[{_id:"a",username:"username",email:"email",password:"password",price:9.99,preference:"VIP"},{_id:"b",username:"username",email:"email",password:"password",price:10.99,preference:"VIP"},{_id:"c",username:"username",email:"email",password:"password",price:7.99,preference:"VIP+"}]

db.collection.aggregate([{$group:{_id:"$preference","value":{$sum:"$price"},}},{$group:{_id:null,"results":{$push:"$$ROOT"}}},{$project:{_id:0,"results":"$results","ok":{$toInt:"1"}}}])