맵-리듀스에서 애그리게이션 파이프라인으로의 전환

var mapFunction1 = function() {
   emit(this.cust_id, this.price);
};
var reduceFunction1 = function(keyCustId, valuesPrices) {
   return Array.sum(valuesPrices);
};
db.orders.mapReduce(
   mapFunction1,
   reduceFunction1,
   { out: "map_reduce_example" }
)

db.orders.aggregate([
   { $group: { _id: "$cust_id", value: { $sum: "$price" } } },
   { $out: "agg_alternative_1" }
])

db.orders.aggregate( [
   { $project: { emit: { key: "$cust_id", value: "$price" } } },  // equivalent to the map function
   { $group: {                                                    // equivalent to the reduce function
         _id: "$emit.key",
         valuesPrices: { $accumulator: {
                     init: function() { return 0; },
                     initArgs: [],
                     accumulate: function(state, value) { return state + value; },
                     accumulateArgs: [ "$emit.value" ],
                     merge: function(state1, state2) { return state1 + state2; },
                     lang: "js"
         } }
   } },
   { $out: "agg_alternative_2" }
] )

var mapFunction2 = function() {
      for (var idx = 0; idx < this.items.length; idx++) {
         var key = this.items[idx].sku;
         var value = { count: 1, qty: this.items[idx].qty };
         emit(key, value);
      }
};
var reduceFunction2 = function(keySKU, countObjVals) {
   reducedVal = { count: 0, qty: 0 };
   for (var idx = 0; idx < countObjVals.length; idx++) {
         reducedVal.count += countObjVals[idx].count;
         reducedVal.qty += countObjVals[idx].qty;
   }
   return reducedVal;
};
var finalizeFunction2 = function (key, reducedVal) {
   reducedVal.avg = reducedVal.qty/reducedVal.count;
   return reducedVal;
};
db.orders.mapReduce(
   mapFunction2,
   reduceFunction2,
   {
      out: { merge: "map_reduce_example2" },
      query: { ord_date: { $gte: new Date("2020-03-01") } },
      finalize: finalizeFunction2
   }
   );

db.orders.aggregate( [
   { $match: { ord_date: { $gte: new Date("2020-03-01") } } },
   { $unwind: "$items" },
   { $group: { _id: "$items.sku", qty: { $sum: "$items.qty" }, orders_ids: { $addToSet: "$_id" } }  },
   { $project: { value: { count: { $size: "$orders_ids" }, qty: "$qty", avg: { $divide: [ "$qty", { $size: "$orders_ids" } ] } } } },
   { $merge: { into: "agg_alternative_3", on: "_id", whenMatched: "replace",  whenNotMatched: "insert" } }
] )

db.orders.aggregate( [
      { $match: { ord_date: {$gte: new Date("2020-03-01") } } },
      { $unwind: "$items" },
      { $project: { emit: { key: "$items.sku", value: { count: { $literal: 1 }, qty: "$items.qty" } } } },
      { $group: {
            _id: "$emit.key",
            value: { $accumulator: {
               init: function() { return { count: 0, qty: 0 }; },
               initArgs: [],
               accumulate: function(state, value) {
                  state.count += value.count;
                  state.qty += value.qty;
                  return state;
               },
               accumulateArgs: [ "$emit.value" ],
               merge: function(state1, state2) {
                  return { count: state1.count + state2.count, qty: state1.qty + state2.qty };
               },
               finalize: function(state) {
                  state.avg = state.qty / state.count;
                  return state;
               },
               lang: "js"}
            }
      } },
      { $merge: {
         into: "agg_alternative_4",
         on: "_id",
         whenMatched: "replace",
         whenNotMatched: "insert"
      } }
] )