Chapter 0: Introduction and Aggregation Concepts

• Aggregation is a pipeline
• Pipelines are composed of one or more stages
• Stages use one or more expressions
• Expressions are functions

Aggregation

db.movies.aggregate([
  {$match: {}},
  {$project: {}},
  {$group: {}}
])

Pipeline

const pipeline = [
  {$match: {}},
  {$project: {}},
  {$group: {}}
]

Stage

{$match: {title: "CEO"}}

Add Expression in MongoDB(add function in aggregation)

{$add :[ "$a", "$b"]}

Introduction to the MongoDB Aggregation Framework

• Expressive filtering
• Powerful data transformation
• Statistical utilities & data analysis
• Single operation

Atlas Requirement

To connect to M121 course Atlas Cluster, using the mongo shell, you will need to use the following connection command:

mongo "mongodb://cluster0-shard-00-00-jxeqq.mongodb.net:27017,cluster0-shard-00-01-jxeqq.mongodb.net:27017,cluster0-shard-00-02-jxeqq.mongodb.net:27017/aggregations?replicaSet=Cluster0-shard-0" --authenticationDatabase admin --ssl -u m121 -p aggregations --norc

Once you’ve connected, list the existing collections of the aggregations database. Your output should be similar to this one:

Cluster0-shard-0:PRIMARY> show collections
air_airlines
air_alliances
air_routes
bronze_banking
customers
employees
exoplanets
gold_banking
icecream_data
movies
nycFacilities
silver_banking
solarSystem
stocks
system.views

The Concept of Pipelines

Pipelines are a composition of stages. Stages are configurable to produce desired transformations.

Documents flow through the stages like parts in an assembly line or water through a pipe.

The Aggregation Framework provides us many stages to filter and transform our data. Documents flow through the pipeline, passing from one stage to the next.

Aggregation Structure and Syntax

db.userColl.aggregate([{stage1}, {stage2}, ..., {stageN}], {options})

// simple first example
db.solarSystem.aggregate([{
  "$match": {
    "atmosphericComposition": { "$in": [/O2/] },
    "meanTemperature": { $gte: -40, "$lte": 40 }
  }
}, {
  "$project": {
    "_id": 0,
    "name": 1,
    "hasMoons": { "$gt": ["$numberOfMoons", 0] }
  }
}], { "allowDiskUse": true});

Aggregation Pipeline Quick Reference

Some expressions can only be used in certain stages.