如何使用MongoDB根据数组元素对记录进行分组 [英] How to group records based on array elements using MongoDB

问题描述

我有一个数据收集，其中包含一组以下格式的记录.

I have a data collection which contains a set of records in the following format.

{
    "_id" : 22,
    "title" : "3D User Interfaces with Java 3D",
    "isbn" : "1884777902",
    "pageCount" : 520,
    "publishedDate" : ISODate("2000-08-01T07:00:00Z"),
    "thumbnailUrl" : "https://s3.amazonaws.com/AKIAJC5RLADLUMVRPFDQ.book-thumb-images/barrilleaux.jpg",
    "longDescription" : "Description",
    "status" : "PUBLISH",
    "authors" : [
        "Jon Barrilleaux"
    ],
    "categories" : [
        "Java",
        "Computer Graphics"
    ]
},
{
    "_id" : 23,
    "title" : "Specification by Example",
    "isbn" : "1617290084",
    "pageCount" : 0,
    "publishedDate" : ISODate("2011-06-03T07:00:00Z"),
    "thumbnailUrl" : "https://s3.amazonaws.com/AKIAJC5RLADLUMVRPFDQ.book-thumb-images/adzic.jpg",
    "status" : "PUBLISH",
    "authors" : [
        "Gojko Adzic"
    ],
    "categories" : [
        "Software Engineering"
    ]
}

请注意，类别"是一个数组.

Please note that the 'categories' is an array.

我想统计每个类别的已出版书籍.我尝试了以下解决方案，但将整个阵列视为一组.

I want to count the published books for each category. I tried the following solution, but it treated the entire array as one group.

db.books.aggregate([
    {
        $group:{_id:"$categories", total:{$sum:1}}
    }
])

相反，我想计算类别"数组中每个单独类别值的记录数.

Instead of so, I want to count the number of records for each individual category value inside 'categories' array.

推荐答案

您应首先使用

You should first use $unwind which outputs one document for each element in the array.

db.books.aggregate([
  { 
    $unwind : "$categories"
  },
  {
    $group : { _id : "$categories", total: { $sum: 1 } }
  }   
])

这篇关于如何使用MongoDB根据数组元素对记录进行分组的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！