perf: use fast path for cloning document with only primitive values re:

#14719

lib/document.js

@@ -28,6 +28,7 @@ const getKeysInSchemaOrder = require('./helpers/schema/getKeysInSchemaOrder');
 const getSubdocumentStrictValue = require('./helpers/schema/getSubdocumentStrictValue');
 const handleSpreadDoc = require('./helpers/document/handleSpreadDoc');
 const immediate = require('./helpers/immediate');
+const isBsonType = require('./helpers/isBsonType');
 const isDefiningProjection = require('./helpers/projection/isDefiningProjection');
 const isExclusive = require('./helpers/projection/isExclusive');
 const inspect = require('util').inspect;
@@ -3810,6 +3811,14 @@ Document.prototype.$toObject = function(options, json) {
   // `clone` is necessary here because `utils.options` directly modifies the second input.
   const defaultOptions = Object.assign({}, baseOptions, schemaOptions[path]);
 
+  const hasOnlyPrimitiveValues = !this.$__.populated && !this.$__.wasPopulated && (this._doc == null || Object.values(this._doc).every(v => {
+    return v == null
+      || typeof v !== 'object'
+      || (utils.isNativeObject(v) && !Array.isArray(v))
+      || isBsonType(v, 'ObjectId')
+      || isBsonType(v, 'Decimal128');
+  }));
+
   // If options do not exist or is not an object, set it to empty object
   options = utils.isPOJO(options) ? { ...options } : {};
   options._calledWithOptions = options._calledWithOptions || { ...options };
@@ -3824,7 +3833,9 @@ Document.prototype.$toObject = function(options, json) {
   }
 
   options.minimize = _minimize;
-  options._seen = options._seen || new Map();
+  if (!hasOnlyPrimitiveValues) {
+    options._seen = options._seen || new Map();
+  }
 
   const depopulate = options._calledWithOptions.depopulate
     ?? options._parentOptions?.depopulate
@@ -3853,7 +3864,27 @@ Document.prototype.$toObject = function(options, json) {
   // to save it from being overwritten by sub-transform functions
   // const originalTransform = options.transform;
 
-  let ret = clone(this._doc, options) || {};
+  let ret;
+  if (hasOnlyPrimitiveValues) {
+    // Fast path: if we don't have any nested objects or arrays, we only need a
+    // shallow clone.
+    ret = this._doc
+      ? options.minimize ? (minimize({ ...this._doc }) || {}) : { ...this._doc }
+      : {};
+    if (!options.minimize || options.flattenObjectIds) {
+      const keys = Object.keys(ret);
+      for (const key of keys) {
+        // If `minimize` is false, still need to remove undefined keys
+        if (!options.minimize && ret[key] === undefined) {
+          delete ret[key];
+        } else if (options.flattenObjectIds && isBsonType(ret[key], 'ObjectId')) {
+          ret[key] = ret[key].toJSON();
+        }
+      }
+    }
+  } else {
+    ret = clone(this._doc, options) || {};
+  }
 
   options._skipSingleNestedGetters = true;
   const getters = options._calledWithOptions.getters

test/document.unit.test.js

@@ -60,8 +60,6 @@ describe('toObject()', function() {
   it('doesnt crash with empty object (gh-3130)', function() {
     const d = new Stub();
     d._doc = undefined;
-    assert.doesNotThrow(function() {
-      d.toObject();
-    });
+    d.toObject();
   });
 });

test/model.populate.test.js

@@ -9449,7 +9449,6 @@ describe('model: populate:', function() {
         children: [{ type: 'ObjectId', ref: 'Child' }]
       }));
 
-
       const children = await Child.create([{ name: 'Luke' }, { name: 'Leia' }]);
 
       let doc = await Parent.create({ children, child: children[0] });