不能在嵌套数组中使用 $slice

Question

我尝试的是 $slice 数组中的一个数组。

这是我尝试过的，但失败了。

db.collection.find({
  name: "admin",
  "datasets.name": "test"
},
{
  "datasets.$.datapoints": {
    $slice: [
      0,
      3
    ]
  }
})

输入数据示例

[
  {
    "name": "admin",
    "datasets": [
      {
        "name": "test",
        "datapoints": [
          1,
          6,
          4,
          3,
          8,
          5,
          3
        ],
        "_id": {
          "$oid": "619288f16733758444a28728"
        }
      },
      {
        "name": "more datasets",
        "datapoints": [
          1,
          2,
          3,
          4,
          5,
          6,
          7,
          8
        ],
        "_id": {
          "$oid": "619289086733758444a2872a"
        }
      }
    ]
  }
]

游乐场链接：

https://mongoplayground.net/p/pmGpk9jtqZ2

输出应该是：

{ name: "test", datapoints: [1,6,4] }

Answer 1

你可以得到你想要的结果

[
  {
    "_id": ObjectId("5a934e000102030405000000"),
    "datasets": [
      {
        "_id": ObjectId("619288f16733758444a28728"),
        "datapoints": [
          1,
          6,
          4
        ],
        "name": "test"
      },
      {
        "_id": ObjectId("619289086733758444a2872a"),
        "datapoints": [
          1,
          2,
          3
        ],
        "name": "more datasets"
      }
    ],
    "name": "admin"
  }
]

与

db.collection.find({
  name: "admin",
  "datasets.name": "test"
},
{
  "datasets.datapoints": {
    $slice: [
      0,
      3
    ]
  }
})

您只需要省略位置运算符。