Database

db={"users":[{_id:"5f9c50dcfac1f091400225e3",email:"Peter.Parker@gmail.com",details:{fname:"Peter Test",lname:"Fulton"},},{_id:"5fa432bfb91fab7db60c70eb",email:"Spidy@xxx.com",details:{fname:"Frodo",lname:"Baggins"},},{_id:"5fa8ec7d3ce22610e5d15190",email:"tommy@xxx.com",details:{fname:"Tommy",lname:"test"},},{_id:"5fc38bb0b3683651be970180",email:"jerry@xxx.io",},{_id:"5fd2340cc443d155ab38383b",email:"Dexter@xxx.io",details:{fname:"Dexter",lname:"Lab"},}],"discussions":[{_id:"5fb2abd6b14fa5683979df58",tags:["javascritp","css","html"],title:"Why is this inline-block element pushed downward?",post:"Test Post",learnerId:"5f9c50dcfac1f091400225e3",}]}

Query

db.users.aggregate([{$project:{"details.fname":1,"details.lname":1,email:1,_id:1}},{$lookup:{from:"discussions",localField:"_id",foreignField:"learnerId",as:"discussions"}}])

Result