Node + Mongo: adding record to array inside collection

I'm writing a blogging system. Now I'm working on comments for posts. Here is the schema for every single post:

{
"topic": "Post Topic",
"post": "<p>\r\n\tPost text</p>\r\n",
"time_added": 1343167025,
"author": "Ashley Brooks",
"_id": {
    "$oid": "500f1a315759c73805000001"
}

}

Now, I'd like to add a comment for this post. I suppose the best way of storing comments is smth like:

{
"topic": "Post Topic",
"post": "<p>\r\n\tPost text</p>\r\n",
"time_added": 1343167025,
"author": "Ashley Brooks",
"_id": {
    "$oid": "500f1a315759c73805000001"
},
"comments" : [
    {
        "author":"James Brown",
        "comment":"My awesome comment"
    },
    {
        "author":"Jimmy White",
        "comment":"And this is my comment"
    }
]
}

I've read some docs for Mongo, however I could'n find a proper way of adding new comments. This is how I'm doing now:

exports.post = function(req, res) {
if (req.currentUser) {
    db.collection("posts", function (err, collection) {
        var obj_id = BSON.ObjectID.createFromHexString(req.params.id);
        console.log(obj_id);
        collection.findAndModify(
            {_id: obj_id}, // query for a post
            {$push: {
                comments: [ {author: req.body.comment, name: "testname" } ]
                }
            },
            function(err, object) {
                if (err){
                    console.warn(err.message);
                }else{
                    res.redirect('/');
                }
            });
    });
} else {
    res.redirect('/');
}

};

It returns me such error:

exception: must specify remove or update

What I'm doing wrong?

P.S. BTW, I think it would be nice to mark separate comments with any unique id or smth like that. How can I specify Mongo to add an obj_id parameter for every adding comment? Thanks in advance.