Skip to main content

MongoDB Tutorials : Aggregation Framework



MongoDB Aggregation Framework groups set of documents together and performs certain operation on this grouped documents and returns results in the form of documents.
MongoDB aggregation answer to those query which requires grouping of documents.


Aggregation framework works on three type of model
  • Aggregation pipeline
  • Map Reduce
  • Single Purpose Aggregation Operations


Now let see how MongoDB Aggregation Framework works with simple example.

Suppose you are MongoDB application developer in a respected company and you have been given a MongoDB database that holds information about human population which is distributed according to cities and their states.

here is one sample document.


{
        "city" : "ACMAR",
        "loc" : [
                -86.51557,
                33.584132
        ],
        "pop" : 6055,
        "state" : "AL",
        "_id" : "35004"
}


for this tutorial collection name is zips.

on first day your manager come and asks you to calculate total population of state DC.

your simplest approach to solve above query would be find all documents which have DC as state. then iterate them one by one and add population to a variable.
here is the a sample script

MongoDB script 1

 var record = db.zips.find( { "state" : "DC"} )
 var sum = 0;
 record.forEach(function(rec){ sum = sum + rec.pop })
 print("Total population of State DC = " + sum)

Total population of State DC = 606900

This works fine and given task is done.
later, next day your manager comes and tells you to find total population of each states.

now this query is little tricky, but you would modify you first MongoDB script and make it is given below.

MongoDB script 2


 var states = db.zips.distinct("state")
 
 states.forEach(function(state){

   var records = db.zips.find({"state" : state});
   var sum = 0;

   records.forEach(function(record){
     sum = sum + record.pop;
   });

   print( state + " population: " + sum);
 });

yes, it will give you expected result. no doubt you are a good MongoDB developer.

but you have put extra efforts get result. it does not mean that you are not a good developer, rather you are not aware of MongoDB aggregation framework which produce expected result in one query.

following is aggregation query


db.zips.aggregate([ { "$group" : { "_id" : "$state", "population" : { "$sum" :
 "$pop" } }} ])
it will give you same result which in equivalent of second MongoDB script.
now let see how it works.

First look out the group clause of aggregate query.

db.zips.aggregate([ { "$group" : { "_id" : "$state" } } ])
 
 
it will group collections according to state



 
as documents are grouped now we can perform calculation on them like summation of population.


db.zips.aggregate([ { "$group" : { "_id" : "$state", "population" : { "$sum" :
 "$pop" } }} ])



hence you get expected result in one query using MongoDB Aggregation Framework.




Comments

Subscribe for latest tutorial updates

* indicates required

Popular posts from this blog

How to enable SQL logs in Spring Boot application?

This tutorial will demonstrate how to enable and disable SQL log statements in a Spring Boot application in order to debug SQL flow. Problem You will frequently need to debug the SQL statement while developing a Spring Boot application with a SQL database. SQL debug logs can assist you figure out what's wrong with JPA statements and whether or not there's a problem with database connectivity. Example  If you've built custom Spring data JPA methods and need to know what SQL statement is being utilized behind them, return repository . findByUsernameIn ( usernames ); Then you can enable Hibernet debug mode to log SQL statements. Solution Update the application.yml file with the Hibernet configuration as logging: level: org: hibernate: DEBUG or application.properties as logging.level.org.hibernate=DEBUG The SQL statement will appear in the application logs after modifying the configuration file and restarting the application. 2022-04-07 08:41...

Preload Images Using Javascript

Preload Image is technique which helps browser to render images with minimum delay. Today we will see example of image gallery. Gallery contain 5 images which load images one after another whenever user clicks next and back button. This is a basic image gallery scenario which is used in all possible website, then why we need preloaded images for this gallery? Now a day’s most of website becoming faster and user expectation is increasing. Suppose your website doesn’t use preload image technique for gallery and a user visits any image gallery from Google Plus or Facebook and visits your website gallery. Then that user always prefer those websites rather than yours. Why? Because your website load one image at a time. When user click on next button, then only gallery load image and user has wait till it get loaded. To avoid this situation gallery needs to download all images ...

JSON Tutorials : Getting Started

JSON is widely accepted text formatted structured data. JSON stands for " JavaScript Object Notation ". In general JSON can represent 1. Object of database record. 2. Object to represent a list of HTML elements. 3. Result of search query. 4. Response of an Ajax call. Here you can see JSON is used in many different areas and for many different scenarios. This means it has simple data structure. most of programming languages adopt it and it can flow easily from one connection to another. You can find JSON office definition here JSON Official Site . JSON is represented by two structural types, which includes two primitive types. Structural types Array : A sequential list of primitive data types between square brackets [ ] Object : Collection of key, value pair stored inside curly braces { }, where value would be primitive data type Primitive types : There are two primitive types key and value. " key " should be string and " value (data type)...