[{"model":"Honda Civic","license_plate":"ABC-1234","attributes":{"rented":"YES"}},{"model":"Toyota Camry","license_plate":"ABC-5678","attributes":{"rented":"YES"}},{"model":"Honda Civic","license_plate":"DEF-1001","attributes":{"rented":"no"}},{"model":"Toyota Camry","license_plate":"ABC-5679","attributes":{"rented":"YES"}},]

db.collection.aggregate([{$group:{_id:{model:"$model",rented:"$attributes.rented"},count:{$sum:1}}},{$project:{model:"$_id.model",stauts:{$cond:[{$eq:["$_id.rented","YES"]},"rented","available"]},count:1,_id:0}}])