MongoDB字符串转整数

Question

我想将MongoDB值字段的字符串转换为整数，然后计算平均值。

这是我的JSON：

"_id" : ObjectId("5c49f398fc0078178c76705b"),     // my json data
    "Time_Created" : ISODate("2019-01-24T17:19:20.205Z"),   // date
    "Test_ID" : "1",
    "data" : [
        {
            "Device_id" : "1",
            "Total_wires" : "5",
            "Cables" : [
                {
                    "TAG" : "4001",
                    "Value" : "24.3"
                },
                {
                    "TAG" : "4002",
                    "Value" : "21.3"
                },
                {
                    "TAG" : "4003",
                    "Value" : "21.3"
                },
                {
                    "TAG" : "4004",
                    "Value" : "21.3"
                },                                                            
                {
                    "TAG" : "4005",                                        
                    "Value" : "100.3"
                }
            ]
        }
    ]
}

我正在使用以下查询语句提取平均值

1. Query:

   db.collection_name.aggregate( [{  '$project': { 'values': '$data.Cables.Value', }}, { '$unwind': '$values' }, { '$addFields': { 'avgValue': { '$avg': { $toInt: '$values' } }  } } ] )
   

   But I get this error:

   2019-01-29T11:06:05.642-0800 E QUERY [js] Error: command failed:
   {
   "ok" : 0,
   "errmsg" : "Unsupported conversion from array to int in $convert
   with no onError value",
   "code" : 241,
   "codeName" : "ConversionFailure"
   } : aggregate failed :
   _getErrorWithCode@src/mongo/shell/utils.js:25:13
   doassert@src/mongo/shell/assert.js:18:14
   _assertCommandWorked@src/mongo/shell/assert.js:534:17
   assert.commandWorked@src/mongo/shell/assert.js:618:16
   DB.prototype._runAggregate@src/mongo/shell/db.js:260:9
   DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1062:12
   @(shell):1:1

2. I also tried the following:

   db.collection_name.aggregate( 
   [{  '$project': { 'values': '$data.Cables.Value', }}, 
     { '$unwind': '$values' }, 
     { '$addFields': { 'avgValue': { '$avg': { 'input': '$values', 'to': 
     'int' } }  } } 
   ] )
   

   But I'm also getting this output:

   { "_id" : ObjectId("5c509604fc007808c427edcb"), "values" :
   [ "24.3", "23.3", "25.3", "31.3", "90.3" ], "avgValue" : null }

预期值应该是数值字段的平均值。
我需要某种嵌套文档值的转换。

输入的字符串值必须转换为整数或小数，然后计算它们的平均值。

然后，这将用于C#代码生成管道。

var pipeline = new[] 
{ 
  project, unwind, addfields 
};

- Sam

Answer 1

使用toDouble而不是toInt，因为您的值不是整数。请尝试以下步骤。

[
    {
        '$project': {
            'values': '$data.Cables.Value'
        }
    }, {
        '$unwind': {
            'path': '$values'
        }
    }, {
        '$project': {
            'values': {
                '$map': {
                    'input': '$values', 
                    'as': 'value', 
                    'in': {
                        '$toDouble': '$$value'
                    }
                }
            }
        }
    }, {
        '$addFields': {
            'avgValue': {
                '$avg': '$values'
            }
        }
    }
]

- simagix