Recursive mongoose model doesn't properly store my data - node.js

Question

While studying node I am trying to create a little commentthread application. Since the book I am using is a little outdated, I had to adjust the model provided in the book to get the application running. However, I think something is still wrong with my model, because a part of the data is stored as object Object. Does anyone see the problem in the model?
var mongoose = require('mongoose'),
Schema = mongoose.Schema;
var ReplySchema = new Schema();
ReplySchema.add({
username: String,
subject: String,
timestamp: { type: Date, default: Date.now },
body: String,
replies:[ReplySchema]
}, { _id: true });
var CommentThreadSchema = new Schema({
title: String,
replies:[ReplySchema]
});
mongoose.model('Reply', ReplySchema);
mongoose.model('CommentThread', CommentThreadSchema);
The result in Mongo:
{ _id: 56c8c91b011c7db2608159d6,
'[object Object]timestamp': Sat Feb 20 2016 21:14:19 GMT+0100 (CET), '[object Object]replies': [] }
The controller
var mongoose = require('mongoose'),
CommentThread = mongoose.model('CommentThread'),
Reply = mongoose.model('Reply');
exports.getComment = function(req, res) {
CommentThread.findOne({ _id: req.query.commentId })
.exec(function(err, comment) {
if (!comment){
res.json(404, {msg: 'CommentThread Not Found.'});
} else {
res.json(comment);
}
});
};
exports.addComment = function(req, res) {
CommentThread.findOne({ _id: req.body.rootCommentId })
.exec(function(err, commentThread) {
if (!commentThread){
res.json(404, {msg: 'CommentThread Not Found.'});
} else {
var newComment = Reply(req.body.newComment);
newComment.username = generateRandomUsername();
addComment(req, res, commentThread, commentThread,
req.body.parentCommentId, newComment);
}
});
};
function addComment(req, res, commentThread, currentComment,
parentId, newComment){
if (commentThread.id == parentId){
console.log(newComment);
commentThread.replies.push(newComment);
updateCommentThread(req, res, commentThread);
} else {
for(var i=0; i< currentComment.replies.length; i++){
var c = currentComment.replies[i];
if (c._id == parentId){
c.replies.push(newComment);
var replyThread = commentThread.replies.toObject();
updateCommentThread(req, res, commentThread);
break;
} else {
addComment(req, res, commentThread, c,
parentId, newComment);
}
}
}
};
function updateCommentThread(req, res, commentThread){
CommentThread.update({ _id: commentThread.id },
{$set:{replies:commentThread.replies}})
.exec(function(err, savedComment){
if (err){
res.json(404, {msg: 'Failed to update CommentThread.'});
} else {
res.json({msg: "success"});
}
});
}
function generateRandomUsername(){
//typically the username would come from an authenticated session
var users=['Mojos', 'Milo', 'Mihaal', 'Haly', 'MilodeBaesz', 'Mihaly'];
return users[Math.floor((Math.random()*5))];
}