1. Aggregation

2. Pipeline
Unix pipeline
Piping command line operations
ls -al | grep '^d‘ |

3. Pipeline
• Piping aggregation operations
$match | $sort | $skip | more
Stream of documents Result document

4. Example
{
title : “mongo DB " ,
author : "bob" ,
posted : new Date() ,
pageViews : 5 ,
tags : [ "fun" , "good" , "fun" ] ,
language : “English”,
other : { foo : 5 }
}

5. 1. Count – return number of documents in collection.
> db.user.count();
you can also pass query, and MongoDB will count number of result for
that query.
2. Distinct
The distinct commands find all the distinct values for given key in
collection.
db.runCommand ( {“distinct” : “collection name” ,”key” : “key field” } )
3. Match
>db.blog.aggregate({$match : {pageViews : {$gt : 1 ,$lt :50 } } } )
4. Limit
> db.blog.aggregate({$limit : 5 } )
5. Sort
>db.blog.find( ).sort({title : 1} )

6. 6. $project
Include, exclude or rename fields
Insert computed fields
Create sub-document fields
selecting and excluding fields
>db.blog.aggregate{
{
“$project” : { _id : 0 , title : 1 , language : 1}
}
}

7. {
_id: 375,
title: “mongo DB",
ISBN: "9781857150193",
available: true,
pages: 218,
subjects: [
“Test",
“Pune",
"1920s"
],
language: "English"
}
{
title: “mongo DB,
language:
"English"
}

8. Insert computed field
Db.blog.aggregate( {
“$project” : { title : 1 ,
newPageViews : { $add : *“$pageViews” ,10]}
}
}

9. Rename Page Views
db.blog.aggregate( { title : 1,
newPageViews : “$pageViews”
} )
Create and Populate new Field that hold subfields
db.blog.aggregate( {"$project" : {
title : 1 ,
stats:{pv : "$pageViews" ,
foo : "$other.foo" }
}
})

10. Groups
Each document in collection has following form
{
"_id": “413710",
"city": “Pune",
"state": “MH",
"pop": 5574,
"loc": [ -74.016323, 40.710537 ]
}

11. Group
db.blog.aggregate( { $group :
{ _id : "$state",
totalPop : { $sum : "$pop" }
}
},
{
$match : {totalPop : { $gte : 10*1000 } }
} )

12. Usage and Limit
• collection.aggregate() method
– Mongo shell
– Most drivers
• aggregate database command

13. Limitations
Result limited by BSON document size
Final command result
Intermediate shard results
Pipeline operator memory limits
Some BSON types unsupported
Binary, Code, deprecated types

14. Map Reduce
• Map reduce operation handle complex
aggregation task.