db={"transport":[{"_id":"GFT008","TRUCK":{"registration":"GFT008","capacity":40000,"weight":15000,"status":"AVAILABLE"}},{"_id":"PKR008","TRUCK":{"registration":"PKR008","capacity":22000,"weight":8800,"status":"AVAILABLE"}},{"_id":"QRT834","TRUCK":{"registration":"QRT834","capacity":5550,"weight":400,"status":"USED"}}],"employee":[{"_id":"6","EMPLOYEE":{"e#":"6","name":"Michael Jones","dob":"05-OCT-65","address":"23 Waterloo Ave. Surry Hills, NSW 2502","hiredate":"12-JAN-93","position":"mechanic","licence":"7773","status":"ON_LEAVE","experience":"STANDARD","maintenances":[{"registration":"QRT834","time":"40","maintenance date":"12-JUN-99"},{"registration":"QRT834","time":"40","maintenance date":"15-JUN-98"},{"registration":"SYF777","time":"30","maintenance date":"01-AUG-98"},{"registration":"SYF777","time":"30","maintenance date":"05-AUG-95"},{"registration":"SYF777","time":"30","maintenance date":"06-AUG-00"},{"registration":"LUCY01","time":"200","maintenance date":"12-MAR-97"},{"registration":"SYF777","time":"30","maintenance date":"02-AUG-00"},{"registration":"PKR768","time":"200","maintenance date":"12-AUG-00"},{"registration":"QRT834","time":"200","maintenance date":"30-JUN-00"},{"registration":"SYF777","time":"300","maintenance date":"02-AUG-02"},{"registration":"PKR768","time":"460","maintenance date":"12-AUG-02"},{"registration":"LUCY01","time":"40","maintenance date":"29-JUL-02"},{"registration":"QRT834","time":"40","maintenance date":"30-JUN-02"}]}}]}

db.employee.aggregate([{$unwind:"$EMPLOYEE.maintenances"},{$lookup:{from:"transport",localField:"EMPLOYEE.maintenances.registration",foreignField:"TRUCK.registration",as:"matches"}},{$match:{"matches":{$size:1}}},{$group:{_id:"$EMPLOYEE.maintenances.registration",}}])