MongoDB分组并从不同文档中减去值 [英] MongoDB Group and Subtract Values from Different Documents

问题描述

我的申请中有以下文件

{
   "timestamp": ISODate("2015-09-17T21:14:35.0Z"),
   "sensor": "gas-in",
   "value": 2.5,
},
{
   "timestamp": ISODate("2015-09-17T21:14:35.0Z"),
   "sensor": "gas-out",
   "value": 2.0,
},
{
   "timestamp": ISODate("2015-09-17T21:20:35.0Z"),
   "sensor": "gas-in",
   "value": 6.3,
},
{
   "timestamp": ISODate("2015-09-17T21:20:35.0Z"),
   "sensor": "gas-out",
   "value": 0.8,
}

...等等...

我如何返回按时间戳分组的(加油)-(加油)差异? 我正在尝试编写一个将其返回如下的函数:

How can I return the difference (gas-in) - (gas-out) grouped per timestamp? I am trying to write a function that returns it like this:

{
   "timestamp": ISODate("2015-09-17T21:14:35.0Z"),
   "sensor": "gas-difference",
   "value": 0.5, // is calculated by 2.5 - 2.0
},
{
   "timestamp": ISODate("2015-09-17T21:20:35.0Z"),
   "sensor": "gas-difference",
   "value": 5.5, // is calculated by 6.3 - 0.8
},

...等等...

我玩过聚合函数和$ subtract，读过其他SO问题，但是它们似乎并没有解决我的问题.在其他问题中，他们似乎知道要相互减去2个文档，但就我而言，我不知道文档的数量，$ timestamp列"是两个文档的匹配标识符.

I have played around with aggregation functions and $subtract, read other SO questions but they do not seem to have the answer to my question. In the other questions they seem to know which to 2 document to subtract from each other, but in my case I don't know the number of documents and the $timestamp "column" is the matching identifier for the two documents.

有人可以帮助我解决这个问题吗?谢谢！

Can anybody help me with solving this? Thanks!

推荐答案

您首先需要基于条件 $sum 在每个值的分组中的 $cond 运算符上.然后，您可以分别 $subtract :

What you first need is a conditional $sum based on the $cond operator in the grouping for each value. Then you can separately $subtract:

db.gas.aggregate([
    { "$group": {
        "_id": "$timestamp",
        "gas-in": { 
            "$sum": { 
                "$cond": [
                    { "$eq": [ "$sensor", "gas-in" ] },
                    "$value", 
                    0
                ]
            }
        },
        "gas-out": { 
            "$sum": { 
                "$cond": [
                    { "$eq": [ "$sensor", "gas-out"] },
                    "$value", 
                    0
                ]
            }
        },
    }},
    { "$project": {
        "gasdifference": { "$subtract": [ "$gas-in", "$gas-out" ] }
    }}
])

哪个给出结果:

{ "_id" : ISODate("2015-09-17T21:20:35Z"), "gasdifference" : 5.5 }
{ "_id" : ISODate("2015-09-17T21:14:35Z"), "gasdifference" : 0.5 }

另一种方法是将排气"值设为单个负值:

The alternate is just make the "gas-out" values negative for a single stage:

db.gas.aggregate([
    { "$group": {
        "_id": "$timestamp",
        "gasdifference": { 
            "$sum": { 
                "$cond": [
                    { "$eq": [ "$sensor", "gas-in" ] },
                    "$value", 
                    { "$subtract": [ 0, "$value" ] }
                ]
            }
        }
    }}
])

这将更加高效.

如果您有两个以上的传感器"值，则只需嵌套" $cond语句:

If you have more than two possible "sensor" values then you just "nest" the $cond statements:

db.gas.aggregate([
    { "$group": {
        "_id": "$timestamp",
        "gasdifference": { 
            "$sum": { 
                "$cond": [
                    { "$eq": [ "$sensor", "gas-in" ] },
                    "$value", 
                    { "$cond": [
                        { "$eq": [ "$sensor", "gas-out" ] },
                        { "$subtract": [ 0, "$value" ] },
                        0
                    ]}
                ]
            }
        }
    }}
])

由于它们是三元"运算符(if-then-else)，因此任何进一步的逻辑都属于"else"条件.

Since they are "ternary" operators ( if-then-else ) then any further logic falls within the "else" condition.

这篇关于MongoDB分组并从不同文档中减去值的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！