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

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 }
])