User.findOneAndUpdate(
    {
        'posts._id': req.params.id
    },
    {
        $push: {
            'posts.$.comments': {
                from: {
                    _id: req.user._id,
                    username: req.user.username,
                },
                body: req.body.commentBody
            }
        }
    },
    err => {
        if (err) console.log(err)
        // Do something with ObjectId
    }
);

var mypost = doc.posts.find(function(post){
    return post._id.toString()==req.params.id
});

console.log(mypost.comments[mypost.comments.length]._id)