Overview

Aggregation framework

Aggregation framework in MongoDB is a powerful feature that allows you to build complex queries that you normally cannot build with the common operators and functions. The framework achieves this by putting your documents into a multi-stage pipeline that transforms the documents in the desired result.

Pipeline

The pipeline consists of a series of stages where each stage receives the output of the previous stage. The section contains samples for the following stages:

Stage	Description
Project	Adds new fields or removes existing ones
Match	Filters the documents
Group	Groups documents by an identifier expression providing accumulator results
Unwind	Deconstructs an array field
Pagination (limit, skip)	Skips the first n documents and limits the total results
Bucket	Produces a distribution among documents based on a specified expression and specified boundaries

Aggregation framework