MongoDB在嵌套数组上的连接

Question

我目前正在研究一个小型聊天轮盘赌应用程序。为此，我在后端使用MongoDB和NodeJS。要列出所有聊天频道，我想加入两个收藏集。第一个（聊天）的元素具有嵌套数组：

{
  title: 'example 1',
  country: 'UK',
  ...
  chat: [
    {
      details: 'chat1 infos',
      channel: [
        '5ea3257d4b861a5aa0d03111',
        '5ea3257d4b861a5aa0d03122'
      ]
    },
    {
      details: 'chat2 infos',
      channel: [
        '5ea3257d4b861a5aa0d03333'
      ]
    }
  ]
}

第二个（通道）存储所有详细信息：

{
  _id: '5ea3257d4b861a5aa0d03111',
  title: 'Channel1',
  details: 'Channel1 Details'
},
{
  _id: '5ea3257d4b861a5aa0d03222',
  title: 'Channel2',
  details: 'Channel2 Details'
},
{
  _id: '5ea3257d4b861a5aa0d03333',
  title: 'Channel3',
  details: 'Channel13 Details'
}

结果应如下所示：

{
  title: 'example 1',
  country: 'UK',
  ...
  chat: [
    {
      details: 'chat1 infos',
      channel: [
        {
          _id: '5ea3257d4b861a5aa0d03111',
          title: 'Channel1',
          details: 'Channel1 Details'
        },
        {
          _id: '5ea3257d4b861a5aa0d03222',
          title: 'Channel2',
          details: 'Channel2 Details'
        }
      ]
    },
    {
      details: 'chat2 infos',
      channel: [
        {
          _id: '5ea3257d4b861a5aa0d03333',
          title: 'Channel3',
          details: 'Channel13 Details'
        }
      ]
    }
  ]
}

我尝试通过使用$ lookup：

db.collection('chats').aggregate([
  { "$lookup": {
    "from": "channel",
    "localField": "chat.channel",
    "foreignField": "_id",
    "as": "chat.channel"
  } }
]);

但是，我将每个频道的所有结果合并在一起。无论通道中包含哪个ID。如何加入集合以获取描述的结果？

Answer 1

您可以遵循此部分

您可以使用填充

 let _doc =await firstModel.find().populate("channels")

Answer 2

尝试一下

db.chats.aggregate([
  {
    $unwind: "$chat" // we need this as we have an array of chats, and each chat element has an array of channels
  },
  {
    $lookup: {
      from: "channels", // will use pipeline in lookup to get rid of multiple group stages
      let: {
        channelIds: "$chat.channel"
      },
      pipeline: [
        {
          $match: {
            $expr: {
              $in: [
                "$_id", // _id of channel
                "$$channelIds" // channel ids from chat.channel array
              ]
            }
          }
        }
      ],
      as: "chat.channel"
    }
  },
  {
    $group: {
      _id: "$_id", // group the unwinded chat array
      title: {
        $first: "$title"
      },
      country: {
        $first: "$country"
      },
      chat: {
        $push: "$chat"
      }
    }
  }
])

检查此Mongo Playground