map-reduce

db.inventory.insertMany([
   { item: "journal", qty: 25, size: { h: 14, w: 21, uom: "cm" }, status: "A" },
   { item: "journal", qty: 50, size: { h: 8.5, w: 11, uom: "in" }, status: "A" },
   { item: "paper", qty: 100, size: { h: 8.5, w: 11, uom: "in" }, status: "A" },
   { item: "planner", qty: 75, size: { h: 22.85, w: 30, uom: "cm" }, status: "D" },
   { item: "postcard", qty: 45, size: { h: 10, w: 15.25, uom: "cm" }, status: "A" }
]);

# 合计
db.inventory.mapReduce(
  function(){
    emit(this.item,this.qty);
  },
  function(key,values){
    return Array.sum(values);
  },
  {
    query:{status:"A"},
    out:"inventory_totals"
  }
)

# 平均值

db.inventory.mapReduce(
  function(){
    emit(this.item,this.qty);
  },
  function(key,values){
    return Array.avg(values);
  },
  {
    query:{status:"A"},
    out:"inventory_avg"
  }
)

​​https://api.mongodb.com/js/3.3.11/symbols/Array.html​​