Database Manual / Aggregation Operations / Map-Reduce

Troubleshoot the Map Function

Note

Aggregation Pipeline as Alternative to Map-Reduce

Starting in MongoDB 5.0, map-reduce is deprecated:

  • Instead of map-reduce, you should use an aggregation pipeline. Aggregation pipelines provide better performance and usability than map-reduce.
  • You can rewrite map-reduce operations using aggregation pipeline stages, such as $group, $merge, and others.
  • For map-reduce operations that require custom functionality, you can use the $accumulator and $function aggregation operators. You can use those operators to define custom aggregation expressions in JavaScript.

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.

Verify Key and Value Pairs

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("emit");
        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") } );
    map.apply(myDoc);

  4. Verify the key and value pair is as you expected.

    emit
    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 = myCursor.next();
        print ("document _id= " + tojson(doc._id));
        map.apply(doc);
        print();
    }
  6. Verify the key and value pairs are as you expected.

Tip

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().