Map-Reduce

Map-Reduce </core/map-reduce/> is a data processing paradigm for condensing large volumes of data into aggregated results.

A map-reduce operation is issued on a collection view, as obtained from Collection#find method, by calling the map_reduce method on the view. The map_reduce method takes three arguments: the mapper, the reducer and map-reduce options. The mapper and the reducer must be provided as strings containing JavaScript functions.

For example, given the following collection with values 1 through 10:

coll = client['foo']
10.times do |i|
  coll.insert_one(v: i)
end

The following invocation will sum up the values less than 6:

coll.find(v: {'$lt' => 6}).map_reduce(
  'function() { emit(null, this.v) }',
  'function(key, values) { return Array.sum(values) }',
).first['value']
# => 15.0

The map_reduce method returns an instance of ::Mongo::Collection::View::MapReduce - a map-reduce view which holds the parameters to be used for the operation. To execute the operation, either iterate the results (by using e.g. each, first or to_a on the view object) or invoke the execute method. The execute method issues the map-reduce operation but does not return the result set from the server, and is primarily useful for when the output of the operation is directed to a collection as follows:

coll.find(...).map_reduce(...).out('destination_collection').execute

Note that:

• If the results of map-reduce are not directed to a collection, they are said to be retrieved inline. In this case the entire result set must fit in the 16 MiB BSON document size limit.
• If the results of map-reduce are directed to a collection, and the map-reduce view is iterated, the driver automatically retrieves the entire collection and returns its contents as the result set. The collection is retrieved without sorting. If map-reduce is performed into a collection that is not empty, the driver will return the documents as they exist in the collection after the map-reduce operation completes, which may include the documents that were in the collection prior to the map-reduce operation.

coll.find(...).map_reduce(...).out('destination_collection').each do |doc|
  # ...
end

coll.find(...).map_reduce(...).out(replace: 'destination_collection', db: 'db_name').each do |doc|
  # ...
end

Given a map-reduce view, it can be configured using the following methods:

The following accessor methods are defined on the view object: