Introduction to MongoDB collection.mapReduce() Method

The mapReduce() method in MongoDB is a powerful data processing tool that can perform custom aggregation and processing on data in a collection. The method takes two JavaScript functions as parameters: one for mapping the data and another for aggregating the mapped results. These functions are the map function and the reduce function, respectively. mapReduce() method returns a new collection or outputs to the console depending on the options used.


The syntax of the mapReduce() method is as follows:

  function () {
    emit(key, value)
  function (key, values) {
    return reduceFunction
    out: "collection",
    query: document,
    sort: document,
    limit: number

Where collection represents the name of the collection to aggregate, map and reduce are the mapping and aggregation functions, out specifies the name of the output collection, query specifies the query conditions, sort specifies the sort conditions, and limit specifies the maximum number of returned results.

Use Cases

The mapReduce() method is commonly used to perform aggregation operations on large datasets, such as calculating the total number of orders per user or the total sales amount per region. Due to the ability to customize the mapping and aggregation functions, mapReduce() method can handle various complex aggregation requirements.


Here is an example of using the mapReduce() method to aggregate the orders collection:

var mapFunction = function () {
  emit(this.user_id, { count: 1 })

var reduceFunction = function (key, values) {
  var count = 0
  values.forEach(function (value) {
    count += value.count
  return { count: count }

db.orders.mapReduce(mapFunction, reduceFunction, { out: "order_totals" })


In the above code, the mapFunction function is used to map the data by grouping each order by user ID and outputting a key-value pair where the key is the user ID and the value is an object containing the order count. The reduceFunction function is used to aggregate the data by calculating the total number of orders for each user. After executing the mapReduce() method, the result is output to the order_totals collection and then viewed using the find() method.


The mapReduce() method is a powerful aggregation tool in MongoDB that can handle various complex aggregation requirements. By customizing the mapping and aggregation functions, various statistical and analytical functions can be achieved. When using the mapReduce() method, it is important to note that if the data is large, it may require some calculation time and resources, and server optimization may be necessary.