Mongodb组和项目运营商 [英] Mongodb group and project operators

问题描述

我正在尝试按"group_name"分组，并使用聚合框架通过php驱动程序选择"group_name"和"group_id"字段:

I'm trying to group by "group_name" and select fields "group_name" and "group_id" with php driver using aggregation framework:

Array
(
    [$project] => Array
        (
            [group_name] => 1
            [group_id] => 1
        )

    [$group] => Array
        (
            [_id] => $group_name
            [total_sum] => Array
                (
                    [$sum] => 1
                )

        )

)

我得到这个:[errmsg] => exception: A pipeline stage specification object must contain exactly one field. 但是，当我仅使用$project或$group运算符时，它就可以正常工作.

I'm getting this: [errmsg] => exception: A pipeline stage specification object must contain exactly one field. However, when I'm using only $project or $group operator it works just fine.

推荐答案

问题是您将$ project和$ group分组在一起.

The issue is your grouping your $project and $group together.

您的数组应该更像

{
  $project: {
    group_name: 1,
    group_id: 1
  },
},
{   
  $group:{
    _id:{
      group_name:'$group_name',
    },
    total_sum:{$sum:1}
  }
}    

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