cannot get PUT to work on Nodejs / express

Kept at it and finally figured it out! Here is my PUT function.

app.put('/people/:id', function (req, res) {
    var person = new PeopleModel({
        first_name: req.body.first_name,
        last_name: req.body.last_name,
                    // etc etc
    });

    var upsertData = person.toObject();

    delete upsertData._id;      

    return PeopleModel.update({ _id: req.params.id }, upsertData, {upsert: true}, function(err) {
          if (!err) {
              return res.send("updated");
          } else {
              console.log(err);
              return res.send(404, { error: "Person was not updated." });
          }
    });
});

I'm going to try and figure out how to auto detect what fields need to be updated, as just for testing i stored one var.