fix(document): avoid massive perf degradation when saving new doc with 10 level deep subdocs

Fix Automattic#14897

Author: vkarpov15

lib/document.js

@@ -2724,7 +2724,9 @@ function _getPathsToValidate(doc, pathsToValidate, pathsToSkip) {
       }
 
       if (doc.$isModified(fullPathToSubdoc, null, modifiedPaths) &&
-            !doc.isDirectModified(fullPathToSubdoc) &&
+            // Avoid using isDirectModified() here because that does additional checks on whether the parent path
+            // is direct modified, which can cause performance issues re: gh-14897
+            !doc.$__.activePaths.getStatePaths('modify').hasOwnProperty(fullPathToSubdoc) &&
             !doc.$isDefault(fullPathToSubdoc)) {
         paths.add(fullPathToSubdoc);
 

test/document.test.js

@@ -13905,6 +13905,27 @@ describe('document', function() {
     const objectWithGetters = result.toObject({ getters: true, virtuals: false });
     assert.strictEqual(objectWithGetters.level1.level2.level3.property, 'TESTVALUE');
   });
+
+  it('handles inserting and saving large document with 10-level deep subdocs (gh-14897)', async function() {
+    const levels = 10;
+
+    let schema = new Schema({ test: { type: String, required: true } });
+    let doc = { test: 'gh-14897' };
+    for (let i = 0; i < levels; ++i) {
+      schema = new Schema({ level: Number, subdocs: [schema] });
+      doc = { level: (levels - i), subdocs: [{ ...doc }, { ...doc }] };
+    }
+
+    const Test = db.model('Test', schema);
+    const savedDoc = await Test.create(doc);
+
+    let cur = savedDoc;
+    for (let i = 0; i < levels - 1; ++i) {
+      cur = cur.subdocs[0];
+    }
+    cur.subdocs[0] = { test: 'updated' };
+    await savedDoc.save();
+  });
 });
 
 describe('Check if instance function that is supplied in schema option is available', function() {