feat: pathsToSave option to save() function

lib/model.js

@@ -298,7 +298,6 @@ Model.prototype.$__handleSave = function(options, callback) {
   if (!saveOptions.hasOwnProperty('session') && session != null) {
     saveOptions.session = session;
   }
-
   if (this.$isNew) {
     // send entire doc
     const obj = this.toObject(saveToObjectOptions);
@@ -511,6 +510,7 @@ function generateVersionError(doc, modifiedPaths) {
  * @param {Number} [options.wtimeout] sets a [timeout for the write concern](https://www.mongodb.com/docs/manual/reference/write-concern/#wtimeout). Overrides the [schema-level `writeConcern` option](https://mongoosejs.com/docs/guide.html#writeConcern).
  * @param {Boolean} [options.checkKeys=true] the MongoDB driver prevents you from saving keys that start with '$' or contain '.' by default. Set this option to `false` to skip that check. See [restrictions on field names](https://docs.mongodb.com/manual/reference/limits/#mongodb-limit-Restrictions-on-Field-Names)
  * @param {Boolean} [options.timestamps=true] if `false` and [timestamps](https://mongoosejs.com/docs/guide.html#timestamps) are enabled, skip timestamps for this `save()`.
+ * @param {Array} [options.pathsToSave] An array of paths that tell mongoose to only validate and save the paths in `pathsToSave`.
  * @throws {DocumentNotFoundError} if this [save updates an existing document](https://mongoosejs.com/docs/api/document.html#Document.prototype.isNew) but the document doesn't exist in the database. For example, you will get this error if the document is [deleted between when you retrieved the document and when you saved it](documents.html#updating).
  * @return {Promise}
  * @api public
@@ -736,8 +736,11 @@ function handleAtomics(self, where, delta, data, value) {
  */
 
 Model.prototype.$__delta = function() {
-  const dirty = this.$__dirty();
-
+  let dirty = this.$__dirty();
+  if (this.$__.saveOptions && this.$__.saveOptions.pathsToSave) {
+    const pathsToSave = this.$__.saveOptions.pathsToSave;
+    dirty = dirty.filter(x => pathsToSave.includes(x.path));
+  }
   const optimisticConcurrency = this.$__schema.options.optimisticConcurrency;
   if (optimisticConcurrency) {
     if (Array.isArray(optimisticConcurrency)) {

test/model.test.js

@@ -2537,6 +2537,18 @@ describe('Model', function() {
       assert.ok(!doc.$__.$versionError);
       assert.ok(!doc.$__.saveOptions);
     });
+    it('should only save paths specificed in the `pathsToSave` array (gh-9583)', async function() {
+      const schema = new Schema({ name: String, age: Number, weight: Number, location: String });
+      const Test = db.model('Test', schema);
+      await Test.create({ name: 'Test Testerson', age: 1, weight: 180, location: 'Florida' });
+      const doc = await Test.findOne();
+      doc.name = 'Test';
+      doc.age = 100;
+      await doc.save({ pathsToSave: ['name'] });
+      const check = await Test.findOne();
+      assert.equal(check.name, 'Test');
+      assert.equal(check.age, 1);
+    });
   });