如何将MongoDB聚合查询的data数组转为根层级结果？

现有聚合查询 #

db.demo.aggregate([
{
    '$group':
        {   '_id': null, 
            "Bytes Sent":{"$sum": "$sentBytes"},
            "Bytes Received":{"$sum":"$recivedBytes"}
        }
},
{
    "$project":{
        "data":[
                {"Count":"$Bytes Sent","Name":"Bytes Sent"},
                {"Count":"$Bytes Received","Name":"Bytes Received"}
               ] 
           }
       }
  ])

当前输出结果 #

{
"_id" : null,
"data" : [
    {
        "Count" : NumberLong("22893994106"),
        "Name" : "Bytes Sent"
    },
    {
        "Count" : NumberLong("16574618640"),
        "Name" : "Bytes Received"
    }
]}

需求 #

将data数组中的内容直接作为根层级结果输出，格式如下：

{
    "Count" : NumberLong("22893994106"),
    "Name" : "Bytes Sent"
},
{
    "Count" : NumberLong("16574618640"),
    "Name" : "Bytes Received"
}

解决方案 #

在原聚合管道末尾添加两个阶段：$unwind和$replaceRoot（或$replaceWith），修改后的完整聚合语句如下：

db.demo.aggregate([
  {
    '$group': {
      '_id': null, 
      "Bytes Sent": {"$sum": "$sentBytes"},
      "Bytes Received": {"$sum": "$recivedBytes"}
    }
  },
  {
    "$project": {
      "data": [
        {"Count": "$Bytes Sent", "Name": "Bytes Sent"},
        {"Count": "$Bytes Received", "Name": "Bytes Received"}
      ]
    }
  },
  // 展开data数组，将数组每个元素转为独立文档
  { "$unwind": "$data" },
  // 将data字段的内容替换为根文档，去掉外层包装
  { "$replaceRoot": { "newRoot": "$data" } }
])

步骤说明

1. $unwind阶段：把包含data数组的单个文档拆分成多个文档，每个文档对应数组中的一个元素，此时每个文档结构为{ "_id": null, "data": {...} }
2. $replaceRoot阶段：将每个文档的data字段内容提升为根层级，最终输出就是数组中的每个独立对象

注：MongoDB 4.2及以上版本可以用$replaceWith替代$replaceRoot，写法更简洁：{ "$replaceWith": "$data" }，效果完全一致。

内容的提问来源于stack exchange，提问作者Avinash Utekar