On this page
Pass the name of the collection to the
<collection>) to use as the source documents to perform the map-reduce operation.
Views do not support map-reduce operations.
The command also accepts the following parameters:
Field Type Description
collection The name of the collection on which you want to perform map-reduce. This collection will be filtered using
querybefore being processed by the
keyand emits the
See Requirements for the map Function for more information.
valuesassociated with a particular
See Requirements for the reduce Function for more information.
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.
document Optional. Specifies the selection criteria using query operators for determining the documents input to the
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.
number Optional. Specifies a maximum number of documents for the input into the
Optional. Follows the
reducemethod and modifies the output.
See Requirements for the finalize Function for more information.
document Optional. Specifies global variables that are accessible in the
Optional. Specifies whether to convert intermediate data into BSON format between the execution of the
- 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.
reducefunction, which can result in faster execution.
- You can only use
jsModefor result sets with fewer than 500,000 distinct
keyarguments to the mapper’s
Optional. Specifies whether to include the
timinginformation in the result information. Set
trueto include the
mapReduceto bypass document validation during the operation. This lets you insert documents that do not meet the validation requirements.
New in version 3.2.
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:
When specifying collation, the
localefield 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.
document Optional. A document that expresses the write concern to use when outputing to a collection. Omit to use the default write concern.
The following is a prototype usage of 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:
map function has the following requirements:
- In the
mapfunction, reference the current document as
thiswithin the function.
mapfunction should not access the database for any reason.
mapfunction 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.
mapfunction may optionally call
emit(key,value)any number of times to create an output document associating
map function will call
emit(key,value) either 0 or 1 times depending on the value of the input document’s
map function may call
emit(key,value) multiple times depending on the number of elements in the input document’s
reduce function has the following prototype:
reduce function exhibits the following behaviors:
reducefunction should not access the database, even to perform read operations.
reducefunction should not affect the outside system.
- MongoDB will not call the
reducefunction for a key that has only a single value. The
valuesargument is an array whose elements are the
valueobjects that are “mapped” to the
- MongoDB can invoke the
reducefunction more than once for the same key. In this case, the previous output from the
reducefunction for that key will become one of the input values to the next
reducefunction invocation for that key.
reducefunction can access the variables defined in the
- The inputs to
reducemust 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
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
valueemitted by the
reducefunction must be associative. The following statement must be true: