Mongodb聚合管道如何限制组推送

Question

我无法使用聚合管道限制组函数中推送元素的数量。这可能吗？小例子：

数据：

[
    {
        "submitted": date,
        "loc": { "lng": 13.739251, "lat": 51.049893 },
        "name": "first",
        "preview": "my first"
    },
    {
        "submitted": date,
        "loc": { "lng": 13.639241, "lat": 51.149883 },
        "name": "second",
        "preview": "my second"
    },
    {
        "submitted": date,
        "loc": { "lng": 13.715422, "lat": 51.056384 },
        "name": "nearpoint2",
        "preview": "my nearpoint2"
    }
]

这是我的聚合管道：

var pipeline = [
    //I want to limit the data to a certain area
    { $match: {
        loc: {
            $geoWithin: {
                $box: [
                    [locBottomLeft.lng, locBottomLeft.lat],
                    [locUpperRight.lng, locUpperRight.lat]
                ]
            }
        }
    }},
    // I just want to get the latest entries  
    { $sort: { submitted: -1 } },
    // I group by name
    {
      $group: {
          _id: "$name",
          // get name
          submitted: { $max: "$submitted" },
          // get the latest date
          locs: { $push: "$loc" },
          // push every loc into an array THIS SHOULD BE LIMITED TO AN AMOUNT 5 or 10
          preview: { $first: "$preview" }
      }
    },
    // Limit the query to at least 10 entries.
    { $limit: 10 }
];

如何将locs 数组限制为10 或任何其他大小？我用$each 和$slice 尝试了一些东西，但这似乎不起作用。

Answer 1

从Mongo 5.2^{release schedule}开始，这是新的$topN聚合累加器的完美用例：

// { submitted: ISODate("2021-12-05"), group: "group1", value: "plop" }
// { submitted: ISODate("2021-12-07"), group: "group2", value: "smthg" }
// { submitted: ISODate("2021-12-06"), group: "group1", value: "world" }
// { submitted: ISODate("2021-12-12"), group: "group1", value: "hello" }
db.collection.aggregate([
  { $group: {
    _id: "$group",
    top: { $topN: { n: 2, sortBy: { submitted: -1 }, output: "$value" } }
  }}
])
// { _id: "group1", top: [ "hello", "world" ] }
// { _id: "group2", top: [ "smthg" ] }

这应用了$topN 组累积：

• 为每个组获取前 2 个 (n: 2) 元素
• 前 2 名，由 sortBy: { submitted: -1 } 定义（倒序）
• 并为每个分组记录提取字段value (output: "$value")

Answer 2

我通过 (1) 允许在小组阶段推送所有值，然后 (2) 在后续的 $project 阶段添加一个 $filter 来解决这个问题。在 $filter 中，消除所有具有不合格值的数组成员。

https://docs.mongodb.com/manual/reference/operator/aggregation/filter/

Answer 3

您可以通过将$slice 运算符直接传递给$push 来实现此目的。

  var pipeline = [{
    //I want to limit the data to a certain area
    $match: {
        loc: {
            $geoWithin: {
                $box: [
                    [locBottomLeft.lng, locBottomLeft.lat],
                    [locUpperRight.lng, locUpperRight.lat]
                ]
            }
        }
    }
},
// I just want to get the latest entries  
{
    $sort: {
        submitted: -1
    }
},
// I group by name
{
    $group: {
        _id: "$name",
        < --get name
        submitted: {
            $max: "$submitted"
        },
        < --get the latest date
        locs: {
            $push: {
              $slice: 10
            }
        },
        < --push every loc into an array THIS SHOULD BE LIMITED TO AN AMOUNT 5 or 10
        preview: {
            $first: "$preview"
        }
    }
},
//Limit the query to at least 10 entries.
{
    $limit: 10
}
];

Answer 4

假设左下坐标和右上坐标分别为[0, 0]和[100, 100]。从 MongoDB 3.2 开始，您可以使用 $slice 运算符返回您想要的数组子集。

db.collection.aggregate([
    { "$match": { 
        "loc": { 
            "$geoWithin":  { 
                "$box": [ 
                    [0, 0], 
                    [100, 100]
                ]
            }
        }}
    }},
    { "$group": { 
        "_id": "$name",
        "submitted": { "$max": "$submitted" }, 
        "preview": { "$first": "$preview" }
        "locs": { "$push": "$loc" }
    }}, 
    { "$project": { 
        "locs": { "$slice": [ "$locs", 5 ] },
        "preview": 1,
        "submitted": 1
    }},
    { "$limit": 10 }
])