蒙哥如何从单个文档计算对象数组？

Question

我尝试过，但是我无法使查询计数，例如 假设我们有一个像这样的文件：

{
    "_id" : ObjectId("24926523r3423423423423rr"),
    "name" : "My super team",
    "stadium" : "Capital",
    "players" : [
            {
                    "name"    : "PlayerName1",
                    "number"  : "1",
                    "country" : "BRA"
            },
            {
                    "name"    : "PlayerName2",
                    "number"  : "2",
                    "country" : "FRA"
            },
            {
                    "name"    : "PlayerName3",
                    "number"  : "3",
                    "country" : "BRA"
            },
            {
                    "name"    : "PlayerName4",
                    "number"  : "4",
                    "country" : "ARG"
            },
            {
                    "name"    : "PlayerName5",
                    "number"  : "5",
                    "country" : "GER"
            },
            {
                    "name"    : "PlayerName6",
                    "number"  : "6",
                    "country" : "BRA"
            },
    ]
}

问题是如何计算“我的超级团队”团队中的所有“ BRA”球员（我有很多团队。任何想法？我尝试将$ size和聚合组合在一起，但失败了，或者更好如果您有技巧如何使用Morphia处理该查询？