Mongodb:在组之后的聚合中使用记录计数

问题描述

我试图存储输入到管道中的记录总数，以便在以后的计算中使用该数目.如何获取输入数量，然后展开，然后在以后的计算中使用输入数量?

I'm trying to store the total number of records input to the pipe so I can use the number in a later calculation. How do I grab the number of inputs, then unwind, then use the number of inputs later in my calcs?

我可以通过以下方式获取电话号码:

I can get the number by doing this:

db.articles.aggregate([
  {
    $count: "totalArticles"
  }
]}

我可以通过执行以下操作获得所需的其余数据:

I can get the rest of the data I want by doing this:

db.articles.aggregate([
  {
    $unwind: "$concepts"
  },
  {
    $group: {
      _id:    "$concepts.text",
      count: {
        $sum: 1
      },
      average: {
        $avg: "$concepts.relevance"
      },
    }
  }
])

我真正想做的是:

db.articles.aggregate([
  {
    $count: "totalArticles"
  },
  {
    $unwind: "$concepts"
  },
  {
    $group: {
      _id:    "$concepts.text",
      count: {
        $sum: 1
      },
      average: {
        $avg: "$concepts.relevance"
      }
    }
  },
  {
    $project: {
      count: "$count",
      percent: {
        $divide: [ "$count", "$totalArticles" ]
      }
    }
  },
  {
    $sort: {
      count: -1
    }
  }
])

推荐答案

您可以使用以下聚合查询.

You can use below aggregation query.

初始$group计算总计数，而$push将概念字段转换为数组字段. $$ROOT访问整个文档.

Initial $group to calculate the total count while $push the concepts field into array field. $$ROOT to access the whole doc.

在下一个$group中保留文章总数.

Retain the total articles count in next $group.

一切都保持原样.

db.articles.aggregate([
  {"$group":{
    "_id":null,
    "totalArticles":{"$sum":1},
    "concepts":{"$push":"$$ROOT.concepts"}
  }},
  {"$unwind":"$concepts"},
  {"$group":{
    "_id":"$concepts.text",
    "totalArticles":{"$first":"$totalArticles"},
    "count":{"$sum":1},
    "average":{"$avg":"$concepts.relevance"}
  }},
  {"$project":{
      "count": "$count",
      "percent": {
        "$divide": [ "$count", "$totalArticles" ]
      }
    }
  },
  {"$sort": {"count": -1}}
])

$facets也是一种选择，您可以在两个单独的管道中进行两个查询，然后合并以继续其余阶段.

$facets is also an option where you can two queries in two separate pipeline followed by merge to continue with rest of stages.

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