MongoDB聚合框架$ group可以返回值数组吗?

问题描述

聚合函数在MongoDB中用于输出格式的灵活性如何?

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，我正在使用此聚合函数返回一个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}}}

哪一个很好，并且速度很快，但是是否有一种格式化输出的方式，因此每个日期时间/指标只有一个数组?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Aggregation Framework可以将两个字段组合成值数组，但绝对不是那么简单(至少在MongoDB 2.2中如此) .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).

这里是一个例子:

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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