db={"user":[{"_id":1,"name":"John"}],"payment":[{"_payment_id":12345,"_user_id":1,"payment_date":"2020-05-15T00:00:00Z","desc":"30-day Sub","payment_method":"Paypal"}]}

db.user.aggregate([{"$lookup":{"from":"payment","localField":"_id","foreignField":"_user_id","as":"payment_docs"}}])