mongodb-lectures/7.MONGO_AGGREGATION.md

Mongo - Aggregation

Lesson Objectives

1. Explain what aggregation is
2. Explain the Mongo Aggregation Pipeline
3. Explain some basic aggregation functions
4. Explain $out

Explain what aggregation is

Explain the Mongo Aggregation Pipeline

1. db.collectionName.aggregate([<stage>, <stage>]);
2. http://docs.mongodb.org/manual/_images/aggregation-pipeline.png

Explain some basic aggregation functions

1. { $match : { weight : { $lt : 600 } } }
2. { $sort : { salary : -1 } }
3. { $limit : 1 }
4. { $skip : 1 }
5. { $out : 'collectionName'}
6. $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' } } }
7. { $unwind : '$loves' }
   • db.employees.aggregate([{$unwind : '$loves'}, {$group : { _id : '$loves', num : {$sum : 1}, names : {$addToSet : '$name'}}}])
8. $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?