MongoDB: The Definitive Guide, 3rd Edition

Chapter 7. Introduction to the Aggregation Framework

Many applications require data analysis of one form or another. MongoDB provides powerful support for running analytics natively using the aggregation framework. In this chapter, we introduce this framework and some of the fundamental tools it provides. We’ll cover:

• The aggregation framework

• Aggregation stages

• Aggregation expressions

• Aggregation accumulators

In the next chapter we’ll dive deeper and look at more advanced aggregation features, including the ability to perform joins across collections.

Pipelines, Stages, and Tunables

The aggregation framework is a set of analytics tools within MongoDB that allow you to do analytics on documents in one or more collections.

The aggregation framework is based on the concept of a pipeline. With an aggregation pipeline we take input from a MongoDB collection and pass the documents from that collection through one or more stages, each of which performs a different operation on its inputs (Figure 7-1). Each stage takes as input whatever the stage before it produced as output. The inputs and outputs for all stages are documents—a stream of documents, if you will.

Figure 7-1. The aggregation pipeline

If you’re familiar with pipelines in a Linux shell, such as bash, this is a very similar idea. Each stage has a specific job that it does. It expects a specific form of document and produces a specific ...