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Join between two collections in MongoDB

$lookup command used to perform join between two collections. This is new feature of MongoDB version 3.2

$lookup perform the left outer join between two collections in same database. For the each input document $lookup add a new array fields whose element matches from joined collection.


       from: <collection to join>,
       localField: <field from the input documents>,
       foreignField: <field from the documents of the "from" collection>,
       as: <output array field>

$lookup takes a document with following fields
from: Its take collection name in same database to perform joined.
localField: Its takes the document in $lookup to perform equally match with foreignField from the documents which is field of collection where we are performing aggregation.
foreighFiled: Its take the field of “from” collection’s.
as: This is new array field contains the matching documents from the “from” collection.
Example: Suppose we have a collection of employee.

Collection EMP

{_id:5,Name:"Ashish",dep:"Syste Admin"}

Collection DEP

{_id:14,depName:"Syste Admin",Manager:"Mar5"}

We have two collections EMP and DEP and we want to know about employees and their managers run following query.

          from: "DEP",
          localField: "dep",
          foreignField: "depName",
          as: "EMp_Manager"


{ "_id" : 1, "Name" : "Manish", "dep" : "HR", "EMp_Manager" : [ { "_id" : 10, "depName" : "HR", "Manager" : "Mar1" } ] } 
{ "_id" : 2, "Name" : "Raj", "dep" : "Salesforce", "EMp_Manager" : [ { "_id" : 11, "depName" : "Salesforce", "Manager" : "Mar2" } ] }
{ "_id" : 3, "Name" : "Dilip", "dep" : "Database", "EMp_Manager" : [ { "_id" : 12, "depName" : "Database", "Manager" : "Mar3" } ] }
{ "_id" : 4, "Name" : "Vipul", "dep" : "UI", "EMp_Manager" : [ { "_id" : 13, "depName" : "UI", "Manager" : "Mar4" } ] }
{ "_id" : 5, "Name" : "Ashish", "dep" : "Syste Admin", "EMp_Manager" : [ { "_id" : 14, "depName" : "Syste Admin", "Manager" : "Mar5" } ] }
{ "_id" : 7, "Name" : "Pankaj", "EMp_Manager" : [] }

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