fix(setDefaultsOnInsert): check child filter paths before applying defaults

lib/helpers/setDefaultsOnInsert.js

@@ -22,7 +22,7 @@ module.exports = function(filter, schema, castedDoc, options) {
   }
 
   const keys = Object.keys(castedDoc || {});
-  const updatedKeys = {};
+  const updatedKeys = Object.create(null);
   const updatedValues = {};
   const numKeys = keys.length;
 
@@ -55,6 +55,14 @@ module.exports = function(filter, schema, castedDoc, options) {
       }
     }
     updatedKeys[path] = true;
+    // Also mark all parent prefixes so child-path lookups are O(1).
+    // e.g. 'extraProps.location' also marks 'extraProps'
+    const pieces = path.split('.');
+    let cur = pieces[0];
+    for (let j = 1; j < pieces.length; ++j) {
+      updatedKeys[cur] = true;
+      cur += '.' + pieces[j];
+    }
   }
 
   if (options?.overwrite && !hasDollarUpdate) {

test/model.findOneAndUpdate.test.js

@@ -959,6 +959,44 @@ describe('model: findOneAndUpdate:', function() {
       assert.equal(1, count);
     });
 
+    it('includes dot-notation filter paths in upserted document with sub-schema default (gh-16030)', async function() {
+      const addressSchema = new Schema({ city: String });
+      const userSchema = new Schema({
+        firstName: String,
+        lastName: String,
+        address: { type: addressSchema, default: {} }
+      });
+      const User = db.model('User', userSchema);
+
+      const foundUser = await User.findOneAndUpdate(
+        { 'address.city': 'New York' },
+        { $setOnInsert: { firstName: 'John' }, $set: { lastName: 'Smith' } },
+        { upsert: true, new: true, setDefaultsOnInsert: true });
+
+      assert.equal(foundUser.lastName, 'Smith');
+      assert.equal(foundUser.firstName, 'John');
+      assert.equal(foundUser.address.city, 'New York');
+    });
+
+    it('applies sub-schema default on upsert when filter has no dot-notation path (gh-16030)', async function() {
+      const addressSchema = new Schema({ city: String });
+      const userSchema = new Schema({
+        firstName: String,
+        lastName: String,
+        address: { type: addressSchema, default: { city: 'Chicago' } }
+      });
+      const User = db.model('User', userSchema);
+
+      const foundUser = await User.findOneAndUpdate(
+        { firstName: 'John' },
+        { $set: { lastName: 'Smith' } },
+        { upsert: true, new: true, setDefaultsOnInsert: true });
+
+      assert.equal(foundUser.lastName, 'Smith');
+      assert.equal(foundUser.firstName, 'John');
+      assert.equal(foundUser.address.city, 'Chicago');
+    });
+
     it('skips setting defaults within maps (gh-7909)', async function() {
       const socialMediaHandleSchema = Schema({ links: [String] });
       const profileSchema = Schema({