db={"questions":[{"_id":1,"name":"Question 1"},{"_id":2,"name":"Question 2"},{"_id":3,"name":"Question 3"},{"_id":4,"name":"Question 4"},{"_id":5,"name":"Question 5"}],"tests":[{"_id":1,"name":"Test 1","questions":[1]},{"_id":2,"name":"Test 2","questions":[1,2,3]},{"_id":3,"name":"Test 3","questions":[5]}]}

db.questions.aggregate([{"$lookup":{"from":"tests","localField":"_id","foreignField":"questions","as":"usedIn"}},{"$project":{"usedIn":{"$size":"$usedIn"},"name":1}}])