So I am sending data of a mission, the startdate and the finish date. However I am not able to put any changes into the database as it believes I get an immutable error with mongoDB... I would like to stick with using the .then method for my js code.

My other methods are working properly, I just can't get this update method right...

```
app.put('/missions/:id', function (req, res) {
  if (req.user) {
    model.Mission.findById(req.params.id).then(function(Mission){
      console.log("req.body.secret_mission: ",req.body.secret_mission)
      Mission['secret_mission'] = req.body.secret_mission;
      Mission['start'] = req.body.start;
      Mission['complete'] = req.body.complete;

      Mission.update().then(function(){
        res.set('Access-Control-Allow-Origin', '*');
        res.sendStatus(201);
      });
  });
}else{
    res.sendStatus(401);
  }
});

```

[Error I receive in the command line](https://i.stack.imgur.com/Qdjjh.png)