db={employees:[{"_id":"621eedae92979fd8f0e9451d","name":"Pallab Koley","shifts":{"_id":"62636b9fcbda6d2b17f5cae0","month":"2022-05","shift":[{"date":"2022-05-01","shiftId":"622bb0f4b88dc92e3c2cac56"}]}},{"_id":"62626a7446ba9a911a623b37","name":"Pinki Das","shifts":{"_id":"62636ba4cbda6d2b17f5cae1","month":"2022-05","shift":[{"date":"2022-05-01","shiftId":"622bb0f4b88dc92e3c2cac56"}]}}],shifts:[{_id:"622bb0f4b88dc92e3c2cac56",name:"Day"}]}

db.employees.aggregate([{"$lookup":{"from":"shifts","localField":"shifts.shift.shiftId","foreignField":"_id","as":"shifts.shift.shiftId"}}])