fixed; only save the depopulated ObjectId

lib/types/array.js

@@ -303,7 +303,7 @@ MongooseArray.prototype.splice = function () {
 MongooseArray.prototype.toObject = function (options) {
   if (options && options.depopulate && this[0] instanceof Document) {
     return this.map(function (doc) {
-      return doc.toObject(options);
+      return doc._id;
     });
   }