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？我尝试使用$each和$slice，但这似乎不起作用。

Answer 1

假设左下角坐标和右上角坐标分别为[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的子集。

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 }
])

Answer 2

您可以通过直接在$slice处传递$push运算符来实现此目的。

  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 3

我通过 (1) 允许在小组阶段推送所有值，然后 (2) 将 $filter 添加到后续 $project 阶段解决了这个问题。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/

Mongodb聚合管道如何限制组推

3 个答案: