mapReduce¶

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Definition
Syntax
Requirements for the map Function
Requirements for the reduce Function
Requirements for the finalize Function
out Options
Required Access
Restrictions
Map-Reduce Examples
Output
Additional Information

Definition¶

mapReduce¶: The mapReduce command allows you to run map-reduce aggregation operations over a collection.

Aggregation Pipeline as Alternative

Aggregation pipeline provides better performance and a more coherent interface than map-reduce, and map-reduce expressions can be rewritten using aggregation pipeline operators, such as $group, $merge, etc.

For map-reduce expressions that require custom functionality, MongoDB provides the $accumulator and $function aggregation operators starting in version 4.4. These operators provide users with the ability to define custom aggregation expressions in JavaScript.

For examples of aggregation alternatives to map-reduce operations, see Map-Reduce Examples. See also Map-Reduce to Aggregation Pipeline.

Syntax¶

Note

Starting in version 4.4, MongoDB ignores the verbose option.

Starting in version 4.2, MongoDB deprecates:

The map-reduce option to create a new sharded collection as well as the use of the sharded option for map-reduce. To output to a sharded collection, create the sharded collection first. MongoDB 4.2 also deprecates the replacement of an existing sharded collection.
The explicit specification of nonAtomic: false option.

The mapReduce command has the following syntax:

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db.runCommand( {
     mapReduce: <string>,
     map: <string or JavaScript>,
     reduce: <string or JavaScript>,
     finalize: <string or JavaScript>,
     out: <output>,
     query: <document>,
     sort: <document>,
     limit: <number>,
     scope: <document>,
     jsMode: <boolean>,
     verbose: <boolean>,
     bypassDocumentValidation: <boolean>,
     collation: <document>,
     writeConcern: <document>,
     comment: <any>
} )

Command Fields¶

The command takes the following fields as arguments:

Field	Type	Description
mapReduce	string	The name of the collection on which you want to perform map-reduce. This collection will be filtered using `query` before being processed by the `map` function. Note Views do not support map-reduce operations.
map	JavaScript or String	A JavaScript function that associates or “maps” a `value` with a `key` and emits the `key` and value `pair`. You can specify the function as BSON type JavaScript (i.e. BSON type 13) or String (i.e. BSON type 2). See Requirements for the map Function for more information.
reduce	JavaScript or String	A JavaScript function that “reduces” to a single object all the `values` associated with a particular `key`. You can specify the function as BSON type JavaScript (i.e. BSON type 13) or String (i.e. BSON type 2). See Requirements for the reduce Function for more information.
out	string or document	Specifies where to output the result of the map-reduce operation. You can either output to a collection or return the result inline. On a primary member of a replica set you can output either to a collection or inline, but on a secondary, only inline output is possible. See out Options for more information.
query	document	Optional. Specifies the selection criteria using query operators for determining the documents input to the `map` function.
sort	document	Optional. Sorts the input documents. This option is useful for optimization. For example, specify the sort key to be the same as the emit key so that there are fewer reduce operations. The sort key must be in an existing index for this collection.
limit	number	Optional. Specifies a maximum number of documents for the input into the `map` function.
finalize	JavaScript or String	Optional. A JavaScript function that modifies the output after the `reduce` function. You can specify the function as BSON type JavaScript (i.e. BSON type 13) or String (i.e. BSON type 2). See Requirements for the finalize Function for more information.
scope	document	Optional. Specifies global variables that are accessible in the `map`, `reduce` and `finalize` functions.
jsMode	boolean	Optional. Specifies whether to convert intermediate data into BSON format between the execution of the `map` and `reduce` functions. Defaults to `false`. If `false`: Internally, MongoDB converts the JavaScript objects emitted by the `map` function to BSON objects. These BSON objects are then converted back to JavaScript objects when calling the `reduce` function. The map-reduce operation places the intermediate BSON objects in temporary, on-disk storage. This allows the map-reduce operation to execute over arbitrarily large data sets. If `true`: Internally, the JavaScript objects emitted during `map` function remain as JavaScript objects. There is no need to convert the objects for the `reduce` function, which can result in faster execution. You can only use `jsMode` for result sets with fewer than 500,000 distinct `key` arguments to the mapper’s `emit()` function.
verbose	boolean	Optional. Specifies whether to include the `timing` information in the result information. Set `verbose` to `true` to include the `timing` information. Defaults to `false`. Starting in MongoDB 4.4, this option is ignored. The result information always excludes the `timing` information. You can view timing information by running `explain` with the `mapReduce` command in the `"executionStats"` or `"allPlansExecution"` `verbosity` modes.
bypassDocumentValidation	boolean	Optional. Enables `mapReduce` to bypass document validation during the operation. This lets you insert documents that do not meet the validation requirements. New in version 3.2. Note If the output option is set to `inline`, no document validation occurs. If the output goes to a collection, `mapReduce` observes any validation rules which the collection has and does not insert any invalid documents unless the `bypassDocumentValidation` parameter is set to true.
collation	document	Optional. Specifies the collation to use for the operation. Collation allows users to specify language-specific rules for string comparison, such as rules for lettercase and accent marks. The collation option has the following syntax: copy collation: { locale: <string>, caseLevel: <boolean>, caseFirst: <string>, strength: <int>, numericOrdering: <boolean>, alternate: <string>, maxVariable: <string>, backwards: <boolean> } When specifying collation, the `locale` field is mandatory; all other collation fields are optional. For descriptions of the fields, see Collation Document. If the collation is unspecified but the collection has a default collation (see `db.createCollection()`), the operation uses the collation specified for the collection. If no collation is specified for the collection or for the operations, MongoDB uses the simple binary comparison used in prior versions for string comparisons. You cannot specify multiple collations for an operation. For example, you cannot specify different collations per field, or if performing a find with a sort, you cannot use one collation for the find and another for the sort. New in version 3.4.
writeConcern	document	Optional. A document that expresses the write concern to use when outputing to a collection. Omit to use the default write concern.
`comment`	any	Optional. A user-provided comment to attach to this command. Once set, this comment appears alongside records of this command in the following locations: mongod log messages, in the `attr.command.cursor.comment` field. Database profiler output, in the `command.comment` field. `currentOp` output, in the `command.comment` field. A comment can be any valid BSON type (string, integer, object, array, etc). New in version 4.4.

Usage¶

The following is a prototype usage of the mapReduce command:

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var mapFunction = function() { ... };
var reduceFunction = function(key, values) { ... };

db.runCommand(
               {
                 mapReduce: <input-collection>,
                 map: mapFunction,
                 reduce: reduceFunction,
                 out: { merge: <output-collection> },
                 query: <query>
               }
             )

JavaScript in MongoDB

Although mapReduce uses JavaScript, most interactions with MongoDB do not use JavaScript but use an idiomatic driver in the language of the interacting application.

Requirements for the `map` Function¶

The map function is responsible for transforming each input document into zero or more documents. It can access the variables defined in the scope parameter, and has the following prototype:

copy

function() {
   ...
   emit(key, value);
}

The map function has the following requirements:

In the map function, reference the current document as this within the function.
The map function should not access the database for any reason.
The map function should be pure, or have no impact outside of the function (i.e. side effects.)
The map function may optionally call emit(key,value) any number of times to create an output document associating key with value.
In MongoDB 4.2 and earlier, a single emit can only hold half of MongoDB’s maximum BSON document size. MongoDB removes this restriction starting in version 4.4.
Starting in MongoDB 4.4, mapReduce no longer supports the deprecated BSON type JavaScript code with scope (BSON type 15) for its functions. The map function must be either BSON type String (BSON type 2) or BSON type JavaScript (BSON type 13). To pass constant values which will be accessible in the map function, use the scope parameter.

The use of JavaScript code with scope for the map function has been deprecated since version 4.2.1.

The following map function will call emit(key,value) either 0 or 1 times depending on the value of the input document’s status field:

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function() {
    if (this.status == 'A')
        emit(this.cust_id, 1);
}

The following map function may call emit(key,value) multiple times depending on the number of elements in the input document’s items field:

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function() {
    this.items.forEach(function(item){ emit(item.sku, 1); });
}

Requirements for the `reduce` Function¶

The reduce function has the following prototype:

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function(key, values) {
   ...
   return result;
}

The reduce function exhibits the following behaviors:

The reduce function should not access the database, even to perform read operations.
The reduce function should not affect the outside system.
MongoDB will not call the reduce function for a key that has only a single value. The values argument is an array whose elements are the value objects that are “mapped” to the key.
MongoDB can invoke the reduce function more than once for the same key. In this case, the previous output from the reduce function for that key will become one of the input values to the next reduce function invocation for that key.
The reduce function can access the variables defined in the scope parameter.
The inputs to reduce must not be larger than half of MongoDB’s maximum BSON document size. This requirement may be violated when large documents are returned and then joined together in subsequent reduce steps.
Starting in MongoDB 4.4, mapReduce no longer supports the deprecated BSON type JavaScript code with scope (BSON type 15) for its functions. The reduce function must be either BSON type String (BSON type 2) or BSON type JavaScript (BSON type 13). To pass constant values which will be accessible in the reduce function, use the scope parameter.

The use of JavaScript code with scope for the reduce function has been deprecated since version 4.2.1.

Because it is possible to invoke the reduce function more than once for the same key, the following properties need to be true:

the type of the return object must be identical to the type of the value emitted by the map function.

the reduce function must be associative. The following statement must be true:

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reduce(key, [ C, reduce(key, [ A, B ]) ] ) == reduce( key, [ C, A, B ] )

the reduce function must be idempotent. Ensure that the following statement is true:

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reduce( key, [ reduce(key, valuesArray) ] ) == reduce( key, valuesArray )

the reduce function should be commutative: that is, the order of the elements in the valuesArray should not affect the output of the reduce function, so that the following statement is true:
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```
reduce( key, [ A, B ] ) == reduce( key, [ B, A ] )
```

Requirements for the `finalize` Function¶

The finalize function has the following prototype:

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function(key, reducedValue) {
   ...
   return modifiedObject;
}

The finalize function receives as its arguments a key value and the reducedValue from the reduce function. Be aware that:

The finalize function should not access the database for any reason.
The finalize function should be pure, or have no impact outside of the function (i.e. side effects.)
The finalize function can access the variables defined in the scope parameter.
Starting in MongoDB 4.4, mapReduce no longer supports the deprecated BSON type JavaScript code with scope (BSON type 15) for its functions. The finalize function must be either BSON type String (BSON type 2) or BSON type JavaScript (BSON type 13). To pass constant values which will be accessible in the finalize function, use the scope parameter.

The use of JavaScript code with scope for the finalize function has been deprecated since version 4.2.1.

`out` Options¶

You can specify the following options for the out parameter:

Output to a Collection¶

This option outputs to a new collection, and is not available on secondary members of replica sets.

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out: <collectionName>

Output to a Collection with an Action¶

Note

Starting in version 4.2, MongoDB deprecates:

The map-reduce option to create a new sharded collection as well as the use of the sharded option for map-reduce. To output to a sharded collection, create the sharded collection first. MongoDB 4.2 also deprecates the replacement of an existing sharded collection.
The explicit specification of nonAtomic: false option.

This option is only available when passing a collection that already exists to out. It is not available on secondary members of replica sets.

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out: { <action>: <collectionName>
        [, db: <dbName>]
        [, sharded: <boolean> ]
        [, nonAtomic: <boolean> ] }

When you output to a collection with an action, the out has the following parameters:

<action>: Specify one of the following actions:
- replace
  
  Replace the contents of the <collectionName> if the collection with the <collectionName> exists.
- merge
  
  Merge the new result with the existing result if the output collection already exists. If an existing document has the same key as the new result, overwrite that existing document.
- reduce
  
  Merge the new result with the existing result if the output collection already exists. If an existing document has the same key as the new result, apply the reduce function to both the new and the existing documents and overwrite the existing document with the result.
db:

Optional. The name of the database that you want the map-reduce operation to write its output. By default this will be the same database as the input collection.
sharded:

Note

Starting in version 4.2, the use of the sharded option is deprecated.

Optional. If true and you have enabled sharding on output database, the map-reduce operation will shard the output collection using the _id field as the shard key.

If true and collectionName is an existing unsharded collection, map-reduce fails.
nonAtomic:

Note

Starting in MongoDB 4.2, explicitly setting nonAtomic to false is deprecated.

Optional. Specify output operation as non-atomic. This applies only to the merge and reduce output modes, which may take minutes to execute.

By default nonAtomic is false, and the map-reduce operation locks the database during post-processing.

If nonAtomic is true, the post-processing step prevents MongoDB from locking the database: during this time, other clients will be able to read intermediate states of the output collection.

Output Inline¶

Perform the map-reduce operation in memory and return the result. This option is the only available option for out on secondary members of replica sets.

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out: { inline: 1 }

The result must fit within the maximum size of a BSON document.

Required Access¶

If your MongoDB deployment enforces authentication, the user executing the mapReduce command must possess the following privilege actions:

Map-reduce with {out : inline} output option:

find

Map-reduce with the replace action when outputting to a collection:

find,
insert,
replace

Map-reduce with the merge or reduce actions when outputting to a collection:

The readWrite built-in role provides the necessary permissions to perform map-reduce aggregation.

Restrictions¶

MongoDB drivers automatically set afterClusterTime for operations associated with causally consistent sessions. Starting in MongoDB 4.2, the mapReduce command no longer support afterClusterTime. As such, mapReduce cannot be associatd with causally consistent sessions.

Map-Reduce Examples¶

In the mongo shell, the db.collection.mapReduce() method is a wrapper around the mapReduce command. The following examples use the db.collection.mapReduce() method:

Aggregation Pipeline as Alternative

Aggregation pipeline provides better performance and a more coherent interface than map-reduce, and various map-reduce expressions can be rewritten using aggregation pipeline operators, such as $group, $merge, etc.

For map-reduce expressions that require custom functionality, MongoDB provides the $accumulator and $function aggregation operators starting in version 4.4. These operators provide users with the ability to define custom aggregation expressions in JavaScript.

The example below includes aggregation pipeline alternatives without custom aggregation expressions. For alternatives that use custom expressions, see Map-Reduce to Aggregation Pipeline Translation Examples.

Create a sample collection orders with the following documents:

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db.orders.insertMany([
   { _id: 1, cust_id: "Ant O. Knee", ord_date: new Date("2020-03-01"), price: 25, items: [ { sku: "oranges", qty: 5, price: 2.5 }, { sku: "apples", qty: 5, price: 2.5 } ], status: "A" },
   { _id: 2, cust_id: "Ant O. Knee", ord_date: new Date("2020-03-08"), price: 70, items: [ { sku: "oranges", qty: 8, price: 2.5 }, { sku: "chocolates", qty: 5, price: 10 } ], status: "A" },
   { _id: 3, cust_id: "Busby Bee", ord_date: new Date("2020-03-08"), price: 50, items: [ { sku: "oranges", qty: 10, price: 2.5 }, { sku: "pears", qty: 10, price: 2.5 } ], status: "A" },
   { _id: 4, cust_id: "Busby Bee", ord_date: new Date("2020-03-18"), price: 25, items: [ { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" },
   { _id: 5, cust_id: "Busby Bee", ord_date: new Date("2020-03-19"), price: 50, items: [ { sku: "chocolates", qty: 5, price: 10 } ], status: "A"},
   { _id: 6, cust_id: "Cam Elot", ord_date: new Date("2020-03-19"), price: 35, items: [ { sku: "carrots", qty: 10, price: 1.0 }, { sku: "apples", qty: 10, price: 2.5 } ], status: "A" },
   { _id: 7, cust_id: "Cam Elot", ord_date: new Date("2020-03-20"), price: 25, items: [ { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" },
   { _id: 8, cust_id: "Don Quis", ord_date: new Date("2020-03-20"), price: 75, items: [ { sku: "chocolates", qty: 5, price: 10 }, { sku: "apples", qty: 10, price: 2.5 } ], status: "A" },
   { _id: 9, cust_id: "Don Quis", ord_date: new Date("2020-03-20"), price: 55, items: [ { sku: "carrots", qty: 5, price: 1.0 }, { sku: "apples", qty: 10, price: 2.5 }, { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" },
   { _id: 10, cust_id: "Don Quis", ord_date: new Date("2020-03-23"), price: 25, items: [ { sku: "oranges", qty: 10, price: 2.5 } ], status: "A" }
])

Return the Total Price Per Customer¶

Perform the map-reduce operation on the orders collection to group by the cust_id, and calculate the sum of the price for each cust_id:

Define the map function to process each input document:
- In the function, this refers to the document that the map-reduce operation is processing.
- The function maps the price to the cust_id for each document and emits the cust_id and price pair.
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```
var mapFunction1 = function() {
   emit(this.cust_id, this.price);
};
```
Define the corresponding reduce function with two arguments keyCustId and valuesPrices:
- The valuesPrices is an array whose elements are the price values emitted by the map function and grouped by keyCustId.
- The function reduces the valuesPrice array to the sum of its elements.
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```
var reduceFunction1 = function(keyCustId, valuesPrices) {
   return Array.sum(valuesPrices);
};
```
Perform map-reduce on all documents in the orders collection using the mapFunction1 map function and the reduceFunction1 reduce function.
copy
```
db.orders.mapReduce(
   mapFunction1,
   reduceFunction1,
   { out: "map_reduce_example" }
)
```
This operation outputs the results to a collection named map_reduce_example. If the map_reduce_example collection already exists, the operation will replace the contents with the results of this map-reduce operation.

Query the map_reduce_example collection to verify the results:

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db.map_reduce_example.find().sort( { _id: 1 } )

The operation returns the following documents:

{ "_id" : "Ant O. Knee", "value" : 95 }
{ "_id" : "Busby Bee", "value" : 125 }
{ "_id" : "Cam Elot", "value" : 60 }
{ "_id" : "Don Quis", "value" : 155 }

Aggregation Alternative¶

Using the available aggregation pipeline operators, you can rewrite the map-reduce operation without defining custom functions:

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db.orders.aggregate([
   { $group: { _id: "$cust_id", value: { $sum: "$price" } } },
   { $out: "agg_alternative_1" }
])

The $group stage groups by the cust_id and calculates the value field (See also $sum). The value field contains the total price for each cust_id.

The stage output the following documents to the next stage:
```
{ "_id" : "Don Quis", "value" : 155 }
{ "_id" : "Ant O. Knee", "value" : 95 }
{ "_id" : "Cam Elot", "value" : 60 }
{ "_id" : "Busby Bee", "value" : 125 }
```
Then, the $out writes the output to the collection agg_alternative_1. Alternatively, you could use $merge instead of $out.

Query the agg_alternative_1 collection to verify the results:

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db.agg_alternative_1.find().sort( { _id: 1 } )

The operation returns the following documents:

{ "_id" : "Ant O. Knee", "value" : 95 }
{ "_id" : "Busby Bee", "value" : 125 }
{ "_id" : "Cam Elot", "value" : 60 }
{ "_id" : "Don Quis", "value" : 155 }

Calculate Order and Total Quantity with Average Quantity Per Item¶

In this example, you will perform a map-reduce operation on the orders collection for all documents that have an ord_date value greater than or equal to 2020-03-01. The operation groups by the item.sku field, and calculates the number of orders and the total quantity ordered for each sku. The operation then calculates the average quantity per order for each sku value and merges the results into the output collection. When merging results, if an existing document has the same key as the new result, the operation overwrites the existing document. If there is no existing document with the same key, the operation inserts the document.

Define the map function to process each input document:
- In the function, this refers to the document that the map-reduce operation is processing.
- For each item, the function associates the sku with a new object value that contains the count of 1 and the item qty for the order and emits the sku and value pair.
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```
 var mapFunction2 = function() {
    for (var idx = 0; idx < this.items.length; idx++) {
       var key = this.items[idx].sku;
       var value = { count: 1, qty: this.items[idx].qty };

       emit(key, value);
    }
};
```
Define the corresponding reduce function with two arguments keySKU and countObjVals:
- countObjVals is an array whose elements are the objects mapped to the grouped keySKU values passed by map function to the reducer function.
- The function reduces the countObjVals array to a single object reducedValue that contains the count and the qty fields.
- In reducedVal, the count field contains the sum of the count fields from the individual array elements, and the qty field contains the sum of the qty fields from the individual array elements.
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```
var reduceFunction2 = function(keySKU, countObjVals) {
   reducedVal = { count: 0, qty: 0 };

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

   return reducedVal;
};
```
Define a finalize function with two arguments key and reducedVal. The function modifies the reducedVal object to add a computed field named avg and returns the modified object:
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```
var finalizeFunction2 = function (key, reducedVal) {
  reducedVal.avg = reducedVal.qty/reducedVal.count;
  return reducedVal;
};
```
Perform the map-reduce operation on the orders collection using the mapFunction2, reduceFunction2, and finalizeFunction2 functions.
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```
db.orders.mapReduce(
   mapFunction2,
   reduceFunction2,
   {
     out: { merge: "map_reduce_example2" },
     query: { ord_date: { $gte: new Date("2020-03-01") } },
     finalize: finalizeFunction2
   }
 );
```
This operation uses the query field to select only those documents with ord_date greater than or equal to new Date("2020-03-01"). Then it output the results to a collection map_reduce_example2.

If the map_reduce_example2 collection already exists, the operation will merge the existing contents with the results of this map-reduce operation. That is, if an existing document has the same key as the new result, the operation overwrites the existing document. If there is no existing document with the same key, the operation inserts the document.

Query the map_reduce_example2 collection to verify the results:

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db.map_reduce_example2.find().sort( { _id: 1 } )

The operation returns the following documents:

{ "_id" : "apples", "value" : { "count" : 3, "qty" : 30, "avg" : 10 } }
{ "_id" : "carrots", "value" : { "count" : 2, "qty" : 15, "avg" : 7.5 } }
{ "_id" : "chocolates", "value" : { "count" : 3, "qty" : 15, "avg" : 5 } }
{ "_id" : "oranges", "value" : { "count" : 6, "qty" : 58, "avg" : 9.666666666666666 } }
{ "_id" : "pears", "value" : { "count" : 1, "qty" : 10, "avg" : 10 } }

Aggregation Alternative¶

Using the available aggregation pipeline operators, you can rewrite the map-reduce operation without defining custom functions:

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db.orders.aggregate( [
   { $match: { ord_date: { $gte: new Date("2020-03-01") } } },
   { $unwind: "$items" },
   { $group: { _id: "$items.sku", qty: { $sum: "$items.qty" }, orders_ids: { $addToSet: "$_id" } }  },
   { $project: { value: { count: { $size: "$orders_ids" }, qty: "$qty", avg: { $divide: [ "$qty", { $size: "$orders_ids" } ] } } } },
   { $merge: { into: "agg_alternative_3", on: "_id", whenMatched: "replace",  whenNotMatched: "insert" } }
] )

The $match stage selects only those documents with ord_date greater than or equal to new Date("2020-03-01").

The $unwinds stage breaks down the document by the items array field to output a document for each array element. For example:

{ "_id" : 1, "cust_id" : "Ant O. Knee", "ord_date" : ISODate("2020-03-01T00:00:00Z"), "price" : 25, "items" : { "sku" : "oranges", "qty" : 5, "price" : 2.5 }, "status" : "A" }
{ "_id" : 1, "cust_id" : "Ant O. Knee", "ord_date" : ISODate("2020-03-01T00:00:00Z"), "price" : 25, "items" : { "sku" : "apples", "qty" : 5, "price" : 2.5 }, "status" : "A" }
{ "_id" : 2, "cust_id" : "Ant O. Knee", "ord_date" : ISODate("2020-03-08T00:00:00Z"), "price" : 70, "items" : { "sku" : "oranges", "qty" : 8, "price" : 2.5 }, "status" : "A" }
{ "_id" : 2, "cust_id" : "Ant O. Knee", "ord_date" : ISODate("2020-03-08T00:00:00Z"), "price" : 70, "items" : { "sku" : "chocolates", "qty" : 5, "price" : 10 }, "status" : "A" }
{ "_id" : 3, "cust_id" : "Busby Bee", "ord_date" : ISODate("2020-03-08T00:00:00Z"), "price" : 50, "items" : { "sku" : "oranges", "qty" : 10, "price" : 2.5 }, "status" : "A" }
{ "_id" : 3, "cust_id" : "Busby Bee", "ord_date" : ISODate("2020-03-08T00:00:00Z"), "price" : 50, "items" : { "sku" : "pears", "qty" : 10, "price" : 2.5 }, "status" : "A" }
{ "_id" : 4, "cust_id" : "Busby Bee", "ord_date" : ISODate("2020-03-18T00:00:00Z"), "price" : 25, "items" : { "sku" : "oranges", "qty" : 10, "price" : 2.5 }, "status" : "A" }
