$group 之后未显示 $project $lookup 值答案

【问题标题】：$project $lookup value not shown after $group$group 之后未显示 $project $lookup 值
【发布时间】：2020-04-04 13:21:52
【问题描述】：

我有 2 个相关的收藏，我想做一个 $lookup。

开关

{ 
    "_id" : ObjectId("5e8453c095c85ca0c33a9461"), 
    "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"), 
    "relay" : NumberInt(1), 
    "name" : "Lampu Tengah", 
    "voltage" : 80.0, 
    "duration" : null, 
    "status" : true, 
    "triggered_by" : ObjectId("5e5fd642fce106005319e884"), 
    "created_at" : ISODate("2020-04-01T15:41:36.588+0000"), 
    "updated_at" : ISODate("2020-04-01T22:59:39.261+0000")
}
{ 
    "_id" : ObjectId("5e8454bc95c85ca0c33a9463"), 
    "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"), 
    "relay" : NumberInt(2), 
    "name" : "Kipas Angin", 
    "voltage" : 100.0, 
    "duration" : null, 
    "status" : true, 
    "triggered_by" : ObjectId("5e5fd642fce106005319e884"), 
    "created_at" : ISODate("2020-04-01T15:45:48.099+0000"), 
    "updated_at" : ISODate("2020-04-01T15:45:48.099+0000")
}

power_usage_month

{ 
    "_id" : ObjectId("5e87edffffba850e8d72ce27"), 
    "switch_id" : ObjectId("5e8453c095c85ca0c33a9461"), 
    "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"), 
    "current" : 19.345, 
    "time_minutes" : NumberInt(123), 
    "created_at" : ISODate("2020-04-02T15:01:37.521+0000"), 
    "updated_at" : ISODate("2020-04-02T15:01:37.521+0000")
}
{ 
    "_id" : ObjectId("5e87ee06ffba850e8d72ce28"), 
    "switch_id" : ObjectId("5e8454bc95c85ca0c33a9463"), 
    "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"), 
    "current" : 17.5, 
    "time_minutes" : NumberInt(123), 
    "created_at" : ISODate("2020-04-03T20:35:09.870+0000"), 
    "updated_at" : ISODate("2020-04-03T20:35:09.871+0000")
}
{ 
    "_id" : ObjectId("5e87ee0cffba850e8d72ce29"), 
    "switch_id" : ObjectId("5e8453c095c85ca0c33a9461"), 
    "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"), 
    "current" : 19.345, 
    "time_minutes" : NumberInt(124), 
    "created_at" : ISODate("2020-04-04T01:45:00.000+0000"), 
    "updated_at" : ISODate("2020-04-04T01:45:00.000+0000")
}
{ 
    "_id" : ObjectId("5e87ee13ffba850e8d72ce2a"), 
    "switch_id" : ObjectId("5e8454bc95c85ca0c33a9463"), 
    "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"), 
    "current" : 17.5, 
    "time_minutes" : NumberInt(124), 
    "created_at" : ISODate("2020-04-04T01:45:00.000+0000"), 
    "updated_at" : ISODate("2020-04-04T01:45:00.000+0000")
}
{ 
    "_id" : ObjectId("5e87ee18ffba850e8d72ce2b"), 
    "switch_id" : ObjectId("5e8453c095c85ca0c33a9461"), 
    "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"), 
    "current" : 19.345, 
    "time_minutes" : NumberInt(125), 
    "created_at" : ISODate("2020-04-04T01:45:00.000+0000"), 
    "updated_at" : ISODate("2020-04-04T01:45:00.000+0000")
}
{ 
    "_id" : ObjectId("5e87ee20ffba850e8d72ce2c"), 
    "switch_id" : ObjectId("5e8454bc95c85ca0c33a9463"), 
    "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"), 
    "current" : 17.5, 
    "time_minutes" : NumberInt(125), 
    "created_at" : ISODate("2020-04-04T01:45:00.000+0000"), 
    "updated_at" : ISODate("2020-04-04T01:45:00.000+0000")
}
{ 
    "_id" : ObjectId("5e87ee26ffba850e8d72ce2d"), 
    "switch_id" : ObjectId("5e8453c095c85ca0c33a9461"), 
    "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"), 
    "current" : 19.345, 
    "time_minutes" : NumberInt(126), 
    "created_at" : ISODate("2020-04-04T01:45:00.000+0000"), 
    "updated_at" : ISODate("2020-04-04T01:45:00.000+0000")
}
{ 
    "_id" : ObjectId("5e87ee2dffba850e8d72ce2e"), 
    "switch_id" : ObjectId("5e8454bc95c85ca0c33a9463"), 
    "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"), 
    "current" : 17.5, 
    "time_minutes" : NumberInt(126), 
    "created_at" : ISODate("2020-04-04T01:45:00.000+0000"), 
    "updated_at" : ISODate("2020-04-04T01:45:00.000+0000")
}

