Mongoose aggregate to get total amount

Mongoose Aggregate

var mongoose = require('mongoose');
app.service('donation').Model
.aggregate([
    {
      $match: {
        eventid: new mongoose.Types.ObjectId(req.body.event)
      }
    },
    {
        $group: {
            _id: '$eventid',  //$region is the column name in collection
            total: {$sum: '$amount'},

        }
    }
], function (err, result) {
    if (err) {
        console.log(err);
    } else {
        res.json('$'+number_format((result[0].total/100),2));
    }
});

thanks