MongoDB:聚合函数中的字符串字段的parseFloat($ avg)

Question

我在Mongo中存储一些包含任意字段的数据.此字段存储为字符串,但可以包含二进制数据,整数,浮点数或其他任何内容(直到应用程序和另一个字段,"类型").无论如何,假设我确定我正在查询的文档在此字段中存储浮动值键入字符串,我想知道这些值在特定日期范围内的平均值.不幸的是,根据$ avg文档,它忽略了任何非数字值.有没有办法强制parseFloat或类似的东西在运行时转换文档值来安抚$ avg运算符？

样本文件结构:

{
    "_id": ObjectId("540f4e29f287300b9097663d"),
    "DateReceived": ISODate("2014-09-09T18:59:53.717Z"),
    "Message": {
        "Version": 1,
        "Source": {
            "Device": "MS02166",
            "Application": "Performance Counter",
            "Category": "Disk - Reads/sec",
            "OriginDate": ISODate("2014-09-09T18:59:51.256Z"),
            "Level": 2
        },
        "PayloadText": "0"
    }
}

我平均的字段是Message.PayloadText(在这个例子中它是0,但是有非零文档)

我能够生成的聚合管道查询是:

[{
    "$match": {
        "Message.Source.Application": "Performance Counter"
    }
}, {
    "$match": {
        "Message.Source.Category": "Processor"
    }
}, {
    "$match": {
        "Message.Source.Level": 2
    }
}, {
    "$match": {
        "Message.Source.Device": "MS02166"
    }
}, {
    "$match": {
        "DateReceived": {
            "$gte": ISODate("2014-09-14T07:00:00Z"),
            "$lt": ISODate("2014-09-15T07:00:00Z")
        }
    }
}, {
    "$group": {
        "_id": {
            "hour": {
                "$hour": "$DateReceived"
            }
        },
        "AvgUsage": {
            "$avg": "$Message.PayloadText"
        },
        "MinUsage": {
            "$min": "$Message.PayloadText"
        },
        "MaxUsage": {
            "$max": "$Message.PayloadText"
        }
    }
}]

我试图做这样的事情"$avg": "parseFloat($Message.PayloadText)",并"$avg": "$parseFloat($Message.PayloadText)"没有用.奇怪的是,$ min和$ max运算符产生了一个有意义的结果(下面的样本结果).

{ "_id" : { "hour" : 7 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.37442" }
{ "_id" : { "hour" : 8 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.37442" }
{ "_id" : { "hour" : 9 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.37442" }
{ "_id" : { "hour" : 10 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.858578" }
{ "_id" : { "hour" : 11 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.999424" }
{ "_id" : { "hour" : 12 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.999424" }
{ "_id" : { "hour" : 21 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.999424" }
{ "_id" : { "hour" : 16 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.37442" }
{ "_id" : { "hour" : 15 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.37442" }
{ "_id" : { "hour" : 23 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.37442" }
{ "_id" : { "hour" : 18 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "94.02982" }
{ "_id" : { "hour" : 0 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.37442" }
{ "_id" : { "hour" : 20 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.37442" }
{ "_id" : { "hour" : 22 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "98.48484" }
{ "_id" : { "hour" : 13 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.37442" }
{ "_id" : { "hour" : 17 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "98.76543" }
{ "_id" : { "hour" : 14 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "97.18308" }
{ "_id" : { "hour" : 19 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.37442" }
{ "_id" : { "hour" : 1 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.37442" }
{ "_id" : { "hour" : 4 }, "AvgUsage" : 0, "MinUsage" : "0", "MaxUsage" : "9.37442" }

Answer 1

如果你想要平均值,你需要存储一个数字.没有任何类型转换函数可以让您从数字的字符串表示映射到数字.$min并且$max因为在每个BSON类型(包括字符串)和不同的BSON类型之间定义了一个顺序,所以MongoDB总能找到任何字段的最大值.