From 84e262e6650e9df57ca349a5037b53460951636e Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Sun, 2 Oct 2022 16:40:56 -0400
Subject: [PATCH 1/2] fix(model): avoid saving applied defaults if path is
 deselected

Fix #12414
---
 lib/helpers/projection/isPathExcluded.js |  4 ++++
 lib/model.js                             | 20 ++++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/lib/helpers/projection/isPathExcluded.js b/lib/helpers/projection/isPathExcluded.js
index e0424b52365..e8f126b22da 100644
--- a/lib/helpers/projection/isPathExcluded.js
+++ b/lib/helpers/projection/isPathExcluded.js
@@ -12,6 +12,10 @@ const isDefiningProjection = require('./isDefiningProjection');
  */
 
 module.exports = function isPathExcluded(projection, path) {
+  if (projection == null) {
+    return false;
+  }
+
   if (path === '_id') {
     return projection._id === 0;
   }
diff --git a/lib/model.js b/lib/model.js
index 9abbeeb14f0..420e01eb486 100644
--- a/lib/model.js
+++ b/lib/model.js
@@ -51,11 +51,13 @@ const {
   getRelatedDBIndexes,
   getRelatedSchemaIndexes
 } = require('./helpers/indexes/getRelatedIndexes');
+const isPathExcluded = require('./helpers/projection/isPathExcluded');
 const decorateDiscriminatorIndexOptions = require('./helpers/indexes/decorateDiscriminatorIndexOptions');
 const isPathSelectedInclusive = require('./helpers/projection/isPathSelectedInclusive');
 const leanPopulateMap = require('./helpers/populate/leanPopulateMap');
 const modifiedPaths = require('./helpers/update/modifiedPaths');
 const parallelLimit = require('./helpers/parallelLimit');
+const parentPaths = require('./helpers/path/parentPaths');
 const prepareDiscriminatorPipeline = require('./helpers/aggregate/prepareDiscriminatorPipeline');
 const pushNestedArrayPaths = require('./helpers/model/pushNestedArrayPaths');
 const removeDeselectedForeignField = require('./helpers/populate/removeDeselectedForeignField');
@@ -760,6 +762,19 @@ Model.prototype.$__delta = function() {
       }
     }
 
+    // If this path is set to default, and either this path or one of
+    // its parents is excluded, don't treat this path as dirty.
+    if (this.$isDefault(data.path) && this.$__.selected) {
+      if (data.path.indexOf('.') === -1 && isPathExcluded(this.$__.selected, data.path)) {
+        continue;
+      }
+
+      const pathsToCheck = parentPaths(data.path);
+      if (pathsToCheck.find(path => isPathExcluded(this.$__.isSelected, path))) {
+        continue;
+      }
+    }
+
     if (divergent.length) continue;
     if (value === undefined) {
       operand(this, where, delta, data, 1, '$unset');
@@ -798,6 +813,11 @@ Model.prototype.$__delta = function() {
   if (this.$__.version) {
     this.$__version(where, delta);
   }
+
+  if (Object.keys(delta).length === 0) {
+    return [where, null];
+  }
+
   return [where, delta];
 };
 

From d061da33066f1fc63f5be46af004415a89c271fd Mon Sep 17 00:00:00 2001
From: Valeri Karpov <val@karpov.io>
Date: Mon, 3 Oct 2022 11:57:48 -0400
Subject: [PATCH 2/2] test: add test coverage for #12414

---
 test/document.test.js | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/test/document.test.js b/test/document.test.js
index 84cafc22463..ca2aca278d9 100644
--- a/test/document.test.js
+++ b/test/document.test.js
@@ -11881,6 +11881,24 @@ describe('document', function() {
     const addedDoc = await Model.findOne({ name: 'Test' });
     assert.strictEqual(addedDoc.count, 1);
   });
+
+  it('avoids overwriting array if saving with no changes with array deselected (gh-12414)', async function() {
+    const schema = new mongoose.Schema({
+      name: String,
+      tags: [String]
+    });
+    const Test = db.model('Test', schema);
+
+    const { _id } = await Test.create({ name: 'Mongoose', tags: ['mongodb'] });
+
+    const doc = await Test.findById(_id).select('name');
+    assert.deepStrictEqual(doc.getChanges(), {});
+    await doc.save();
+
+    const rawDoc = await Test.collection.findOne({ _id });
+    assert.ok(rawDoc);
+    assert.deepStrictEqual(rawDoc.tags, ['mongodb']);
+  });
 });
 
 describe('Check if instance function that is supplied in schema option is availabe', function() {