Mongodb 在嵌套文档上使用mongo映射Reduce

我的文件结构如下：

{
  "country_id" : 328,
  "country_name" : "Australien",
  "cities" : [{
      "city_id" : 19398,
      "city_name" : "Bondi Beach (Sydney)"
    }, {
      "city_id" : 31102,
      "city_name" : "Double Bay (Sydney)"
    }, {
      "city_id" : 31101,
      "city_name" : "Rushcutters Bay (Sydney)"
    }, {
      "city_id" : 817,
      "city_name" : "Sydney"
    }, {
      "city_id" : 31022,
      "city_name" : "Wolly Creek (Sydney)"
    }, {
      "city_id" : 18851,
      "city_name" : "Woollahra"
    }],
  "regions" : {
    "region_id" : 796,
    "region_name" : "Australien: New South Wales (Sydney)"
  }
}

对于分面导航，我想计算属性country\u id、cities.city\u id、regions\u region\u id，我想我可以使用map/reduce实现这一点

对于给定的结构，这可能吗

---

也许有人能给我指出正确的地图/缩略方向。

Mongo地图缩略示例可以在这里找到：

每个唯一国家/地区id、城市id和地区id元组的文档数量非常简单：

> function m() { 
    for(var i in this.cities) {     
         emit({country_id:this.country_id, 
               city_id:this.cities[i].city_id,
               region_id:this.regions.region_id}, 
              1); 
    } }



> function r(id,docs) {
      return Array.sum(docs);
}
> db.loc.mapReduce(m,r,{out:"map_reduce_out"})
{
    "result" : "map_reduce_out",
    "timeMillis" : 5,
    "counts" : {
        "input" : 1,
        "emit" : 6,
        "reduce" : 0,
        "output" : 6
    },
    "ok" : 1,
}
> db.map_reduce_out.find()
{ "_id" : { "country_id" : 328, "city_id" : 817, "region_id" : 796 }, "value" : 1 }
{ "_id" : { "country_id" : 328, "city_id" : 18851, "region_id" : 796 }, "value" : 1 }
{ "_id" : { "country_id" : 328, "city_id" : 19398, "region_id" : 796 }, "value" : 1 }
{ "_id" : { "country_id" : 328, "city_id" : 31022, "region_id" : 796 }, "value" : 1 }
{ "_id" : { "country_id" : 328, "city_id" : 31101, "region_id" : 796 }, "value" : 1 }
{ "_id" : { "country_id" : 328, "city_id" : 31102, "region_id" : 796 }, "value" : 1 }

---

似乎区域应该是一个数组

 "regions" : [{
    "region_id" : 796,
    "region_name" : "Australien: New South Wales (Sydney)"
  }]

“我要计算国家/地区id，…”

似乎您需要此输出

...
{_id:  328, cities: 6, regions: 1},
{_id:  329, cities: 10, regions: 4},
...

尝试以下方法，注意它只能对cities数组求和

db.Country.aggregate(
  { $unwind : "$regions" },  {'$group': {'_id': '$country_id' , 'cities' : { $sum : 1}   } }
)

以下内容将提供与公认答案类似的输出

db.Country.aggregate(
{'$group': {'_id': '$country_id' , 'cities' : { $push: "$cities.city_id" }, 'regions' : {    $push: "$regions.region_id" }   }  }
)

---

为什么要使用map/reduce？使用聚合框架会更直接（更快），我认为这在聚合框架中是不可能的。您是否有上述数据结构的示例？