MongoDB 2.6：聚合中的过滤字段（替代$ function）

Question

在previous post中，我问过如何按MongoDB中的数组元素对文档进行排序。

现在我必须在结果中过滤元素。特别是我想要排除不必要的字段。我找到了$filter函数，但仅适用于3.2以上的MongoDB版本

这里的例子是

样品采集

[
   {
      "_id":ObjectId("5695128ee4b04403440b61c2"),
      "_class":"com.acme.entity.Company",
      "name":"Company Name A",
      "countries":[
         {
            "country_code":"a.b.c",
            "country_name":"Italy",
            "country_weight":4
         },
         {
            "country_code":"b.c.d",
            "country_name":"Germany",
            "country_weight":4
         },
         {
            "country_code":"c.d.e",
            "country_name":"France",
            "country_weight":3
         }
      ]
   },
   {
      "_id":ObjectId("292829287293bcbe440b22bb"),
      "_class":"com.acme.entity.Company",
      "name":"Company Name B",
      "countries":[
         {
            "country_code":"a.b.c",
            "country_name":"Italy",
            "country_weight":6
         }
      ]
   },
   {
      "_id":ObjectId("99922222bbeecc3339900000"),
      "_class":"com.acme.entity.Company",
      "name":"Company Name C",
      "countries":[
         {
            "country_code":"a.b.c",
            "country_name":"Italy",
            "country_weight":9
         },
         {
            "country_code":"c.d.e",
            "country_name":"France",
            "country_weight":3
         },
         {
            "country_code":"l.h.i",
            "country_name":"Holland",
            "country_weight":8
         },
         {
            "country_code":"e.n.g",
            "country_name":"England",
            "country_weight":1
         }
      ]
   }
]

根据country_weight字段过滤和排序数据的查询

db.companies.aggregate([
    { $match: { "countries.country_code": "a.b.c"}},
    { $project: {
        name : 1,
        countries: "$countries",
        aggregatefunction: { 
            $setDifference: [
                { $map: {
                    "input": "$countries",
                    "as": "p",
                    "in": { $cond: [
                            { "$eq": [ "$$p.country_code", "$01.01.01" ] },
                        "$$p",
                        false
                    ]}
                }},
                [false]   
            ]
        }
    }},
    { $sort: { "countries.country_weight": -1 } }
])

通过这个解决方案，我进入了所有公司所在国家/地区。我想过滤国家/地区字段以仅获取已过滤的国家/地区。