Изначальный df
db.users.insertMany([
{name: "milk", var: "1", price: 100, count: 15},
{name: "water", var: "1", price: 55, count: 20},
{name: "bear", var: "2", price: 205, count: 10},
{name: "vodka", var: "2", price: 300, count: 5}
])
Попробовал таким запросом
db.users.aggregate([{
$group: {
_id: "$var",
total: { $multiply: ["$price", "$count"] }
}
}])
Но вылезла ошибка
http://prntscr.com/st1icm