db={"car_parts":[{"_id":1,"name":"shock absorber"},{"_id":2,"name":"hood"},{"_id":3,"name":"steering wheel","sub_parts":[1,/** other car_parts._id s*/2]},{"_id":4,"name":"tire"},],"customers":[{"_id":"customer_1","parts":[{"scanned_parts":[1/** this is car_parts._id 1*/],date:"15.01.2020"},{"scanned_parts":[4],date:"21.05.2021"}]},{"_id":"customer_2","parts":[{"scanned_parts":[4,1],date:"04.02.2021"}]},{"_id":"customer_3","parts":[{"scanned_parts":[3],date:"15.07.2020"}]}]}

db.car_parts.aggregate([{"$lookup":{"from":"customers","localField":"_id","foreignField":"parts.scanned_parts","as":"customers"}},{"$project":{"customers":{$map:{input:"$customers",as:"each",in:"$$each._id"}}}}])