An alternate way to implement JOINs in MongoDB [Update]

Ayush Mishra

11 years ago

Reading Time: 2 minutes

I have updated this blog . As last description was creating some misunderstanding and lot of confusion .

In one of my project , there was a requirement to generate report in .csv format from MongoDB without using any reporting tool and any language .
I had to use only MongoDB query .
In that report , I had to use multiple collection . Since there was not a well structured DB schema .
We are using MongoDB , which is non -relational database . Despite of this fact , there were related collections .

As we know MongoDB is a no-SQL database and doesn’t support joins. But I found an alternate to implement JOINs in MongoDb using Map Reduce.

For Versions below Mongo 2.4:-

Let’s go with a small example :
We have two collections Employee and Department . We have to fetch details of all employees with their department .

1) Create Employee and Department collection .

db.createCollection("employee");
db.employee.insert(
   {
      _id: 1,
      name: { first: 'John', last: 'Backus' },
      city: 'New York',
      department:1
   }
);

db.employee.insert(
    {
      _id: 2,
      name: { first: 'Merry', last: 'Desuja' },
      city: 'London',
      department:2
   }
);
db.createCollection("department");
db.department.insert(
   {
      _id: 1,
      department: 'Manager'
   }
);

db.department.insert(
    {
      _id: 2,
      department: 'Accountant'
   }
);

2) Create a Map . This is a JavaScript function which emits key and value pair and processes for each input document .

var map = function () {
				var output= {firstname:this.name.first, lastname:this.name.last , department:db.department.findOne({_id:this.department}).department}
	    			emit(this._id, output);
				};

3) Create Reduce . This is a JavaScript function which accepts two arguments Key and Value .


var reduce = function(key, values) {
				var outs={ firstname:null , lastname:null , department:null}
 				values.forEach(function(v){
					if(outs.firstname ==null){
						outs.firstname = v.firstname
					}
					if(outs.lastname ==null){
						outs.lastname = v.lastname
					}
					if(outs.department ==null){
						outs.department = v.department
					}
					
				});
				return outs;
			};

4) MongoDB provides mapReduce commmand for Map Reduce operation .

db.employee.mapReduce(map,reduce,{out: 'emp_dept'});

5) ’emp_dept’ is new resulted collection where result will be stored . The Result will be :

{ "_id" : 1, "value" : { "firstname" : "John", "lastname" : "Backus", "department" : "Manager" } }
{ "_id" : 2, "value" : { "firstname" : "Merry", "lastname" : "Desuja", "department" : "Accountant" } }

For Version Mongo 2.4:-

Go to http://blog.knoldus.com/2014/03/12/easiest-way-to-implement-joins-in-mongodb-2-4/

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