MongoDB Aggregation

A query returns documents that are already stored, but many questions ask for a value that no single document holds. How many users are 30 years old? What is the total age of the users in each city? To answer these questions, MongoDB must read many documents, combine them, and calculate a new result. This work is called aggregation.

MongoDB performs aggregation with a pipeline, which is an ordered list of steps that documents pass through. Each step is called a stage. A stage receives documents, performs one job on them, and sends its output to the next stage