Node.JS + Mongo-native: Insert child object to exiting object

My user schema is:

{
    "_id": {"$oid": "515774f2c24007eb57000001"},
    "login": "Ilya",
    "password": "30947a8c9f39b27290a11eabaeec8c89",
}

I'm doing dynamic adding seminars info to user object. After this code:

exports.signSeminar = function(user_id, seminar_id, callback) {
    var user_obj_id = BSON.ObjectID.createFromHexString(String(user_id));
    db.collection("users", function(error, collection) {
        collection.findAndModify({
            _id: user_obj_id
        }, [], {
             $set: {
                "seminars": [{
                    "id:": String(seminar_id)
                }]
             }
        }, {}, function(err, result) {
            callback(200);
        });
    });
};

my object becomes:

{
    "_id": {"$oid": "515774f2c24007eb57000001"},
    "login": "Ilya",
    "password": "30947a8c9f39b27290a11eabaeec8c89",
    "seminars": [
        {
            "id:": "51562509b781ae1d53000001"
        }
    ],
}

After I shell this code twice+ times, it just re-writes children object in "seminars". And I need to insert just another seminar object like this:

{
    "_id": {"$oid": "515774f2c24007eb57000001"},
    "login": "Ilya",
    "password": "30947a8c9f39b27290a11eabaeec8c89",
    "seminars": [
        {
            "id:": "51562509b781ae1d53000001"
        },
        {
            "id:": "#another_object_id#"
        }
    ],
}

What I'm doing wrong? Thanks.

P.S. Tried findAndModify, update, unsert, no luck((

$set sets a value, you want $push to append to an array.

 {
    $push: {
            "seminars": {
                "id:": String(seminar_id)
           }
 }