Troubleshoot the Reduce Function

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Confirm Output Type

Ensure Insensitivity to the Order of Mapped Values
Ensure Reduce Function Idempotence

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 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 mongosh 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.

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);
                      };

Define a sample array of integers:
```
var myTestValues = [ 5, 5, 10 ];
```
Invoke the reduceFunction1 with myTestValues:
```
reduceFunction1('myKey', myTestValues);
```
Verify the reduceFunction1 returned an integer:
```
20
```

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;
                      };

Define a sample array of documents:
```
var myTestObjects = [
                      { count: 1, qty: 5 },
                      { count: 2, qty: 10 },
                      { count: 3, qty: 15 }
                    ];
```
Invoke the reduceFunction2 with myTestObjects:
```
reduceFunction2('myKey', myTestObjects);
```
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.

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 }
              ];

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;
                      };

Invoke the reduceFunction2 first with values1 and then with values2:
```
reduceFunction2('myKey', values1);
reduceFunction2('myKey', values2);
```
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.

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;
                      };

Define a sample key:
```
var myKey = 'myKey';
```

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 } ] )
                       ];

Define a sample values1 array that combines the values passed to reduceFunction2:
```
var values1 = [
                { count: 1, qty: 5 },
                { count: 2, qty: 10 },
                { count: 3, qty: 15 }
              ];
```
Invoke the reduceFunction2 first with myKey and valuesIdempotent and then with myKey and values1:
```
reduceFunction2(myKey, valuesIdempotent);
reduceFunction2(myKey, values1);
```
Verify the reduceFunction2 returned the same result:
```
{ "count" : 6, "qty" : 30 }
```

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