Date('2015-06-01').toString()
     );
   });
+
+  it('calls transform with single ObjectId when populating justOne path underneath array (gh-14073)', async function() {
+    const mySchema = mongoose.Schema({
+      name: { type: String },
+      items: [{
+        _id: false,
+        name: { type: String },
+        brand: { type: mongoose.Schema.Types.ObjectId, ref: 'Brand' }
+      }]
+    });
+
+    const brandSchema = mongoose.Schema({
+      name: 'String',
+      quantity: Number
+    });
+
+    const myModel = db.model('MyModel', mySchema);
+    const brandModel = db.model('Brand', brandSchema);
+    const { _id: id1 } = await brandModel.create({
+      name: 'test',
+      quantity: 1
+    });
+    const { _id: id2 } = await brandModel.create({
+      name: 'test1',
+      quantity: 1
+    });
+    const { _id: id3 } = await brandModel.create({
+      name: 'test2',
+      quantity: 2
+    });
+    const brands = await brandModel.find();
+    const test = new myModel({ name: 'Test Model' });
+    for (let i = 0; i < brands.length; i++) {
+      test.items.push({ name: `${i}`, brand: brands[i]._id });
+    }
+    await test.save();
+
+    const ids = [];
+    await myModel
+      .findOne()
+      .populate([
+        {
+          path: 'items.brand',
+          transform: (doc, id) => {
+            ids.push(id);
+            return doc;
+          }
+        }
+      ]);
+    assert.equal(ids.length, 3);
+    assert.deepStrictEqual(
+      ids.map(id => id.toHexString()),
+      [id1.toString(), id2.toString(), id3.toString()]
+    );
+  });
 });

From 0282f50b5fe696ca9ae869c07f3cec41f2b40cb6 Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Fri, 1 Dec 2023 15:13:07 -0500
Subject: [PATCH 2/2] fix(populate): make sure to call `transform` with the
 correct index, even if no document found

Re: #14073
---
 lib/helpers/populate/assignVals.js | 5 ++++-
 test/model.populate.test.js        | 9 ++++++---
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/lib/helpers/populate/assignVals.js b/lib/helpers/populate/assignVals.js
index 858846b42c0..44de64fbc39 100644
--- a/lib/helpers/populate/assignVals.js
+++ b/lib/helpers/populate/assignVals.js
@@ -39,8 +39,10 @@ module.exports = function assignVals(o) {
   const options = o.options;
   const count = o.count && o.isVirtual;
   let i;
+  let setValueIndex = 0;
 
   function setValue(val) {
+    ++setValueIndex;
     if (count) {
       return val;
     }
@@ -81,12 +83,13 @@ module.exports = function assignVals(o) {
     } else if (o.justOne === false && !Array.isArray(val)) {
       return valueFilter([val], options, populateOptions, _allIds);
     } else if (o.justOne === true && !Array.isArray(val) && Array.isArray(_allIds)) {
-      return valueFilter(val, options, populateOptions, val == null ? val : _allIds[o.rawOrder[val._id]]);
+      return valueFilter(val, options, populateOptions, val == null ? val : _allIds[setValueIndex - 1]);
     }
     return valueFilter(val, options, populateOptions, _allIds);
   }
 
   for (i = 0; i < docs.length; ++i) {
+    setValueIndex = 0;
     const _path = o.path.endsWith('.$*') ? o.path.slice(0, -3) : o.path;
     const existingVal = mpath.get(_path, docs[i], lookupLocalFields);
     if (existingVal == null && !getVirtual(o.originalModel.schema, _path)) {
diff --git a/test/model.populate.test.js b/test/model.populate.test.js
index 4072a2a081a..9f629f28844 100644
--- a/test/model.populate.test.js
+++ b/test/model.populate.test.js
@@ -10811,6 +10811,9 @@ describe('model: populate:', function() {
     for (let i = 0; i < brands.length; i++) {
       test.items.push({ name: `${i}`, brand: brands[i]._id });
     }
+
+    const id4 = new mongoose.Types.ObjectId();
+    test.items.push({ name: '4', brand: id4 });
     await test.save();
 
     const ids = [];
@@ -10825,10 +10828,10 @@ describe('model: populate:', function() {
           }
         }
       ]);
-    assert.equal(ids.length, 3);
+    assert.equal(ids.length, 4);
     assert.deepStrictEqual(
-      ids.map(id => id.toHexString()),
-      [id1.toString(), id2.toString(), id3.toString()]
+      ids.map(id => id?.toHexString()),
+      [id1.toString(), id2.toString(), id3.toString(), id4.toString()]
     );
   });
 });