如何使用聚合 mongoDB 从 2 个集合中获取链接数据

Question

我是 MongoDB 新手，我有 2 个集合：具有以下结构的User和Video 。 当用户喜欢一个视频时，视频的_id将被添加到用户收藏的liked_videos 。 liked_videos是一个 _id 数组

视频集

{
    _id
    mp4_url,
    liked_count
}

用户收藏

{
    _id,
    username,
    password,
    liked_videos: [ // videos _id array ]
}

如何查询以获取用户喜欢的视频？ 像下面这样？

[
    {
    _id: 1
    mp4_url,
    liked_count
    },
    {
    _id: 2
    mp4_url,
    liked_count
    },
    ...
]

谢谢

Answer 1

您需要使用$lookup来加入集合，使用$size来获取数组大小

db.video.aggregate([
  {
    "$lookup": {
      "from": "user",
      "localField": "_id",
      "foreignField": "liked_videos",
      "as": "join_video"
    }
  },
  {
    "$project": {
      liked_count: {
        $size: "$join_video"
      }
    }
  }
])

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Answer 2

试试这个方法

db={ "video": [ { "_id": 1, "mp4_url": "url1", "liked_count": 2, "quantity": 2 }, { "_id": 2, "mp4_url": "url2", "liked_count": 3, "quantity": 1 }, { "_id": 3, "mp4_url": "url2", "liked_count": 3, "quantity": 4 } ], "User": [ { "_id": 1, "username": "almonds", "password": "pwd", "Likedvideos": [ "aaa", "ffff" ] }, { "_id": 2, "username": "almonds", "password": "pwd", "Likedvideos": [ "qqq", "bbbb" ] }, { "_id": 3, "username": "almonds", "password": "pwd", "Likedvideos": [ "ccc", "ffff" ] }, { "_id": 4, "username": "almonds", "password": "pwd", "Likedvideos": [ "bbbb", "ffff" ] }, { "_id": 5, "username": "almonds", "password": "pwd", "Likedvideos": [ "qqq", "ffff" ] }, { "_id": 6 } ] }

询问 ：

db.video.aggregate([
  {
    $match: {
      _id: 1
    }
  },
  {
    "$lookup": {
      "from": "User",
      "localField": "_id",
      "foreignField": "_id",
      "as": "data"
    },
    
  },
  {
    $unwind: "$data"
  },
  {
    $replaceRoot: {
      newRoot: {
        $mergeObjects: [
          "$data",
          "$$ROOT"
        ]
      }
    }
  },
  {
    $project: {
      data: 0
    }
  }
])

输出：

[
  {
    "Likedvideos": [
      "aaa",
      "ffff"
    ],
    "_id": 1,
    "liked_count": 2,
    "mp4_url": "url1",
    "password": "pwd",
    "quantity": 2,
    "username": "almonds"
  }
]

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