Mongo db is document based nosql database. Mongodb insert, reads are faster than mysql but joins are bit slower. Joins in mongo db are implemented using map reduce terminology.

1. How to implement joins in mongo db?
Joins in mongo db are implemented using map reduce terminology.
Lets consider a scenario where you have a collestion of users and these
users do have roles assigned to them. Now consider we have to apply join
to determine role of user.
● First we need to connect to mongo
> mongo
● We need to select database to interact. This command will select
faculties database if it already exists or create new if it doesn’t exists
> use faculties
● Now we need to create collections over this db (which are like tables
in mysql). Let’s create two collections one is users and other one is
roles.
> db.createCollection(‘users’)
> db.createCollection(‘roles’)
● Now next step is to add some record sets to these collections
> db.users.insert({name: ‘Jakes’, role:1})
> db.users.insert({name: ‘Jakes’, role:2})
> db.roles.insert({role_id:1, rolename:’HOD’})
> db.roles.insert({role_id:2, rolename:’Director’})
● Now lets start process of applying join. For this first we need to create
map function.
> var map = function()
{
var output = {name: this.name,
rolename:db.roles.findOne({role_id:this.role}).rolename}
emit(this._id, output)

2. };
● Now time to create map function
> var reduce = { function(key, values) {
var outs = {name:null, rolename:null}
values.forEach(function(v) {
if(outs.name == null) { outs.name = v.name }
if(outs.rolename == null) {outs.rolename = v.rolename }
});
return outs;
};
● Now time to apply map reduce over the collection to be joined.
> db.users.mapReduce(map,reduce,{out:’result_coll’})
● Now result of join will be stored in result_coll, to check the output you
can query result_coll.
> db.result_coll.find()
[Thanks for reading]
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Author: Prasoon Sharma
Email: Prasoon.sharma1983@gmail.com