db={"department":[{"_id":1,"name":"dep1"},{"_id":2,"name":"dep2"},{"_id":3,"name":"dep3"}],"student":[{"department":1,"name":"student1"},{"department":1,"name":"student2"},{"department":2,"name":"student3"}]}

db.department.aggregate([{"$lookup":{"from":"student","localField":"_id","foreignField":"department","as":"departments"}}])