db.collection. mapReduce ( map, reduce, {<out>, <query>, <sort>, <limit>, <finalize>, <scope>, <jsMode>, <verbose>} )

The db.collection.mapReduce() method provides a wrapper around the mapReduce command.


Views do not support map-reduce operations.

db.collection.mapReduce() has the following syntax:

                           out: <collection>,
                           query: <document>,
                           sort: <document>,
                           limit: <number>,
                           finalize: <function>,
                           scope: <document>,
                           jsMode: <boolean>,
                           verbose: <boolean>,
                           bypassDocumentValidation: <boolean>

db.collection.mapReduce() takes the following parameters:

Parameter Type Description
map function

A JavaScript function that associates or “maps” a value with a key and emits the key and value pair.

See Requirements for the map Function for more information.

reduce function

A JavaScript function that “reduces” to a single object all the values associated with a particular key.

See Requirements for the reduce Function for more information.

options document A document that specifies additional parameters to db.collection.mapReduce().
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.

The following table describes additional arguments that db.collection.mapReduce() can accept.

Field Type Description
out string or document

Specifies the location of the result of the map-reduce operation. You can output to a collection, output to a collection with an action, or output inline. You may output to a collection when performing map-reduce operations on the primary members of the set; on secondary members you may only use the inline output.

See out Options for more information.

query document Specifies the selection criteria using query operators for determining the documents input to the map function.
sort document 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 Specifies a maximum number of documents for the input into the map function.
finalize function

Optional. Follows the reduce method and modifies the output.

See Requirements for the finalize Function for more information.

scope document Specifies global variables that are accessible in the map, reduce and finalize functions.
jsMode boolean

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

Specifies whether to include the timing information in the result information. Set verbose to true to include the timing information.

Defaults to false.

collation document


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:

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.


map-reduce operations, the group command, and $where operator expressions cannot access certain global functions or properties, such as db, that are available in the mongo shell.

The following JavaScript functions and properties are available to map-reduce operations, the group command, and $where operator expressions:

Available Properties Available Functions  

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:

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.)
  • A single emit can only hold half of MongoDB’s maximum BSON document size.
  • The map function may optionally call emit(key,value) any number of times to create an output document associating key with value.

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:

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:

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

Requirements for the reduce Function

The reduce function has the following prototype: