Nodejs MongoDB join

lookup() method is used between 2 or more collections for join in MongoDB. lookup() method used in the aggregate() method.

Suppose you have 2 collections departments and employees

departments collections

[
  { _id: 1, name: "Engineer", status: 1 },
  { _id: 2, name: "Manager", status: 1 },
  { _id: 3, name: "Peon", status: 1 }
]

employees collections

[
  { _id: 1, name: "John", department_id: 1 }
]

Now, get the employee’s details with the department name.

create the collections_join.js file

var MongoClient = require('mongodb').MongoClient;

let url="mongodb://localhost:27017/";
var options = {
  useNewUrlParser: true,
  useUnifiedTopology: true,  
}
MongoClient.connect(url,options, function (err, connection_obj) {
  try{
     if (err) throw err;
     var dbo = connection_obj.db("users_management");
     dbo.collection("employees").aggregate([
      { $lookup:
         {
           from: 'departments',
           localField: 'department_id',
           foreignField: '_id',
           as: 'departments_details'
         }
       }
      ]).toArray(function(err, res) {
      if (err) throw err;
       console.log("documents show successfully");
       console.log(JSON.stringify(res));
       connection_obj.close();
    });
  }catch(err){

    console.log("connection fails");
  
  }

Now, run the collections_join.js file.

node collections_join.js