如何向mongodb查找添加ObjectId参数

我正在进行查找，其中本地字段是ObjectId，外部字段是ObjectId的数组。执行查找时会出现以下错误：

arguments to $lookup must be strings

我做过类似的查找，其中外部字段不是数组（而是ObjectId），因此错误似乎不明确。我的数据库由两个集合组成：歌曲和播放列表。一首歌可以属于多个播放列表。我正在尝试编写一个聚合，该聚合返回一首匹配的歌曲，其中包含该歌曲所属的播放列表数组：

歌曲：

[
  {
    songName: "In Da Club",
    _id: ObjectId(1)
  },
  {
    songName: "Happy Birthday",
    _id: ObjectId(2)
  },
  {
    songName: "Ode to Joy",
    _id: ObjectId(3)
  }
]

[
  {
    _id: ObjectId(4)
    playlistName: "PlaylistOne,
    songs: [ObjectId(1), ObjectId(3)]
  },
  {
    _id: ObjectId(5)
    playlistName: "PlaylistTwo,
    songs: [ObjectId(1)]
  }
]

{
  songName: "In Da Club",
  _id: ObjectId(1),
  playlists: [
    {
      _id: ObjectId(4),
      playlistName: "PlaylistOne,
    },
    {
      _id: ObjectId(5),
      playlistName: "PlaylistTwo"
    }
  ]
}

db.songs.aggregate([
  {
    $match: {
      songName: "In Da Club"
    }
  },
  {
    $lookup: {
      from: 'playlists',
      let: { songId: '$_id'},
      pipeline: [
        {
          $match: {
            $expr: {
              {
                $in: ["$$songId", "$songs"]
              }
            }
          }
        }
      ],
      as: 'playlists'
    }
  }
])

播放列表：

[
  {
    songName: "In Da Club",
    _id: ObjectId(1)
  },
  {
    songName: "Happy Birthday",
    _id: ObjectId(2)
  },
  {
    songName: "Ode to Joy",
    _id: ObjectId(3)
  }
]

[
  {
    _id: ObjectId(4)
    playlistName: "PlaylistOne,
    songs: [ObjectId(1), ObjectId(3)]
  },
  {
    _id: ObjectId(5)
    playlistName: "PlaylistTwo,
    songs: [ObjectId(1)]
  }
]

{
  songName: "In Da Club",
  _id: ObjectId(1),
  playlists: [
    {
      _id: ObjectId(4),
      playlistName: "PlaylistOne,
    },
    {
      _id: ObjectId(5),
      playlistName: "PlaylistTwo"
    }
  ]
}

db.songs.aggregate([
  {
    $match: {
      songName: "In Da Club"
    }
  },
  {
    $lookup: {
      from: 'playlists',
      let: { songId: '$_id'},
      pipeline: [
        {
          $match: {
            $expr: {
              {
                $in: ["$$songId", "$songs"]
              }
            }
          }
        }
      ],
      as: 'playlists'
    }
  }
])

期望的结果：

[
  {
    songName: "In Da Club",
    _id: ObjectId(1)
  },
  {
    songName: "Happy Birthday",
    _id: ObjectId(2)
  },
  {
    songName: "Ode to Joy",
    _id: ObjectId(3)
  }
]

[
  {
    _id: ObjectId(4)
    playlistName: "PlaylistOne,
    songs: [ObjectId(1), ObjectId(3)]
  },
  {
    _id: ObjectId(5)
    playlistName: "PlaylistTwo,
    songs: [ObjectId(1)]
  }
]

{
  songName: "In Da Club",
  _id: ObjectId(1),
  playlists: [
    {
      _id: ObjectId(4),
      playlistName: "PlaylistOne,
    },
    {
      _id: ObjectId(5),
      playlistName: "PlaylistTwo"
    }
  ]
}

db.songs.aggregate([
  {
    $match: {
      songName: "In Da Club"
    }
  },
  {
    $lookup: {
      from: 'playlists',
      let: { songId: '$_id'},
      pipeline: [
        {
          $match: {
            $expr: {
              {
                $in: ["$$songId", "$songs"]
              }
            }
          }
        }
      ],
      as: 'playlists'
    }
  }
])

