MongoDB：嵌套聚合答案

【问题标题】：MongoDB: Nested aggregationMongoDB：嵌套聚合
【发布时间】：2021-03-13 12:38:26
【问题描述】：

我有多个收藏：

用户 - 集合架构

历史是用户集合/文档的子文档

name: "jacob"
_id: 5fc53209e70f776378cce0c5,
history: [
  {
     _id: 5fc634bee65f96338a63b9e4,
     article: 5fc5f3e6140646c2024f7963,
     created_at: 2010-12-01T12:19:10.121+00:00
  },
  {
    _id: 5fc634d8e65f96338a63b9e5,
    article: 5fc5faaa8b1fffc1f4dec900,
    created_at: 2010-12-01T12:19:36.102+00:00
  }
]

文章 - 集合架构

{
   _id: 5fc5faaa8b1fffc1f4dec900,
   title: "hello there",
   author: 5fc531cae70f776378cce0c4 // Author is related to user collection
},
{
   _id: 5fc5f3e6140646c2024f7963,
   title: "hello wonderland",
   author: 5fc531cae70f776378cce0c4 // Author is related to user collection
}

可以$lookup历史上的文章，然后$lookup文章的作者吗？并且还按历史日期排序？

期望的输出

...
name: "jacob",
_id: 5fc53209e70f776378cce0c5,
history: [
  {
     _id: 5fc634bee65f96338a63b9e4,
     article: {
        _id: 5fc5faaa8b1fffc1f4dec900,
        title: "hello there",
        author: {
           _id: 5fc531cae70f776378cce0c4,
           name: "melissa"
        }
     },
     created_at: 2010-12-01T12:19:10.121+00:00
  },
  {
    _id: 5fc634d8e65f96338a63b9e5,
    article: {
       _id: 5fc5faaa8b1fffc1f4dec900,
       title: "hello wonderland",
       author: {
          _id: 5fc531cae70f776378ccedsu8,
         name: "omelia",
       }
    },
    created_at: 2010-11-12T2:19:36.102+00:00
  }
]

注意：按日期降序排列

【问题讨论】：

标签： javascript mongodb mongoose nosql

【解决方案1】：

你可以试试，

$unwind解构history数组
$lookup加入文章收藏，让传递文章id，
- $match 匹配文章ID
- $lookup加入用户收藏
- $unwind 解构用户，因为它的数组
- $project 显示必填字段
$unwind解构文章数组
$sort by created_at 日期
$group 按 id 并重构 history 数组

db.users.aggregate([
  { $unwind: "$history" },
  {
    $lookup: {
      from: "articles",
      let: { articleId: "$history.article" },
      pipeline: [
        { $match: { $expr: { $eq: ["$_id", "$$articleId"] } } },
        {
          $lookup: {
            from: "users",
            localField: "author",
            foreignField: "_id",
            as: "author"
          }
        },
        { $unwind: "$author" },
        {
          $project: {
            _id: 1,
            author: { _id: 1, name: 1 },
            title: 1
          }
        }
      ],
      as: "history.article"
    }
  },
  { $unwind: "$history.article" },
  { $sort: { "history.created_at": -1 } },
  {
    $group: {
      _id: "$_id",
      history: { $push: "$history" },
      name: { $first: "$name" }
    }
  }
])

Playground

【讨论】：