现在我想使用 $lookup 和 $project 我要显示的字段“加入”这些集合，但显然 $project 值在我执行 $group 后不会显示。这是我的查询

db.getCollection("power_usages_month").aggregate(
    [
        { 
            "$project" : { 
                "_id" : NumberInt(0), 
                "power_usages_month" : "$$ROOT"
            }
        }, 
        { 
            "$lookup" : { 
                "localField" : "power_usages_month.switch_id", 
                "from" : "switches", 
                "foreignField" : "_id", 
                "as" : "switches"
            }
        }, 
        { 
            "$unwind" : { 
                "path" : "$switches", 
                "preserveNullAndEmptyArrays" : false
            }
        }, 
        { 
            "$group" : { 
                "_id" : "$power_usages_month.switch_id",
                "sum_current" : { 
                    "$sum" : "$power_usages_month.current"
                }
            }
        }, 
        { 
            "$project" : { 
                "switch_id" : "$_id", 
                "device_id" : "$switches.device_id",
                "sum_current" : "$sum_current", 
                "voltage" : "$switches.voltage",
            }
        }
    ], 
    { 
        "allowDiskUse" : true
    }
);

根据结果，"device_id" : "$switches.device_id", 和 "voltage" : "$switches.voltage" 无法按预期出现：

{ 
    "_id" : ObjectId("5e8454bc95c85ca0c33a9463"), 
    "switch_id" : ObjectId("5e8454bc95c85ca0c33a9463"), 
    "sum_current" : 70.0
}
{ 
    "_id" : ObjectId("5e8453c095c85ca0c33a9461"), 
    "switch_id" : ObjectId("5e8453c095c85ca0c33a9461"), 
    "sum_current" : 77.38
}

怎么可能？请告诉我查询的错误..

编辑：这是我想要的结果

{ 
    "_id" : ObjectId("5e8454bc95c85ca0c33a9463"), 
    "switch_id" : ObjectId("5e8454bc95c85ca0c33a9463"),
    "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"),
    "sum_current" : 70.0,
    "voltage" : 80.0

}
{ 
    "_id" : ObjectId("5e8453c095c85ca0c33a9461"), 
    "switch_id" : ObjectId("5e8453c095c85ca0c33a9461"),
    "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"), 
    "sum_current" : 77.38,
    "voltage" : 100.0

}

【问题讨论】：

标签： mongodb nosql bson nosql-aggregation

【解决方案1】：

您可以在分组和$lookup 之后将项目作为最后一步你也可以在小组中处理所有这些，您将在$group 中包含的内容将仅在您的查询中可用，但是假设您需要在该交换机的一个数组中收集每个交换机使用的power_usages_month 文档，但是您不需要来自 power_usages_month 的所有信息，例如您只需要 current 和 time_minutes

你可以这样做

db.powerUsageMonth.aggregate(
    [
        {
            $match: {}
        },
        {
            $lookup: {
                from: 'switches',
                localField: 'switch_id',
                foreignField: '_id',
                as: 'switch'
            }
        },
        {
            $unwind: '$switch'
        },
        {
            $group: {
                _id: '$switch._id',
                sum_current: {
                    $sum: '$current'
                },
                switchInfo: {
                    $first: '$switch'
                },
                powerUsageDocs: {
                    $addToSet: '$$ROOT'
                }
            }
        },
        {
            $project: {
                _id: 1,
                sum_current: 1,
                switchInfo: 1,
                'powerUsageDocs.current': 1,
                'powerUsageDocs.time_minutes': 1,
            }
        }
    ]
)

这将返回一组开关及其总电流和开关信息（如果您不需要所有开关信息，您也可以在此处进行一些投影），以及与此开关相关的电源使用文档（投影后)

结果会是这样的

{
    "_id" : ObjectId("5e8454bc95c85ca0c33a9463"),
    "sum_current" : 70,
    "switchInfo" : {
        "_id" : ObjectId("5e8454bc95c85ca0c33a9463"),
        "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"),
        "relay" : 2,
        "name" : "Kipas Angin",
        "voltage" : 100,
        "duration" : null,
        "status" : true,
        "triggered_by" : ObjectId("5e5fd642fce106005319e884"),
        "created_at" : ISODate("2020-04-01T15:45:48.099Z"),
        "updated_at" : ISODate("2020-04-01T15:45:48.099Z")
    },
    "powerUsageDocs" : [
        {
            "current" : 17.5,
            "time_minutes" : 125
        },
        {
            "current" : 17.5,
            "time_minutes" : 123
        },
        {
            "current" : 17.5,
            "time_minutes" : 126
        },
        {
            "current" : 17.5,
            "time_minutes" : 124
        }
    ]
}
{
    "_id" : ObjectId("5e8453c095c85ca0c33a9461"),
    "sum_current" : 77.38,
    "switchInfo" : {
        "_id" : ObjectId("5e8453c095c85ca0c33a9461"),
        "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"),
        "relay" : 1,
        "name" : "Lampu Tengah",
        "voltage" : 80,
        "duration" : null,
        "status" : true,
        "triggered_by" : ObjectId("5e5fd642fce106005319e884"),
        "created_at" : ISODate("2020-04-01T15:41:36.588Z"),
        "updated_at" : ISODate("2020-04-01T22:59:39.261Z")
    },
    "powerUsageDocs" : [
        {
            "current" : 19.345,
            "time_minutes" : 124
        },
        {
            "current" : 19.345,
            "time_minutes" : 126
        },
        {
            "current" : 19.345,
            "time_minutes" : 123
        },
        {
            "current" : 19.345,
            "time_minutes" : 125
        }
    ]
}

更新

如果您只需要有关交换机的信息，您还需要获得瓦特，即 = 总电流 * 每个开关的电压，我们可以在$project 中做到这一点，我们可以将$multiply 运算符添加到$project 管道中，它将开关的电压乘以我们刚刚在$group 管道中计算的总电流

db.powerUsageMonth.aggregate(
    [
        {
            $match: {}
        },
        {
            $lookup: {
                from: 'switches',
                localField: 'switch_id',
                foreignField: '_id',
                as: 'switch'
            }
        },
        {
            $unwind: '$switch'
        },
        {
            $group: {
                _id: '$switch._id',
                switch_id: { $first: '$switch._id' },
                device_id: { $first: '$switch.device_id' },
                voltage: { $first: '$switch.voltage' },
                sum_current: {
                    $sum: '$current'
                }
            }
        },
        {
            $project: {
                _id: 1,
                switch_id: 1,
                device_id: 1,
                voltage: 1,
                sum_current: 1,
                watt: { $multiply: [ "$voltage", "$sum_current" ] }
            }
        }
    ]
)

这将产生一个形式为

的数组

{
    "_id" : ObjectId("5e8454bc95c85ca0c33a9463"),
    "switch_id" : ObjectId("5e8454bc95c85ca0c33a9463"),
    "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"),
    "voltage" : 100,
    "sum_current" : 70,
    "watt" : 7000
}
{
    "_id" : ObjectId("5e8453c095c85ca0c33a9461"),
    "switch_id" : ObjectId("5e8453c095c85ca0c33a9461"),
    "device_id" : ObjectId("5e7d83efd62c242a11e3ca5e"),
    "voltage" : 80,
    "sum_current" : 77.38,
    "watt" : 6190.4
}

【讨论】：

感谢您的回答，但这似乎不是我想要的结果格式。我编辑了所需的结果格式，请看一下:)
我已经更新了我的答案，你现在可以检查一下吗？另外，不要忘记将 collectionName 'powerUsageMonth' 替换为您自己的
太棒了！谢谢你解决我的问题：D
嗯，再回答一个先生，我想在voltage字段之后添加乘法值，我的查询是watt: {$first: {$multiply: ["$sum_current", '$switch.voltage']}}但它不能调用sum_current组，如何实现？
好的，我刚刚再次更新了我的答案以添加每个开关的瓦特