在单个查询中计算外部和内部嵌入数组 [英] Count Both Outer and Inner embedded array in a single query

问题描述

<代码>{_id: ObjectId("5dbdacc28cffef0b94580dbd"),评论" : [{"_id" : ObjectId("5dbdacc78cffef0b94580dbf"),回复":[{"_id" : ObjectId("5dbdacd78cffef0b94580dc0")},]},]}

如何统计comments中的元素个数并与relies

的个数相加

我的方法是做 2 个这样的查询:

1.回复

的总元素

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

2.统计评论的总元素

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

然后总结一下，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 + reply

解决方案

您可以使用 $reduce 和 $concatArrays 到合并"一个内部的数组数组"到一个列表中并测量$size 其中.然后只需 $add 两个结果一起:

db.posts.aggregate([{ "$match": { _id:ObjectId("5dbdacc28cffef0b94580dbd") } },{$addFields":{totalBoth":{"$add": [{ "$size": "$comments" },{$size":{$减少":{"input": "$comments.replys",初始值": []，在": {"$concatArrays": [ "$$value", "$$this"]}}}}]}}}])

请注意，数组数组"是像 $comments.replies 这样的表达式的结果，因此操作将它们组合成一个可以测量所有元素的数组.

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

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

My approach is do 2 query like this:

1. total elements of replies

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

{ $unwind: "$comments",},

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

])

2. count total elements of comments

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

])

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

解决方案

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" ] 
            }
          }
        }}
      ]
    }
  }}
])

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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