[{"user_id":"ebebc012-082c-4e7f-889c-755d2679bdab","car_1a58db0b-5449-4d2b-a773-ee055a1ab24d":1,"car_37c04124-cb12-436c-902b-6120f4c51782":0,"car_b78ddcd0-1136-4f45-8599-3ce8d937911f":1},{"user_id":"f3eb2a61-5416-46ba-bab4-459fbdcc7e29","car_1a58db0b-5449-4d2b-a773-ee055a1ab24d":1,"car_0d15eae9-9585-4f49-a416-46ff56cd3685":1}]

db.collection.aggregate([{$group:{_id:"$_id",root:{$first:{$objectToArray:"$$ROOT"}}}},{$unwind:"$root"},{$match:{"root.v":1}},{$group:{_id:"$root.k",count:{$sum:1}}}])