我尝试的查询：

[
  {
    songName: "In Da Club",
    _id: ObjectId(1)
  },
  {
    songName: "Happy Birthday",
    _id: ObjectId(2)
  },
  {
    songName: "Ode to Joy",
    _id: ObjectId(3)
  }
]

[
  {
    _id: ObjectId(4)
    playlistName: "PlaylistOne,
    songs: [ObjectId(1), ObjectId(3)]
  },
  {
    _id: ObjectId(5)
    playlistName: "PlaylistTwo,
    songs: [ObjectId(1)]
  }
]

{
  songName: "In Da Club",
  _id: ObjectId(1),
  playlists: [
    {
      _id: ObjectId(4),
      playlistName: "PlaylistOne,
    },
    {
      _id: ObjectId(5),
      playlistName: "PlaylistTwo"
    }
  ]
}

db.songs.aggregate([
  {
    $match: {
      songName: "In Da Club"
    }
  },
  {
    $lookup: {
      from: 'playlists',
      let: { songId: '$_id'},
      pipeline: [
        {
          $match: {
            $expr: {
              {
                $in: ["$$songId", "$songs"]
              }
            }
          }
        }
      ],
      as: 'playlists'
    }
  }
])

这似乎是一个相对简单的查询，我不确定如何绕过“传入查找的参数必须是字符串”错误，因为我的查找是基于ObjectId的。任何帮助都将不胜感激！蒂亚

虽然在版本

3.2

中引入，但在后续更新中对其进行了几项增强：

根据文件：

[
  {
    songName: "In Da Club",
    _id: ObjectId(1)
  },
  {
    songName: "Happy Birthday",
    _id: ObjectId(2)
  },
  {
    songName: "Ode to Joy",
    _id: ObjectId(3)
  }
]

[
  {
    _id: ObjectId(4)
    playlistName: "PlaylistOne,
    songs: [ObjectId(1), ObjectId(3)]
  },
  {
    _id: ObjectId(5)
    playlistName: "PlaylistTwo,
    songs: [ObjectId(1)]
  }
]

{
  songName: "In Da Club",
  _id: ObjectId(1),
  playlists: [
    {
      _id: ObjectId(4),
      playlistName: "PlaylistOne,
    },
    {
      _id: ObjectId(5),
      playlistName: "PlaylistTwo"
    }
  ]
}

db.songs.aggregate([
  {
    $match: {
      songName: "In Da Club"
    }
  },
  {
    $lookup: {
      from: 'playlists',
      let: { songId: '$_id'},
      pipeline: [
        {
          $match: {
            $expr: {
              {
                $in: ["$$songId", "$songs"]
              }
            }
          }
        }
      ],
      as: 'playlists'
    }
  }
])

在启动MongoDB 3.4时，如果localField是一个数组，则可以匹配 针对标量外来字段的数组元素，而不需要 $unwind阶段

MongoDB 3.6增加了对在连接的服务器上执行管道的支持 集合，它允许将多个联接条件指定为 以及不相关的子查询

所以问题可能是mongodb版本太低。无论如何，根据您的要求，您可以使用

$lookup

，它用于单个相等联接，因为数组位于

foreignField

上

{
    $lookup: {
      from: "playlists",
      localField: "_id", // Scalar value
      foreignField: "songs", // Against an array
      as: "playlists"
    }
  }

---

测试：

除了查询中的两个语法错误之外，一切看起来都很好，在这里测试：，您的MongoDB版本是什么？它托管在哪里？啊，好的-我在本地运行MongoDB版本3.6。。。我将升级到最新版本，然后重试。谢谢我在3.2***上尝试一下：&让我知道！你可能会遇到同样的问题，但请告诉我会发生什么……啊，这也行！我甚至没有尝试，因为我认为它不会工作，因为外场是一个数组。。。非常感谢！