在mongoDB v3.6.5中使用聚合时如何将字符串转换为双精度

Question

如果要通过其他方式解决此问题，请让我知道

我尝试了以下汇总方法：

 db.CLV_MATERIAL_AGING.aggregate([ { $match: {"Material_Number" : 
  "000000000810000010"}}, 
{ 
   $group: {
      _id: 'null', 
      total:{
          $sum:{$convert: {'input': '$MR', 'to': 'double'}}     
     },
      count:{
          $sum:1
     }
 }
}])

给我错误：-

  {
    "message" : "Unrecognized expression '$convert'",
     "stack" : "MongoError: Unrecognized expression '$convert'" +
          "at queryCallback 
(/tmp/.mount_nosqlbn5xIBs/app/resources/app.asar/node_modules/mongodb- 
core/lib/cursor.js:247:25)" +
          "at 
/tmp/.mount_nosqlbn5xIBs/app/resources/app.asar/node_modules/mongodb- 
core/lib/connection/pool.js:531:18" +
          "at _combinedTickCallback (internal/process/next_tick.js:131:7)" +
          "at process._tickCallback (internal/process/next_tick.js:180:9)",
 "name" : "MongoError",
 "ok" : 0,
 "errmsg" : "Unrecognized expression '$convert'",
 "code" : 168,
 "codeName" : "InvalidPipelineOperator"
 }

并且$toDouble()在mongoDB v3.6.5中不可用我已经在mongodb v4中进行了尝试，但是在mongodb v3.5.6中没有使用