在单个查询中同时统计外部和内部嵌入式数组 [英] Count Both Outer and Inner embedded array in a single query

问题描述

{
_id: ObjectId("5dbdacc28cffef0b94580dbd"),
 "comments" : [
     {
      "_id" : ObjectId("5dbdacc78cffef0b94580dbf"),
      "replies" : [
                   {
                     "_id" : ObjectId("5dbdacd78cffef0b94580dc0")          
                   },
                 ]
     },
  ]
}

如何计算comments中元素的数量并求和与relies

How to count the number of element in comments and sum with number of relies

我的方法是像这样进行2次查询:

My approach is do 2 query like this:

db.posts.aggregate([
{$match: {_id:ObjectId("5dbdacc28cffef0b94580dbd")}},

{ $unwind: "$comments",},

{$project:{total:{$size:"$comments.replies"} , _id: 0} }

])

2.计算评论的总数

db.posts.aggregate([
{$match: {_id:ObjectId("5dbdacc28cffef0b94580dbd")}},
{$project:{total:{$size:"$comments.replies"} , _id: 0} }

])

然后对两者进行总结，我们有没有更好的解决方案来编写查询，例如返回总元素comments + replies

Then sum up both, do we have any better solution to write the query like return the sum of of total element comments + replies

推荐答案

您可以使用 $concatArrays 合并" 内部的数组数组"到单个列表中，并测量 $add :

You can use $reduce and $concatArrays to "merge" an inner "array of arrays" into a single list and measure the $size of that. Then simply $add the two results together:

db.posts.aggregate([
  { "$match": { _id:ObjectId("5dbdacc28cffef0b94580dbd") } },
  { "$addFields": {
    "totalBoth": {
      "$add": [
        { "$size": "$comments" },
        { "$size": {
          "$reduce": {
            "input": "$comments.replies",
            "initialValue": [],
            "in": {
              "$concatArrays": [ "$$value", "$$this" ] 
            }
          }
        }}
      ]
    }
  }}
])

请注意，数组数组"是像$comments.replies这样的表达式的作用，因此需要将它们制作成一个可以测量所有元素的数组的操作.

Noting that an "array of arrays" is the effect of an expression like $comments.replies, so hence the operation to make these into a single array where you can measure all elements.

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