From 49aa88dd3eb7a01c95eeb453a9bbf3978cc6a8de Mon Sep 17 00:00:00 2001
From: Luca Pizzini <lpizzini7@gmail.com>
Date: Sat, 16 Jul 2022 10:56:04 +0200
Subject: [PATCH] fix: mongoose schema default should not mutate original
 object

Closes #12102
---
 lib/document.js       |  6 ++++--
 test/document.test.js | 25 +++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/lib/document.js b/lib/document.js
index 72c12c75442..4f525addcc2 100644
--- a/lib/document.js
+++ b/lib/document.js
@@ -1149,8 +1149,10 @@ Document.prototype.$set = function $set(path, val, type, options) {
       }
 
       if (utils.isNonBuiltinObject(valForKey) && pathtype === 'nested') {
-        $applyDefaultsToNested(path[key], prefix + key, this);
-        this.$set(prefix + key, path[key], constructing, Object.assign({}, options, { _skipMarkModified: true }));
+        // Create a copy of the object to avoid editing the original
+        const pathKey = { ...path[key] }
+        $applyDefaultsToNested(pathKey, prefix + key, this);
+        this.$set(prefix + key, pathKey, constructing, Object.assign({}, options, { _skipMarkModified: true }));
         continue;
       } else if (strict) {
         // Don't overwrite defaults with undefined keys (gh-3981) (gh-9039)
diff --git a/test/document.test.js b/test/document.test.js
index 160b14fbe3d..1ccc39bd19f 100644
--- a/test/document.test.js
+++ b/test/document.test.js
@@ -2332,6 +2332,31 @@ describe('document', function() {
       assert.equal(res.test, 'test');
       assert.ok(!res.subDoc);
     });
+
+    it('original object nested fields should not be mutated by mongoose schema defaults (gh-12102)', function() {
+      const schema = new mongoose.Schema({
+        topLevelField1: {
+          nestedField1: String,
+          nestedField2: {type: String, default: 'defValue'}
+        },
+        topLevelField2: {type: String, default: 'defValue'}
+      })
+
+      const Model = db.model('Test', schema);
+      
+      const originalData = {
+        topLevelField1: {
+          nestedField1: 'provided'
+        }
+      }
+      
+      const model = new Model(originalData);
+      assert.deepEqual(originalData, {
+        topLevelField1: {
+          nestedField1: 'provided'
+        }
+      })
+    });  
   });
 
   describe('error processing (gh-2284)', async function() {