MongoDB$查找物件陣列

類別

    {
        "_id" : ObjectId("61740086893f048528d166b9"),
        "name": "Category1",
        "tracks" : [ 
            "61c65353565a2d9a1cd3020d", 
            "61c74518962dc3efb96c3438", 
            "61c74775703176a6f72df444"
        ]
    }

曲目

    {
        "_id" : ObjectId("61c65353565a2d9a1cd3020d"),
        "name" : "Track1",
        "categoryId" : ObjectId("61740086893f048528d166b9"),
        "creatorId" : ObjectId("61c6478304e98ed63e8ee7d3"),
        "thumbnailId" : ObjectId("61c65353565a2d9a1cd3020c"),
        "plays" : [],
        "media" : {
            "type" : "wav",
            "url" : ""
        },
        "status" : "approved",
        "downloads" : [],
        "uploadedDate" : 1640387411
    }

假設我有 5 個類別，每個類別有很多曲目 ID，我想為每個類別獲取最后 N 個曲目，所以我在下面使用了這個代碼

    categories.aggregate([
        {
            $project: {
                tracks: { $slice: ["$tracks", -2] },
            },
        },
    ]

回應是

    [
        {
            "_id": "61740086893f048528d166b9",
            "tracks": [
                "61c74518962dc3efb96c3438",
                "61c74775703176a6f72df444"
            ]
        },
        {
            "_id": "61740094893f048528d166c1",
            "tracks": []
        },
        {
            "_id": "617400a0893f048528d166cb",
            "tracks": []
        }
    ]

到目前為止還不錯，但問題是如何將每個類別的軌道從 ID 陣列替換為物件陣列？

我試過$loopup了，但我可能沒有localField正確實施。

預期結果

    [
        {
            "_id": "61740086893f048528d166b9",
            "tracks": [
                {
                    "_id": ObjectId("61c74518962dc3efb96c3438")
                    ...
                },
                {
                    "_id": ObjectId("61c74775703176a6f72df444")
                    ...
                }
            ]
        },
        {
            "_id": "61740094893f048528d166c1",
            "tracks": []
        },
        {
            "_id": "617400a0893f048528d166cb",
            "tracks": []
        }
    ]

***** 更新 *****

我正在嘗試替換creatorId集合中createdBy用戶的物件users

用戶

    {
      "_id": ObjectId("61c6478304e98ed63e8ee7cb"),
      "email": "[email protected]",
      "username": "USER999",
      "tracks": [
        ObjectId("61c65353565a2d9a1cd3020d"),
      ],
    }

預期的結果應該是

    [
        {
            "_id": "61740086893f048528d166b9",
            "tracks": [
                {
                    "_id": ObjectId("61c74518962dc3efb96c3438"),
                    "createdBy": {
                        "_id": "userId"
                        ...
                    },
                    ...
                },
                {
                    "_id": ObjectId("61c74775703176a6f72df444"),
                    "createdBy": {
                        "_id": "userId"
                        ...
                    }
                    ...
                }
            ]
        },
        {
            "_id": "61740094893f048528d166c1",
            "tracks": []
        },
        {
            "_id": "617400a0893f048528d166cb",
            "tracks": []
        }
    ]

除了下面的解決方案ray，我在這里添加了代碼https://mongoplayground.net/p/8AjmnL-vhtz

位于createdBy頂層，但不是在每個軌道下

uj5u.com熱心網友回復：

$lookupTracks是您在集合中找到相應物件的正確方法。為什么您的代碼不起作用是因為您將字串存盤在集合中的tracks陣列中；Categories而_id集合Tracks是 ObjectId。$lookup由于資料型別不匹配，因此不會有任何結果。您可以做的是使用$toObjectIdin a將字串轉換為 ObjectId $map，然后執行$lookup

db.categories.aggregate([
  {
    $project: {
      tracks: {
        $slice: [
          "$tracks",
          -2
        ]
      }
    }
  },
  {
    $project: {
      tracks: {
        "$map": {
          "input": "$tracks",
          "as": "t",
          "in": {
            "$toObjectId": "$$t"
          }
        }
      }
    }
  },
  {
    "$lookup": {
      "from": "tracks",
      let: {
        t: "$tracks"
      },
      pipeline: [
        {
          $match: {
            $expr: {
              "$in": [
                "$_id",
                "$$t"
              ]
            }
          }
        }
      ],
      "as": "tracks"
    }
  }
])

這是Mongo游樂場供您參考。

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