如何在MONGODB中找到Document元素的Count one

我想得到县的确切数量。在我的例子中，属性是3，我尝试了$size聚合，但没有得到想要的答案

{ 
"_id" : ObjectId("234567890"), 
"Version" : "1.1", 
"FileNo" : "1234567", 
"Feature" : [
    "Map", 
    "usr/ammer"
], 
"TVProperties" : [
    "foul"
], 
"Name" : "nice", 
"Countries" : {
    "Properties" : [
        {
            "kyu" : "NAmerica", 
            "triggerString" : "America", 
            "user" : "ME"
        }, 
        {
            "kyu" : "ASIA", 
            "triggerString" : "China", 
            "user" : "You"
        }, 
        {
            "kyu" : "Europe", 
            "triggerString" : "France", 
            "user" : "HE"
        }
    ]
},

---

{“ok”：0.0，“errmsg”：“$size的参数必须是数组，但类型为：missing”，“code”：numberprint（17124），“codeName”：“Location17124”}尝试了与上面相同的错误。此处的国家是“object”type@PrashantSharma属性是一个数组，我已经计算了它的大小

db.collection.aggregate(

    // Pipeline
    [
        // Stage 1
        {
            $project: {
                count: {
                    $size: '$Countries.Properties'
                },
                "Version": 1,
                "FileNo": 1,
                "Feature": 1,
                "TVProperties": 1,
                "Name": 1,
                "Countries": 1

            }
        },

    ]


);