Mongo - MapReduce

Lesson Objectives

1. Explain what MapReduce is and why we have it
2. Explain structure of Map Reduce
3. Explain map function
4. Explain reduce function
5. Explain aggregating multiple values
6. Explain Multiple group by

Explain what MapReduce is and why we have it

Explain structure of Map Reduce

1. db.collectionName.mapReduce(mapFunction, reduceFunction, { query: {}, out:{} })
2. out

• collection name
• { [ replace | inline | merge | reduce ]: 1 }

Explain map function

1. return key=>value pair

const emitter = function () {
  if (this.gender === "m") {
    emit(this.name, { yum: this.loves[0], weight: this.weight });
  }
};
db.employees.mapReduce(emitter, function () {}, { out: "mapTest" });

Explain reduce function

1. if multiple values for a key, how to reduce

const emitter = function () {
  emit(this.gender, this.weight);
};
const reducer = (key, values) => Array.sum(values);
db.employees.mapReduce(emitter, reducer, { out: "mapTest" });

Multiple values

const emitter = function () {
  emit(this.gender, { weights: this.weight, money: this.salary });
};
const reducer = (key, values) => {
  let total_weight = 0;
  let total_salary = 0;
  for (let i = 0; i < values.length; i++) {
    total_weight += values[i].weights;
    total_salary += values[i].money;
  }
  return { total_weight: total_weight, total_salary: total_salary };
};
db.employees.mapReduce(emitter, reducer, { out: "mapTest" });
db.mapTest.find();

Multiple group by

const emitter = function () {
  emit(
    {
      gender: this.gender,
      weight: this.weight,
    },
    this.weight
  );
};