MongoDB Group and Subtract Values from Different Documents

Question

In my application I have the following documents

{
   \"timestamp\": ISODate(\"2015-09-17T21:14:35.0Z\"),
   \"sensor\": \"gas-in\",
   \"value\": 2.5,
},
{
   \         


        

Answer 1

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" ] }
    }}
])

Which gives the results:

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

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" ] }
                ]
            }
        }
    }}
])

And that's going to be more efficient.

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
                    ]}
                ]
            }
        }
    }}
])

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