From 08a30c7eaf6e7ce216b1ad5a7b8394587609cd91 Mon Sep 17 00:00:00 2001
From: Felix Reinhardt <felixre0@users.noreply.github.com>
Date: Mon, 17 Aug 2020 16:19:20 +0200
Subject: [PATCH 1/3] Fix: Retaining null value for populated documents when
 _id is suppressed

---
 lib/helpers/populate/assignVals.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/helpers/populate/assignVals.js b/lib/helpers/populate/assignVals.js
index 798482197db..c47172b0b25 100644
--- a/lib/helpers/populate/assignVals.js
+++ b/lib/helpers/populate/assignVals.js
@@ -243,7 +243,7 @@ function valueFilter(val, assignmentOpts, populateOptions) {
  */
 
 function maybeRemoveId(subdoc, assignmentOpts) {
-  if (assignmentOpts.excludeId) {
+  if (subdoc != null && assignmentOpts.excludeId) {
     if (typeof subdoc.$__setValue === 'function') {
       delete subdoc._doc._id;
     } else {

From bf1a9b492a986221f441f7276f9c944a2d733828 Mon Sep 17 00:00:00 2001
From: Felix Reinhardt <felixre0@users.noreply.github.com>
Date: Mon, 17 Aug 2020 17:10:41 +0200
Subject: [PATCH 2/3] Add regression test for Issue #9336

---
 test/model.populate.test.js | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/test/model.populate.test.js b/test/model.populate.test.js
index ecf2a52e33e..f9055597a40 100644
--- a/test/model.populate.test.js
+++ b/test/model.populate.test.js
@@ -1626,6 +1626,33 @@ describe('model: populate:', function() {
     });
   });
 
+  it('supports `retainNullValues` while supressing _id of subdocument', function() {
+    const BlogPost = db.model('BlogPost', blogPostSchema);
+    const User = db.model('User', userSchema);
+
+    return co(function*() {
+      const user = new User({ name: 'Victor Hugo' });
+      yield user.save();
+      const post = yield BlogPost.create({
+        title: 'Notre-Dame de Paris',
+        fans: []
+      });
+      
+      yield BlogPost.collection.updateOne({ _id: post._id }, {
+        $set: { fans: [user.id] }
+      });
+
+      yield user.delete()
+
+      const returned = yield BlogPost.
+        findById(post._id).
+        populate({ path: 'fans', select: 'name -_id', options: { retainNullValues: true } });
+
+      assert.equal(returned.fans.length, 1);
+      assert.strictEqual(returned.fans[0], null);
+    });
+  });
+
   it('populating more than one array at a time', function(done) {
     const User = db.model('User', userSchema);
     const M = db.model('Test', new Schema({

From 01b48b5d3419fab07624f23bffe72cdf1e8db678 Mon Sep 17 00:00:00 2001
From: Felix Reinhardt <felixre0@users.noreply.github.com>
Date: Mon, 17 Aug 2020 17:27:32 +0200
Subject: [PATCH 3/3] Fixed code style

---
 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 f9055597a40..3fdd83c76ee 100644
--- a/test/model.populate.test.js
+++ b/test/model.populate.test.js
@@ -1637,12 +1637,12 @@ describe('model: populate:', function() {
         title: 'Notre-Dame de Paris',
         fans: []
       });
-      
+
       yield BlogPost.collection.updateOne({ _id: post._id }, {
         $set: { fans: [user.id] }
       });
 
-      yield user.delete()
+      yield user.delete();
 
       const returned = yield BlogPost.
         findById(post._id).