mongoosejs

var taskSchema = new Schema({
    name: String,
    completed: Boolean,
    subtasks: [{name: String, completed: Boolean}],
    description: String
});

var listSchema = new Schema({
    name: String,
    author: String,
    tasks: [
    {
        type: Schema.Types.ObjectId,
        ref: 'Task'
    }
  ]
});


//this is how I'm trying to push a newly created task into the list

    List.findById(req.body.listId, function(err, list) {
        var newTask = new Task({
            name: req.body.newTaskName,
            completed: false
        })
        newTask.save(function (err, task) {
            if (err) {
                console.log(err);
                return res.sendStatus(500);
            };
            list.tasks.push(task._id);
            return res.json({
                _id: task._id
            });
        });
    });

//this is how I'm trying to populate tasks in the list

    List.find({
        author: req.user.username
    }).populate('tasks').exec(function (err, data) {
        //some stuff
        return res.json(result);
    });



nothing is being added to list.tasks, mongodb shell is showing an empty array