Convert String to Double in Mongo

I tried to convert a String to Double with the $convert operator

       ...
       {
        $addFields: {
            uslDouble: {
                $convert: {
                    input: "$currentDimension.usl",
                    to: "double",
                    one rror: "Impossible to convert str to double",
                }
            }
        },
        ...

The output error is explicit:

E QUERY    [thread1] Error: command failed: {
    "ok" : 0,
    "errmsg" : "Unrecognized expression '$convert'",
    "code" : 168,
    "codeName" : "InvalidPipelineOperator"
} : aggregate failed :
_getErrorWithCode@src/mongo/shell/utils.js:25:13

My constraint is that I can't upgrade to mongodb > 3.4.14.

Is there any workaround to convert String to Double ?

CodePudding user response：

That is possible, but I'm not sure about the performance.

The basic idea:

Split the string into whole and fractional parts
Convert each part to a number:
o iterate over the characters in each part
o multiply the running total by 10
o add the value of the current digit
Divide the fractional part by 10^(number of digits)
Add the whole and fractional parts to get the final double
Remove temporary fields

db.collection.aggregate([
  {$addFields: {
      split: {$split: ["$value", "."]}
  }},
  {$addFields: {
      whole: {"$arrayElemAt": ["$split", 0]},
      fraction: {"$arrayElemAt": ["$split", 1]},
  }},
  {$addFields: {
      whole: {$reduce: {
          input: {$range: [0,{$strLenCP: "$whole"}]},
          initialValue: 0,
          in: {
            $add: [
              {$multiply: ["$$value",10]},
              {"$indexOfArray": [
                    ["0","1","2","3","4","5","6","7","8","9"],
                    {$substr: ["$whole","$$this",1]}
              ]}
            ]
          }
      }},
      fractionVal: {
        $reduce: {
          input: {$range: [0,{$strLenCP: "$fraction"}]},
          initialValue: 0,
          in: {
            $add: [
              {$multiply: ["$$value", 10]},
              {"$indexOfArray": [
                  ["0","1","2","3","4","5","6","7","8","9"],
                  {$substr: ["$fraction","$$this",1]}
              ]}
            ]
          }
        }
      }
  }},
  {$addFields: {
      double: {$add: [
          "$whole",
          {$divide: ["$fractionVal",{"$pow": [10,{$strLenCP: "$fraction"}]}]}
      ]}
  }},
  {$project: {
      fraction: 0,
      fractionVal: 0,
      whole: 0,
      split: 0
  }}
])

Playground