# Mongo - Aggregation

## Lesson Objectives
1. Explain what aggregation is
1. Explain the Mongo Aggregation Pipeline
1. Explain some basic aggregation functions
1. Explain $out

## Explain what aggregation is

## Explain the Mongo Aggregation Pipeline
1. `db.collectionName.aggregate([<stage>, <stage>]);`
1. http://docs.mongodb.org/manual/_images/aggregation-pipeline.png

## Explain some basic aggregation functions
1. `{ $match : { weight : { $lt : 600 } } }`
1. `{ $sort : { salary : -1 } }`
1. `{ $limit : 1 }`
1. `{ $skip : 1 }`
1. `{ $out : 'collectionName'}`
1. $group
	- `{ $group : { _id : '$gender' } }`
	- `{ $group : { _id : { 'weight' : '$weight', 'gender' : '$gender' } } } ] );`
	- `{ $group : { _id : '$gender', numOfEachGender : { $sum : 1 } } }`
	- `{ $group : { _id : '$gender', avgSalary : { $avg : '$salary' } } }`
	- `{ $group : { _id : '$weight', namesArray : { $addToSet : '$name' } } }`
1. `{ $unwind : '$loves' }`
	- `db.employees.aggregate([{$unwind : '$loves'}, {$group : { _id : '$loves', num : {$sum : 1}, names : {$addToSet : '$name'}}}])`
1. $project
	- `{ $project : { name : 1, _id : 0 } }`
	- `{ $project : { namezz :'$name' } }`
	- expressions
		- `{ $project : { awesomeName : { $concat : ['$name', ' is awesome', '!']}} }`
		- http://docs.mongodb.org/manual/meta/aggregation-quick-reference/#aggregation-expressions
	- `{ $project : { embedded_atrribute : '$embedded_object.foo' } }`
	- `{ $project : { embedded_object.attribute : 1 } }`
	- arrays?