perf(model): make insertMany() lean option skip hydrating Mongoose docs

Fix #14372

Author: vkarpov15

lib/model.js

@@ -3430,6 +3430,13 @@ Model.$__insertMany = function(arr, options, callback) {
   const results = ordered ? null : new Array(arr.length);
   const toExecute = arr.map((doc, index) =>
     callback => {
+      // If option `lean` is set to true bypass validation and hydration
+      if (lean) {
+        // we have to execute callback at the nextTick to be compatible
+        // with parallelLimit, as `results` variable has TDZ issue if we
+        // execute the callback synchronously
+        return immediate(() => callback(null, doc));
+      }
       if (!(doc instanceof _this)) {
         try {
           doc = new _this(doc);
@@ -3440,13 +3447,6 @@ Model.$__insertMany = function(arr, options, callback) {
       if (options.session != null) {
         doc.$session(options.session);
       }
-      // If option `lean` is set to true bypass validation
-      if (lean) {
-        // we have to execute callback at the nextTick to be compatible
-        // with parallelLimit, as `results` variable has TDZ issue if we
-        // execute the callback synchronously
-        return immediate(() => callback(null, doc));
-      }
       doc.$validate({ __noPromise: true }, function(error) {
         if (error) {
           // Option `ordered` signals that insert should be continued after reaching
@@ -3510,7 +3510,7 @@ Model.$__insertMany = function(arr, options, callback) {
       callback(null, []);
       return;
     }
-    const docObjects = docAttributes.map(function(doc) {
+    const docObjects = lean ? docAttributes : docAttributes.map(function(doc) {
       if (doc.$__schema.options.versionKey) {
         doc[doc.$__schema.options.versionKey] = 0;
       }
@@ -3572,6 +3572,9 @@ Model.$__insertMany = function(arr, options, callback) {
             return !isErrored;
           }).
           map(function setIsNewForInsertedDoc(doc) {
+            if (lean) {
+              return doc;
+            }
             doc.$__reset();
             _setIsNew(doc, false);
             return doc;
@@ -3588,9 +3591,11 @@ Model.$__insertMany = function(arr, options, callback) {
         return;
       }
 
-      for (const attribute of docAttributes) {
-        attribute.$__reset();
-        _setIsNew(attribute, false);
+      if (!lean) {
+        for (const attribute of docAttributes) {
+          attribute.$__reset();
+          _setIsNew(attribute, false);
+        }
       }
 
       if (rawResult) {