mongodb $toInt如何实现数据类型的转化并完成排序

student:{
    name:"jacky",
    idCardNumber:"620102199503025028"
}

let aggregate=[
{
  payload:{$substr: ["$idCardNumber", 6, 8]},
  //$idCardNumber 含义是查询的字段，6代表的是从第几位开始截取 8，代表要截取几位
  brithYear:{
    $toInt:{
    //$toInt 是mongodb的类型转化工具
  		$substr:[{$substr: ["$idCardNumber", 6, 8]},0,4]
    }
  },
  brithMonth:{
    $toInt:{
  	$substr:[{$substr: ["$idCardNumber", 6, 8]},4,2]
		}
  },
  brithDay:{
  	$toInt:{
    $substr:[{$substr: ["$idCardNumber", 6, 8]},6,2]
   }
  },
}]
this.xxxModel.aggregate(aggregate)

/**
 * specifications: The fields to
 *   include or exclude.
 */
{
  payload:{ $function:
               {
                  body: function(idCardNumber) {
                     return idCardNumber.substr(6,8)
                  },
                  args: [ "$idCardNumber" ],
                  lang: "js"
               }
            },
  brithYear:{
    $toInt:  { $function:
               {
                  body: function(idCardNumber) {
                     return idCardNumber.substr(6,4)
                  },
                  args: [ "$idCardNumber" ],
                  lang: "js"
               }
      }
  },
  brithMonth:{
     $toInt:  { $function:
               {
                  body: function(idCardNumber) {
                     return idCardNumber.substr(10,2)
                  },
                  args: [ "$idCardNumber" ],
                  lang: "js"
               }
      }
  },
  brithDay:{
  	 $toInt:  { $function:
               {
                  body: function(idCardNumber) {
                     return idCardNumber.substr(12,2)
                  },
                  args: [ "$idCardNumber" ],
                  lang: "js"
               }
      }
  },
  
}
//执行查询后的结果
{
payload:"19010321"
brithYear:1901
brithMonth:3
brithDay:21
}