Does MongoDB Support Join?

Document-oriented databases such as MongoDB are designed to store de-normalized data. Ideally, there should be no relationship between collections. If the same data is required in two or more documents, it must be repeated.

This can be frustrating, since there are few situations where you never need relational data. Fortunately,
MongoDB 3.2 introduces a new $lookup operator which can perform a LEFT-OUTER-JOIN-like operation on two or more collections.

As of Mongo 3.2 the answers to this question are mostly no longer correct. The new $lookup operator added to the aggregation pipeline is essentially identical to a left outer join:

{
 $lookup:
 {
 from: ,
 localField: ,
 foreignField: ,
 as: 

 }
}

Of course Mongo is not a relational database, and the developers are being careful to recommend specific use cases for $lookup, but at least as of 3.2 doing joins is now possible with MongoDB.

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Ref link: http://stackoverflow.com/questions/2350495/how-do-i-perform-the-sql-join-equivalent-in-mongodb

The $lookup is introduced in the MongoDB version 3.2 and it’s perform a "Left Outer Join" in the same database collection.

The "$lookup" stage does an equality match between a fields.

The "$lookup" stage adds a new array field whose elements are the matching from the "joined" collection.

The "$lookup" is an alternate way to implement Joins in MongoDB.

{
$lookup: {
from: “collection- to-join”,
localField: “Input-field-documents”,
foreignField: “Input-field- from - documents -of - the- "from"- collection”,
as: “Output- field-Array
}
}

//Perform Joins using $lookup

//A collection Users contains:

{ "_id" : 1001, "name" : "Anil Singh", "Age" : 32, "Qualification" : "MCA"}
{ "_id" : 1002, "name" : "Reena Singh", "price" : 26, "Qualification" : "MA" }
{ "_id" : 10033 }


//Other collection Customers contains:

{ "_id" : 1001, "cust_name" : "Aradhya", description: "Customer 1", "terms" : 3 }
{ "_id" : 1002, "cust_name" : "Sunil", description: "Customer 2", "terms" : 4 }
{ "_id" : 1003, "cust_name" : "Sushil", description: "Customer 3", "terms" : 6 }
{ "_id" : 1004, "cust_name" : null, description: "Customer 4" }
{ "_id" : 1005 }

Join operations perform between the users and customer’s collection on the fields "name" and "cust_name".

db.users.aggregate([
{
$lookup:
{
from: "customers",
localField: "name",
foreignField: "cust_name",
as: "users_customers_joins_docs"
}
}
]);

The Operation Result looks as.

{ "_id" : 1001,
"name" : "Anil Sing",
"Age": 32,
"Qualification" : "MCA",
"users_customers_joins_docs" : [
{ "_id" : 1002, "cust_name" : "Sunil", description: "Customer 2", "terms" : 4 } ]
}
{ "_id" : 1002,
"name" : "Reena Singh",
"Age" : 26,
"Qualification" : "MA",
"users_customers_joins_docs" : [
{ "_id" : 1003, "cust_name" : "Sushil", description: "Customer 3", "terms" : 6 }]
}
{ "_id" : 1003
"users_customers_joins_docs" : [
{ "_id" : 1004, "cust_name" : null, description: "Customer 4" }
{ "_id" : 1005 } ]
}

1. https://www.mongodb.com/blog/post/joins-and-other-aggregation-enhancements-coming-in-mongodb-3-2-part-1-of-3-introduction
2. http://stackoverflow.com/questions/2350495/how-do-i-perform-the-sql-join-equivalent-in-mongodb

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