From 2918f79b2cb4f893823bc3401175a4787f3c2c59 Mon Sep 17 00:00:00 2001
From: Mohamed Yousef <moh.y.oraby@gmail.com>
Date: Fri, 9 Aug 2024 21:07:58 +0300
Subject: [PATCH 1/2] fix populate called with lean when multiple documents
 with same foreignField exist

---
 lib/model.js                |  4 ++--
 test/model.populate.test.js | 48 +++++++++++++++++++++++++++++++++++++
 2 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/lib/model.js b/lib/model.js
index b3497669be5..808a595811c 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -4459,7 +4459,7 @@ function _assign(model, vals, mod, assignmentOpts) {
         }
       } else {
         if (_val instanceof Document) {
-          _val = _val._doc._id;
+          _val = _val._id;
         }
         key = String(_val);
         if (rawDocs[key]) {
@@ -4468,7 +4468,7 @@ function _assign(model, vals, mod, assignmentOpts) {
             rawOrder[key].push(i);
           } else if (isVirtual ||
             rawDocs[key].constructor !== val.constructor ||
-            String(rawDocs[key]._doc._id) !== String(val._doc._id)) {
+            String(rawDocs[key]._id) !== String(val._id)) {
             // May need to store multiple docs with the same id if there's multiple models
             // if we have discriminators or a ref function. But avoid converting to an array
             // if we have multiple queries on the same model because of `perDocumentLimit` re: gh-9906
diff --git a/test/model.populate.test.js b/test/model.populate.test.js
index 48427bf6ae4..bbdfadfe281 100644
--- a/test/model.populate.test.js
+++ b/test/model.populate.test.js
@@ -11048,4 +11048,52 @@ describe('model: populate:', function() {
     assert.equal(pet1.owner.name, 'Alice');
     assert.equal(pet2.owner.name, 'Alice');
   });
+
+  it('makes sure that populate works correctly with duplicate foreignField with lean(); (gh-14794)', async function() {
+    const authorSchema = new mongoose.Schema({
+      group: String,
+      name: String
+    });
+
+    const postSchema = new mongoose.Schema({
+      authorGroup: String,
+      title: String,
+      content: String
+    });
+
+    const Author = db.model('Author', authorSchema);
+    const Post = db.model('Post', postSchema);
+
+    await Author.create({ group: 'AUTH1', name: 'John Doe' });
+    await Author.create({ group: 'AUTH2', name: 'Jane Smith' });
+    await Author.create({ group: 'AUTH2', name: 'Will Jons' });
+
+    await Post.create({
+      authorGroup: 'AUTH1',
+      title: 'First Post',
+      content: 'Content 1'
+    });
+
+    await Post.create({
+      authorGroup: 'AUTH2',
+      title: 'Second Post',
+      content: 'Content 2'
+    });
+
+    const posts = await Post.find()
+      .populate({
+        path: 'authorGroup',
+        model: 'Author',
+        select: { _id: 1, name: 1 },
+        foreignField: 'group'
+      })
+      .select({ _id: 1, authorGroup: 1, title: 1 })
+      .lean();
+
+    for (const post of posts) {
+      assert.ok(post.authorGroup._id instanceof mongoose.Types.ObjectId);
+      assert.ok(typeof post.authorGroup.name === 'string');
+    }
+    assert.equal(posts.length, 2);
+  });
 });

From 0cb96b92a2bf549c6e8233b68425622f260460bc Mon Sep 17 00:00:00 2001
From: Mohamed Yousef <moh.y.oraby@gmail.com>
Date: Sun, 11 Aug 2024 02:32:43 +0300
Subject: [PATCH 2/2] check if rawDocs[key] and val are instance of Document

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

diff --git a/lib/model.js b/lib/model.js
index 808a595811c..da841ae28f7 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -4459,7 +4459,7 @@ function _assign(model, vals, mod, assignmentOpts) {
         }
       } else {
         if (_val instanceof Document) {
-          _val = _val._id;
+          _val = _val._doc._id;
         }
         key = String(_val);
         if (rawDocs[key]) {
@@ -4468,7 +4468,7 @@ function _assign(model, vals, mod, assignmentOpts) {
             rawOrder[key].push(i);
           } else if (isVirtual ||
             rawDocs[key].constructor !== val.constructor ||
-            String(rawDocs[key]._id) !== String(val._id)) {
+            (rawDocs[key] instanceof Document ? String(rawDocs[key]._doc._id) : String(rawDocs[key]._id)) !== (val instanceof Document ? String(val._doc._id) : String(val._id))) {
             // May need to store multiple docs with the same id if there's multiple models
             // if we have discriminators or a ref function. But avoid converting to an array
             // if we have multiple queries on the same model because of `perDocumentLimit` re: gh-9906