Pushing object into mongoDB, ["object Object"] is saved

Sorry if this is a repetitive one, I really couldn't solve it with any of the information provided here.

Basically I have this in my User.js mongoDB schema:

notifications: [{
        type: String,
        story: String,
        seen: Boolean,
        createdTime: Date,
        from: {
            name: String,
            username: String,
            id: mongoose.Schema.ObjectId
        }
    }]

After I query the user, this is what I do to push this object:

var notifObj = {

    type: notification.type,
    story: notification.story || ' ',
    seen: false,
    createdTime: new Date(),
    from: {
        name: notification.from.firstName + " " + notification.from.lastName,
        username: notification.from.username,
        id: notification.from._id
    }
};

into the mongoDB database:

user.notifications.push(notifObj);

User.update({
    _id: notification.to
}, user, function(err, data) {

    if (err) {

        deferred.reject({
            err: err
        });
    }

    //Tell sender everything went alrgiht
    deferred.resolve(data);
});

P.S.: I have deferred.resolve instead of res.end(), because I push notifications on some of the requests in a different controller, I don't have a separate route only for notifications. (e.g.: User has a new message, I send the message and push a notification too)