{ "_id" : 5, "cust_id" : "Busby Bee", "ord_date" : ISODate("2020-03-19T00:00:00Z"), "price" : 50, "items" : { "sku" : "chocolates", "qty" : 5, "price" : 10 }, "status" : "A" }
...

The $group stage groups by the items.sku, calculating for each sku:

The qty field. The qty field contains the total qty ordered per each items.sku (See $sum).
The orders_ids array. The orders_ids field contains an array of distinct order _id’s for the items.sku (See $addToSet).

{ "_id" : "chocolates", "qty" : 15, "orders_ids" : [ 2, 5, 8 ] }
{ "_id" : "oranges", "qty" : 63, "orders_ids" : [ 4, 7, 3, 2, 9, 1, 10 ] }
{ "_id" : "carrots", "qty" : 15, "orders_ids" : [ 6, 9 ] }
{ "_id" : "apples", "qty" : 35, "orders_ids" : [ 9, 8, 1, 6 ] }
{ "_id" : "pears", "qty" : 10, "orders_ids" : [ 3 ] }

The $project stage reshapes the output document to mirror the map-reduce’s output to have two fields _id and value. The $project sets:

the value.count to the size of the orders_ids array. (See $size.)
the value.qty to the qty field of input document.
the value.avg to the average number of qty per order. (See $divide and $size.)

{ "_id" : "apples", "value" : { "count" : 4, "qty" : 35, "avg" : 8.75 } }
{ "_id" : "pears", "value" : { "count" : 1, "qty" : 10, "avg" : 10 } }
{ "_id" : "chocolates", "value" : { "count" : 3, "qty" : 15, "avg" : 5 } }
{ "_id" : "oranges", "value" : { "count" : 7, "qty" : 63, "avg" : 9 } }
{ "_id" : "carrots", "value" : { "count" : 2, "qty" : 15, "avg" : 7.5 } }

Finally, the $merge writes the output to the collection agg_alternative_3. If an existing document has the same key _id as the new result, the operation overwrites the existing document. If there is no existing document with the same key, the operation inserts the document.

Query the agg_alternative_3 collection to verify the results:

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db.agg_alternative_3.find().sort( { _id: 1 } )

The operation returns the following documents:

{ "_id" : "apples", "value" : { "count" : 4, "qty" : 35, "avg" : 8.75 } }
{ "_id" : "carrots", "value" : { "count" : 2, "qty" : 15, "avg" : 7.5 } }
{ "_id" : "chocolates", "value" : { "count" : 3, "qty" : 15, "avg" : 5 } }
{ "_id" : "oranges", "value" : { "count" : 7, "qty" : 63, "avg" : 9 } }
{ "_id" : "pears", "value" : { "count" : 1, "qty" : 10, "avg" : 10 } }

Output¶

If you set the out parameter to write the results to a collection, the mapReduce command returns a document in the following form:

MongoDB 4.4+
MongoDB 4.2 and earlier

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{ "result" : "map_reduce_example", "ok" : 1 }

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{
    "result" : <string or document>,
    "timeMillis" : <int>,
    "counts" : {
        "input" : <int>,
        "emit" : <int>,
        "reduce" : <int>,
        "output" : <int>
    },
    "ok" : <int>,
}

If you set the out parameter to output the results inline, the mapReduce command returns a document in the following form:

MongoDB 4.4+
MongoDB 4.2 and earlier

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{
    "results" : [
       {
          "_id" : <key>,
          "value" :<reduced or finalizedValue for key>
       },
       ...
    ],
    "ok" : <int>
}

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{
    "results" : [
       {
          "_id" : <key>,
          "value" :<reduced or finalizedValue for key>
       },
       ...
    ],
    "timeMillis" : <int>,
    "counts" : {
       "input" : <int>,
       "emit" : <int>,
       "reduce" : <int>,
       "output" : <int>
    },
    "ok" : <int>
}

mapReduce.result¶

For output sent to a collection, this value is either:

a string for the collection name if out did not specify the database name, or
a document with both db and collection fields if out specified both a database and collection name.

mapReduce.results¶

For output written inline, an array of resulting documents. Each resulting document contains two fields:

_id field contains the key value,
value field contains the reduced or finalized value for the associated key.

mapReduce.timeMillis¶