如何获取文档的数量并以有效的方式过滤它们？（猫鼬）

Question

我正在实现搜索功能，即在 mongoDB 中查找文档。我想.skip(x)在.limit(x)结果上模拟分页结果，但是我可以获取文档总数（在跳过和限制之前）并立即获取过滤结果吗？

产生预期输出的代码：

db.Datas.find({ type: "Unknown" })
  .then((result) => {
    let count = result.length;
    db.Datas.find({ type: "Unknown" })
      .sort({ createdAt: -1 })
      .skip((req.query.page - 1) * 10)
      .limit(10)
      .then((res) => {
        res.json({ count: count, result: res });
      });
  })
  .catch((err) => {});

但查询两次有点烦人，而且在大型数据库中可能会很慢。我尝试了类似的方法find({}).then(x => { ... }).sort(...) ...，但不起作用，因为它只返回 Promise。

我怎样才能有效地做这些事情？或者只是获取整个文档并跳过，用 JS 方式限制（使用 .splice 等）会更快更高效？

Answer 1

您可以使用$facet聚合来实现此目的。

db.Datas.aggregate([
  {
    $match: {
      "type": "Unknown"
    }
  },
  {
    $sort: {
      createdAt: -1
    }
  },
  {
    $facet: {
      totalRecords: [
        {
          $count: "total"
        }
      ],
      data: [
        {
          $skip: 0
        },
        {
          $limit: 5
        }
      ]
    }
  }
])

操场

假设您有这些文件：

db={
  "Datas": [
    {
      "_id": "5e390fc33285e463a0799689",
      "type": "Known",
      "createdAt": "2020-02-04T06:31:31.311Z",
      "__v": 0
    },
    {
      "_id": "5e390fd03285e463a079968a",
      "type": "Known",
      "createdAt": "2020-02-04T06:31:44.190Z",
      "__v": 0
    },
    {
      "_id": "5e390fda3285e463a079968b",
      "type": "Unknown",
      "createdAt": "2020-02-04T06:31:54.248Z",
      "__v": 0
    },
    {
      "_id": "5e390fdf3285e463a079968c",
      "type": "Unknown",
      "createdAt": "2020-02-04T06:31:59.993Z",
      "__v": 0
    },
    {
      "_id": "5e390fec3285e463a079968d",
      "type": "Unknown",
      "createdAt": "2020-02-04T06:32:12.336Z",
      "__v": 0
    },
    {
      "_id": "5e390ffd3285e463a079968e",
      "type": "Unknown",
      "createdAt": "2020-02-04T06:32:29.670Z",
      "__v": 0
    },
    {
      "_id": "5e3910163285e463a079968f",
      "type": "Unknown",
      "createdAt": "2020-02-04T06:32:54.131Z",
      "__v": 0
    },
    {
      "_id": "5e3910213285e463a0799690",
      "type": "Unknown",
      "createdAt": "2020-02-04T06:33:05.166Z",
      "__v": 0
    }
  ]
}

响应将是这样的：

[
  {
    "data": [
      {
        "__v": 0,
        "_id": "5e3910213285e463a0799690",
        "createdAt": "2020-02-04T06:33:05.166Z",
        "type": "Unknown"
      },
      {
        "__v": 0,
        "_id": "5e3910163285e463a079968f",
        "createdAt": "2020-02-04T06:32:54.131Z",
        "type": "Unknown"
      },
      {
        "__v": 0,
        "_id": "5e390ffd3285e463a079968e",
        "createdAt": "2020-02-04T06:32:29.670Z",
        "type": "Unknown"
      },
      {
        "__v": 0,
        "_id": "5e390fec3285e463a079968d",
        "createdAt": "2020-02-04T06:32:12.336Z",
        "type": "Unknown"
      },
      {
        "__v": 0,
        "_id": "5e390fdf3285e463a079968c",
        "createdAt": "2020-02-04T06:31:59.993Z",
        "type": "Unknown"
      }
    ],
    "totalRecords": [
      {
        "total": 6
      }
    ]
  }
]

正如你所看到的，我们得到了经过过滤、排序、跳过和有限数据的总记录。