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mongodb / v3.6 / reference / tutorial-troubleshoot-reduce-function.html

Troubleshoot the Reduce Function

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The reduce function is a JavaScript function that “reduces” to a single object all the values associated with a particular key during a map-reduce operation. The reduce function must meet various requirements. This tutorial helps verify that the reduce function meets the following criteria:

  • The reduce function must return an object whose type must be identical to the type of the value emitted by the map function.
  • The order of the elements in the valuesArray should not affect the output of the reduce function.
  • The reduce function must be idempotent.

For a list of all the requirements for the reduce function, see mapReduce, or the mongo shell helper method db.collection.mapReduce().

Confirm Output Type

You can test that the reduce function returns a value that is the same type as the value emitted from the map function.

  1. Define a reduceFunction1 function that takes the arguments keyCustId and valuesPrices. valuesPrices is an array of integers:

    var reduceFunction1 = function(keyCustId, valuesPrices) {
                          return Array.sum(valuesPrices);
                      };

  2. Define a sample array of integers:

    var myTestValues = [ 5, 5, 10 ];

  3. Invoke the reduceFunction1 with myTestValues:

    reduceFunction1('myKey', myTestValues);

  4. Verify the reduceFunction1 returned an integer:

    20

  5. Define a reduceFunction2 function that takes the arguments keySKU and valuesCountObjects. valuesCountObjects is an array of documents that contain two fields count and qty:

    var reduceFunction2 = function(keySKU, valuesCountObjects) {
                          reducedValue = { count: 0, qty: 0 };

                          for (var idx = 0; idx < valuesCountObjects.length; idx++) {
                              reducedValue.count += valuesCountObjects[idx].count;
                              reducedValue.qty += valuesCountObjects[idx].qty;
                          }

                          return reducedValue;
                      };

  6. Define a sample array of documents:

    var myTestObjects = [
                      { count: 1, qty: 5 },
                      { count: 2, qty: 10 },
                      { count: 3, qty: 15 }
                    ];

  7. Invoke the reduceFunction2 with myTestObjects:

    reduceFunction2('myKey', myTestObjects);

  8. Verify the reduceFunction2 returned a document with exactly the count and the qty field:

    { "count" : 6, "qty" : 30 }

Ensure Insensitivity to the Order of Mapped Values

The reduce function takes a key and a values array as its argument. You can test that the result of the reduce function does not depend on the order of the elements in the values array.

  1. Define a sample values1 array and a sample values2 array that only differ in the order of the array elements:

    var values1 = [
                { count: 1, qty: 5 },
                { count: 2, qty: 10 },
                { count: 3, qty: 15 }
              ];

var values2 = [
                { count: 3, qty: 15 },
                { count: 1, qty: 5 },
                { count: 2, qty: 10 }
              ];

  2. Define a reduceFunction2 function that takes the arguments keySKU and valuesCountObjects. valuesCountObjects is an array of documents that contain two fields count and qty:

    var reduceFunction2 = function(keySKU, valuesCountObjects) {
                          reducedValue = { count: 0, qty: 0 };

                          for (var idx = 0; idx < valuesCountObjects.length; idx++) {
                              reducedValue.count += valuesCountObjects[idx].count;
                              reducedValue.qty += valuesCountObjects[idx].qty;
                          }

                          return reducedValue;
                      };

  3. Invoke the reduceFunction2 first with values1 and then with values2:

    reduceFunction2('myKey', values1);
reduceFunction2('myKey', values2);

  4. Verify the reduceFunction2 returned the same result:

    { "count" : 6, "qty" : 30 }

Ensure Reduce Function Idempotence

Because the map-reduce operation may call a reduce multiple times for the same key, and won’t call a reduce for single instances of a key in the working set, the reduce function must return a value of the same type as the value emitted from the map function. You can test that the reduce function process “reduced” values without affecting the final value.

  1. Define a reduceFunction2 function that takes the arguments keySKU and valuesCountObjects. valuesCountObjects is an array of documents that contain two fields count and qty:

    var reduceFunction2 = function(keySKU, valuesCountObjects) {
                          reducedValue = { count: 0, qty: 0 };

                          for (var idx = 0; idx < valuesCountObjects.length; idx++) {
                              reducedValue.count += valuesCountObjects[idx].count;
                              reducedValue.qty += valuesCountObjects[idx].qty;
                          }

                          return reducedValue;
                      };

  2. Define a sample key:

    var myKey = 'myKey';

  3. Define a sample valuesIdempotent array that contains an element that is a call to the reduceFunction2 function:

    var valuesIdempotent = [
                         { count: 1, qty: 5 },
                         { count: 2, qty: 10 },
                         reduceFunction2(myKey, [ { count:3, qty: 15 } ] )
                       ];

  4. Define a sample