[{_id:"1",car_id:"A",brand:"honda",accessories:[{parts:6,name:"speaker"},{parts:2,name:"headlight"}]},{_id:"2",car_id:"A",brand:"honda",accessories:[{parts:6,name:"speaker"},{parts:2,name:"headlight"}]},{_id:"3",car_id:"B",brand:"honda",accessories:[{parts:6,name:"speaker"},{parts:2,name:"headlight"}]},{_id:"4",car_id:"C",brand:"honda",accessories:[{parts:6,name:"speaker"},{parts:2,name:"headlight"}]}]

db.collection.aggregate([{$group:{_id:"$car_id",cars:{$push:{k:"$_id",v:{accessories:"$accessories"}}}}},{$group:{_id:null,data:{$push:{k:"$_id",v:{$arrayToObject:"$cars"}}}}},{$replaceRoot:{newRoot:{$arrayToObject:"$data"}}}])