Node.js - 如何计算mongodb中子文档的聚合（总和）？

My collection structure was,

我的收藏结构是,

[{
    "_id": "....",
    "name": "aaaa",
    "level_max_leaves": [{
        level: "ObjectIdString 1",
        max_leaves: 4,
    }]
}, {
    "_id": "....",
    "name": "bbbb",
    "level_max_leaves": [{
        level: "ObjectIdString 1",
        max_leaves: 4,
    }]
}, {
    "_id": "....",
    "name": "cccc",
    "level_max_leaves": [{
        level: "ObjectIdString 1",
        max_leaves: 7,
    }]
}, {
    "_id": "....",
    "name": "dddd",
    "level_max_leaves": [{
        level: "ObjectIdString 2",
        max_leaves: 3,
    }]
}]
[{