db={"driver":[{"_id":"5f9c1d897ea5e246945cd73a","agent_name":"Ratnabh Kumar Rai","agent_email":"ra2614@gmail.com","agent_phone":"70****63331"}],"reviews":[{_id:"5f9d54cb3a3ee10c6829c0a4",order_id:"5f9d40f096e4a506e8684aba",user_id:"5f9bcb66f7a5bf4be0ad9973",driver_id:"5f9c1d897ea5e246945cd73a",rating:3,text:""},{_id:"5f9d54cb3a3ee10c6829c0a5",order_id:"5f9d40f096e4a506e8684aba",user_id:"5f9bcb66f7a5bf4be0ad9973",driver_id:"5f9c1d897ea5e246945cd73a",rating:10,text:""}]}

db.driver.aggregate([{$project:{_id:{$toString:"$_id",},agent_email:"$agent_email"},},{$lookup:{from:"reviews",localField:"_id",foreignField:"driver_id",as:"driver_info",},},/** Makes the driver info flat with other information*/{"$unwind":"$driver_info"},{/** Now you can group them*/$group:{_id:"$_id",/** Calculates avg on rating field*/avg:{"$avg":"$driver_info.rating"},agent_email:{$first:"$agent_email"}}},{$project:{agent_email:1,orderReview:"$driver_info",avg:1},},])