MongoDB: parseFloat for string field in aggregate function ($ avg)

I am storing some data in Mongo which contains an arbitrary field. This field is stored as a string, but can contain binary data, integer, float, or whatever (before the application and another "Type" field). Anyway, assuming that I know for sure that the documents I am querying store floating values ​​in this field, entered as a string, I want to know the average of those values ​​over a specific date range. Unfortunately, according to the $ avg documentation , it ignores any values ​​that are not numeric. Is there a way to force parseFloat or something similar to convert the document value on the fly to appease the $ avg operator?

Example of document structure:

{
    "_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"
    }
}

      

        
        
        
      

    

Field I averaging on equals Message.PayloadText

(in this example it is 0, but there are non-zero documents)

The aggregation pipeline request I was able to create is the following:

[{
    "$match": {
        "Message.Source.Application": "Performance Counter"
    }
}, {
    "$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"
        }
    }
}]

      

        
        
        
      

    

I've tried doing something like "$avg": "parseFloat($Message.PayloadText)"

and to "$avg": "$parseFloat($Message.PayloadText)"

no avail. Oddly enough, the $ min and $ max DO operators produce meaningful results (examples of results below).

{ "_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" }