{"name":"A","pro":[{"name":"A1","total":1},{"name":"A2","total":2},{"name":"A3","total":3}......]}

{"name":"C","pro":[{"name":"C1","total":1},{"name":"C2","total":2},......]}

{"name":"B","pro":[{"name":"B1","total":1},{"name":"B2","total":2},{"name":"B3","total":3}......]}

上面的数据库中，怎么直接求所有total的和？不用分组。

db.collection('CARS').aggregate().group({
_id: null,
total: $.sum('$pro.total')
}).end()

这样测试不成功！