MongoDB 在聚合查询中获取第一个和最后一个文档 [英] MongoDB get first and last document in aggregate query

问题描述

如何根据 time 字段获取第一个和最后一个文档.我可以使用 $group 并获取 $first 和 $last 文档，但我不需要在这里分组，只需获取第一个和最后一个完整文档.也许我可以使用 slice ?此查询不起作用:

How can I get first and last document based on time field. I can use $group and get $first and $last document, but I don't need grouping here, just get first and last full document. Maybe I could use slice? This query doesn't work:

{
  "aggregate": "353469045637980_data",
  "pipeline": [
    {
      "$match": {
        "$and": [
          {
            "time": {
              "$gte": 1461369600
            }
          },
          {
            "time": {
              "$lt": 1461456000
            }
          }
        ]
      }
    },
    {
      "$project": {
        "first": {
          "$slice": 1
        },
        "last": {
          "$slice": -1
        }
      }
    }
  ]
}

推荐答案

好吧，您需要 $group 但您可以简单地使用一个常量(例如 null，请参阅 文档) 为其 id 从而导致单个组.$$ROOT 然后引用文档本身，您可以与 $first 和 $last 一起使用

Well you need $group but you can simply use a constant (e.g. null, see the docs) for its id so that it results in a single group. $$ROOT then refers to the document itself which you can use with $first and $last like so

$group: {
  _id: null,
  first: { $first: "$$ROOT" },
  last: { $last: "$$ROOT" }
}

当然你可以引入更多的 $project 阶段来将数据塑造成一个数组(正如你提到的你想要一个列表)等.

Of course you can introduce further $project stages to shape that data into an array (as you mentioned you want a list) etc.

作为旁注，您可能需要引入 $sort 阶段以确保 $first 和 $last 具有正确的含义.

As a side note you may want to introduce a $sort stage to make sure $first and $last have a proper meaning.

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