如何使用mongodb mapreduce计算简单的移动平均值？

db.stock_prices.insertMany([
  { date: "2022-01-01", price: 10 },
  { date: "2022-01-02", price: 15 },
  { date: "2022-01-03", price: 20 },
  { date: "2022-01-04", price: 25 },
  { date: "2022-01-05", price: 30 },
  { date: "2022-01-06", price: 35 },
  { date: "2022-01-07", price: 40 }
])

var mapFunction = function() {
  emit(this.date, this.price);
};

var reduceFunction = function(key, values) {
  var sum = Array.sum(values);
  var average = sum / values.length;
  return average;
};

db.stock_prices.mapReduce(
  mapFunction,
  reduceFunction,
  { out: "moving_averages" }
)

db.moving_averages.find()