MongoDB 聚合框架 $group 可以返回一个值数组吗? [英] Can the MongoDB aggregation framework $group return an array of values?

问题描述

MongoDB 中用于输出格式化的聚合函数有多灵活?

How flexible is the aggregate function for output formatting in MongoDB?

数据格式:

{
        "_id" : ObjectId("506ddd1900a47d802702a904"),
        "port_name" : "CL1-A",
        "metric" : "772.0",
        "port_number" : "0",
        "datetime" : ISODate("2012-10-03T14:03:00Z"),
        "array_serial" : "12345"
}

现在我正在使用这个聚合函数来返回一个 DateTime 数组、一个指标数组和一个计数:

Right now I'm using this aggregate function to return an array of DateTime, an array of metrics, and a count:

{$match : { 'array_serial' : array, 
                            'port_name' : { $in : ports},
                            'datetime' : { $gte : from, $lte : to}
                        }
                },
               {$project : { port_name : 1, metric : 1, datetime: 1}},
               {$group : { _id : "$port_name", 
                            datetime : { $push : "$datetime"},
                            metric : { $push : "$metric"},
                            count : { $sum : 1}}}

这很好，而且非常快，但是有没有办法格式化输出，以便每个日期时间/指标有一个数组?像这样:

Which is nice, and very fast, but is there a way to format the output so there's one array per datetime/metric? Like this:

[
    {
      "_id" : "portname",
      "data" : [
                ["2012-10-01T00:00:00.000Z", 1421.01],
                ["2012-10-01T00:01:00.000Z", 1361.01],
                ["2012-10-01T00:02:00.000Z", 1221.01]
               ]
    }
]

这将大大简化前端，因为这是图表代码所期望的格式.

This would greatly simplify the front-end as that's the format the chart code expects.

推荐答案

使用聚合框架将两个字段组合成一个值数组是可能的，但绝对不是那么简单(至少在 MongoDB 2.2 中).0).

Combining two fields into an array of values with the Aggregation Framework is possible, but definitely isn't as straightforward as it could be (at least as at MongoDB 2.2.0).

这是一个例子:

db.metrics.aggregate(

    // Find matching documents first (can take advantage of index)
    { $match : {
        'array_serial' : array, 
        'port_name' : { $in : ports},
        'datetime' : { $gte : from, $lte : to}
    }},

    // Project desired fields and add an extra $index for # of array elements
    { $project: {
        port_name: 1,
        datetime: 1,
        metric: 1,
        index: { $const:[0,1] }
    }},

    // Split into document stream based on $index
    { $unwind: '$index' },

    // Re-group data using conditional to create array [$datetime, $metric]
    { $group: {
        _id: { id: '$_id', port_name: '$port_name' },
        data: {
            $push: { $cond:[ {$eq:['$index', 0]}, '$datetime', '$metric'] }
        },
    }},

    // Sort results
    { $sort: { _id:1 } },

    // Final group by port_name with data array and count
    { $group: {
        _id: '$_id.port_name',
        data: { $push: '$data' },
        count: { $sum: 1 }
    }}
)

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