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Troubleshoot the Map Function

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  • Verify Key and Value Pairs


Aggregation Pipeline as Alternative to Map-Reduce

Starting in MongoDB 5.0, map-reduce is deprecated:

For examples of aggregation pipeline alternatives to map-reduce, see:

An aggregation pipeline is also easier to troubleshoot than a map-reduce operation.

The map function is a JavaScript function that associates or “maps” a value with a key and emits the key and value pair during a map-reduce operation.

To verify the key and value pairs emitted by the map function, write your own emit function.

Consider a collection orders that contains documents of the following prototype:

_id: ObjectId("50a8240b927d5d8b5891743c"),
cust_id: "abc123",
ord_date: new Date("Oct 04, 2012"),
status: 'A',
price: 250,
items: [ { sku: "mmm", qty: 5, price: 2.5 },
{ sku: "nnn", qty: 5, price: 2.5 } ]
  1. Define the map function that maps the price to the cust_id for each document and emits the cust_id and price pair:

    var map = function() {
    emit(this.cust_id, this.price);
  2. Define the emit function to print the key and value:

    var emit = function(key, value) {
    print("key: " + key + " value: " + tojson(value));
  3. Invoke the map function with a single document from the orders collection:

    var myDoc = db.orders.findOne( { _id: ObjectId("50a8240b927d5d8b5891743c") } );
  4. Verify the key and value pair is as you expected.

    key: abc123 value:250
  5. Invoke the map function with multiple documents from the orders collection:

    var myCursor = db.orders.find( { cust_id: "abc123" } );
    while (myCursor.hasNext()) {
    var doc =;
    print ("document _id= " + tojson(doc._id));
  6. Verify the key and value pairs are as you expected.


See also:

The map function must meet various requirements. For a list of all the requirements for the map function, see mapReduce, or mongosh helper method db.collection.mapReduce().

← Perform Incremental Map-Reduce