db={"users":[{"user_id":"25","user_name":"john"},{"user_id":"26","user_name":"peter"}],"movies":[{"id":"1","title":"Pk","user_id":"25"},{"id":"2","title":"Master","user_id":"25"}]}

db.users.aggregate([{"$lookup":{"from":"movies","localField":"user_id","foreignField":"user_id","as":"movies"}},{"$set":{"movies":{"$map":{"input":"$movies","in":"$$this.id"}}}}])