db.collection.mapReduce()

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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.

Note

Views do not support map-reduce operations.

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

copy

db.collection.mapReduce(
                         <map>,
                         <reduce>,
                         {
                           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	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.

Note

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
`args` `MaxKey` `MinKey`	`assert()` `BinData()` `DBPointer()` `DBRef()` `doassert()` `emit()` `gc()` `HexData()` `hex_md5()` `isNumber()` `isObject()` `ISODate()` `isString()`	`Map()` `MD5()` `NumberInt()` `NumberLong()` `ObjectId()` `print()` `printjson()` `printjsononeline()` `sleep()` `Timestamp()` `tojson()` `tojsononeline()` `tojsonObject()` `UUID()` `version()`

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.)
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:

copy

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:

copy

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

Requirements for the `reduce` Function

The reduce function has the following prototype:

copy

function(key

Docs

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Title here

db.collection.mapReduce()

Requirements for the `map` Function

Requirements for the `reduce` Function

db.collection.mapReduce()

Requirements for the map Function

Requirements for the reduce Function

Requirements for the `map` Function

Requirements for the `reduce` Function