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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)

};
● 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

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How to implement joins in mongo db

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How to implement joins in mongo db