node.js - 该字段必须是mongo中的累加器对象

Question

我正在执行聚合查询。我的要求是我有以下格式的文档:

{
"name":"a","age":20},
{
"name":"b","age":23},
{
"name":"a","age":26}

我需要对姓名字段中包含“a”的所有姓名的年龄求和。 我正在执行以下操作，但它给了我一个错误:“字段‘名称’必须是累加器对象”:

db.collection('students').aggregate({
        $group: {"name": "a", "sum": {$sum: '$age'}}
    }, {
        $project: {sum: '$sum'}
    }, (e, d) => {
        if (!e) {
            var e = d.stream({transform: JSON.stringify});
            console.log("answer")
            console.log(e);
            deferred.resolve(e);
        } else {
            console.log(e)
        }
    })

Answer 1

我更改了您的聚合查询。请尝试如下:

db.collection('students').aggregate(
     [
       {
          $match: {
                 name: "a"
          }
       },
       {
         $group: {_id: "$name",
                  "sum": {
                     $sum: '$age'
                   }
                 }
        },
        {
          $project: {
                 sum: 1
        }
     ], (e, d) => {
        if (!e) {
            var e = d.stream({transform: JSON.stringify});
            console.log("answer")
            console.log(e);
            deferred.resolve(e);
        } else {
            console.log(e)
        }
    })