MongoDB Map Reduce

What is MongoDB MapReduce?

The mapReduce() method from MongoDB is used to carry out the MapReduce operations. Map and reduce are the two primary functions of this function. The map function is used to group all of the data based on their key-value pairs, and the reduce function is used to perform actions on the grouped data. Typically, only huge data sets were used by this mapReduce() function. Similar to groupBy in SQL, aggregation procedures like max and avg may be performed using MongoDB MapReduce on data using a key. It operates on data in parallel and independently.

MapReduce and JavaScript Function

Custom JavaScript functions are used in MapReduce operations in MongoDB to associate or map values to keys. When more than one value is mapped to a key, the operation reduces all of those values into a single object. A document is inputted into the map function, which outputs key-value pairs. The reduction function produces a reduced result after receiving a key and a set of values as input. Every document in the collection is subjected to the map and reduce functions, and the output is then aggregated to get the result.

MapReduce Results

The MapReduce operation in MongoDB allows users to return the results inline or write the results to a collection. If you save the results of a mapreduce operation to a collection, you can use that collection as an input for additional mapreduce operations that replace, combine, or decrease fresh results with older ones. The result documents from a MongoDB mapreduce operation must not exceed the BSON Document Size limit, which is presently 16 megabytes when returning the results inline.

db.collection.mapReduce()

A MongoDB collection can be subjected to MapReduce operations using the mapReduce() method.

Syntax

The syntax for db.collection.mapReduce() is as follows:

The following parameters are required:

1. map: A JavaScript function that "maps" a value to a key and outputs the key and value pair. Type: Javascript or String
2. reduce: A JavaScript method that "reduces" each value connected to a given key to a single object. Type: Javascript or String
3. options: Document specifying additional parameters to db.collection.mapReduce(). Type: document

Description of additional arguments that db.collection.mapReduce() can accept.

1. out: specifies where the MongoDB mapreduce operation's result will be stored. Type: string or document
2. query: Determines the documents to be used as input for the map function by specifying the selection criteria using query operators. Type: document
3. sort: Input documents are sorted. Type: document
4. limit: Sets a maximum amount of documents that can be used as input for the map function. Type: number
5. finalize: Optional. a JavaScript method that alters the output produced by the reduce function. Type: Javascript or String
6. scope: Specifies global variables that can be accessed by the map, reduce, and finalize functions. Type: document
7. jsMode: Indicates whether intermediate data should be converted to BSON format between map and reduce operations. Type: boolean
8. verbose: Specifies if the time details should be included in the result information. Type: boolean
9. collation: Optional, the collation that will be used for the operation is specified. Type: document
10. bypassDocumentValidation: Optional, it allows mapReduce to skip document validation while performing the operation. Type: boolean

Output

A MapReduce operation's output can either be returned inline or written to a new collection. The output options for the MapReduce process are specified by the out parameter. The output is returned inline if the out option is not supplied.

Examples

Example 1: MongoDB mapreduce example for Aggregating Data

Input Document:

MongoDB MapReduce operation to calculate the total score:

Output:

Explanation:

1. The map function emits a key-value pair for each document in the scores collection. The key is set to a constant string "total" and the value is set to the score field of the document.
2. The reduce function aggregates the values for each key by summing them up.
3. The out option specifies the output collection for the MongoDB MapReduce result. In this case, we use the { inline: 1 } option to return the result as an array of documents rather than storing it in a collection.
4. The result is a single document with a key _id of "total" and a value of 245, which is the sum of the score fields in the input documents.

Example 2: MongoDB mapreduce example for Filtering Data

Input Document:

MapReduce operation to filter data:

Output:

Explanation:

1. The map function filters the documents based on the conditions specified in the if statement. In this case, we only emit the document if the gender field is "male" and the score field is greater than or equal to 75.
2. The reduce function simply returns the array of values for each key (in this case, the _id field).
3. The out option specifies the output collection for the MapReduce result. In this case, we use the { inline: 1 } option to return the result as an array of documents rather than storing it in a collection.
4. The result is an array of two documents that meet the filtering conditions in the map function.

Example 3: MongoDB mapreduce example for Transforming Data

Input Document:

Output:

MapReduce operation to transform data:

Explanation:

1. The map function adds a prefix ("Mr." or "Ms.") to the name field based on the gender field. It then emits a new document with the transformed name field and the original values for the gender field.
2. The reduce function is not used in this example since there is only one value for each key.
3. The out option specifies the output collection for the MapReduce result. In this case, we use the { inline: 1 } option to return the result as an array of documents rather than storing it in a collection.
4. The result is an array of three documents, which have the transformed name field and the original values for the gender field.