db={"students":[{"_id":"1234","name":"Theodo"}],"cards":[{"student_id":"1234","given_date":"11.10.2019"}]}

db.students.aggregate([{"$lookup":{"from":"cards","localField":"_id","foreignField":"student_id","as":"student_card"}},{"$unwind":"$student_card"},{"$set":{"card_given_date":"$student_card.given_date"}},{"$unset":"student_card"}])