db={"users":[{"_id":ObjectId("000000000000000001234566"),"userName":"Juancho","email":"juancho@gmail.com","password":"123","kids":[{"name":"juanchito","age":6}]}],"solicitudesDeNinera":[{"_id":ObjectId("000000000000000000656868"),"requestedBy":ObjectId("000000000000000001234566"),/**este campo relaciona a users mediante un id. En este caso el id del usuario Juancho,*/"date":{"$date":"2021-06-05"},/**"date": 05/06/2021,*/"descroption":"solicito alguien que cuide a mis hijos por dos horas"}]}

db.users.aggregate([{"$lookup":{"from":"solicitudesDeNinera","localField":"_id","foreignField":"requestedBy","as":"output"}}])