From 167f68284098dcb67d733edc540cce596db44d4d Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Tue, 4 Oct 2022 18:26:03 -0400
Subject: [PATCH 1/3] fix(document): set defaults on subdocuments underneath
 init-ed single nested subdocument

Fix #12515
---
 lib/schema/SubdocumentPath.js |  1 +
 test/document.test.js         | 27 +++++++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/lib/schema/SubdocumentPath.js b/lib/schema/SubdocumentPath.js
index e748d6a5026..46ea86673e0 100644
--- a/lib/schema/SubdocumentPath.js
+++ b/lib/schema/SubdocumentPath.js
@@ -174,6 +174,7 @@ SubdocumentPath.prototype.cast = function(val, doc, init, priorVal, options) {
     subdoc = new Constructor(void 0, selected, doc, false, { defaults: false });
     subdoc.$init(val);
     applyDefaults(subdoc, selected);
+    delete subdoc.$__.defaults;
   } else {
     if (Object.keys(val).length === 0) {
       return new Constructor({}, selected, doc, undefined, options);
diff --git a/test/document.test.js b/test/document.test.js
index ca2aca278d9..eebea364b3c 100644
--- a/test/document.test.js
+++ b/test/document.test.js
@@ -11865,6 +11865,33 @@ describe('document', function() {
     assert.strictEqual(called, 1);
   });
 
+  it('applies defaults to pushed subdocs after initing document (gh-12515)', async function() {
+    const animalSchema = new Schema({ title: String });
+    const animalsSchema = new Schema({
+      species: [animalSchema],
+      totalAnimals: Number
+    });
+    const Userschema = new Schema({
+      animals: animalsSchema
+    });
+    const UserModel = db.model('User', Userschema);
+
+    const doc = new UserModel();
+    doc.animals = { totalAnimals: 1 };
+    doc.animals.species = [{ title: 'Lion' }];
+    await doc.save();
+    // once created we fetch it again
+    let user = await UserModel.findById(doc._id);
+
+    // add new animal
+    user.animals.species.push({ title: 'Elephant' });
+    await user.save();
+    assert.ok(user.animals.species[1]._id);
+    user = await UserModel.collection.findOne({ _id: user._id });
+
+    assert.ok(user.animals.species[1]._id);
+  });
+
   it('If the field does not exist, $inc should create it and set is value to the specified one (gh-12435)', async function() {
     const schema = new mongoose.Schema({
       name: String,

From b0edca9f6d2ce957194f8229744140c08b325fb5 Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Wed, 5 Oct 2022 10:25:14 -0400
Subject: [PATCH 2/3] fix: quick fix so `defaults` is unset before init

---
 lib/schema/SubdocumentPath.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/schema/SubdocumentPath.js b/lib/schema/SubdocumentPath.js
index 46ea86673e0..1eb65207d85 100644
--- a/lib/schema/SubdocumentPath.js
+++ b/lib/schema/SubdocumentPath.js
@@ -172,9 +172,9 @@ SubdocumentPath.prototype.cast = function(val, doc, init, priorVal, options) {
   options = Object.assign({}, options, { priorDoc: priorVal });
   if (init) {
     subdoc = new Constructor(void 0, selected, doc, false, { defaults: false });
+    delete subdoc.$__.defaults;
     subdoc.$init(val);
     applyDefaults(subdoc, selected);
-    delete subdoc.$__.defaults;
   } else {
     if (Object.keys(val).length === 0) {
       return new Constructor({}, selected, doc, undefined, options);

From 630bd3738ea237934ac4f246fb97d868412b5d70 Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Wed, 5 Oct 2022 11:04:04 -0400
Subject: [PATCH 3/3] test: cover a couple of extra cases for #12515

---
 test/document.test.js | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/test/document.test.js b/test/document.test.js
index eebea364b3c..2d38adc8629 100644
--- a/test/document.test.js
+++ b/test/document.test.js
@@ -11886,9 +11886,11 @@ describe('document', function() {
     // add new animal
     user.animals.species.push({ title: 'Elephant' });
     await user.save();
+    assert.ok(user.animals.species[0]._id);
     assert.ok(user.animals.species[1]._id);
     user = await UserModel.collection.findOne({ _id: user._id });
 
+    assert.ok(user.animals.species[0]._id);
     assert.ok(user.animals.species[1]._id);
   });