MongoDB - Aggregate max/min/average for multiple variables at once - mongodb

Question

I am logging data into MongoDB in the following format:
{ "_id" : ObjectId("54f2393f80b72b00079d1a53"), "outT" : 10.88, "inT3" : 22.3, "light" : 336, "humidity" : 41.4, "pressure" : 990.31, "inT1" : 22.81, "logtime" : ISODate("2015-02-28T21:55:11.838Z"), "inT2" : 21.5 }
{ "_id" : ObjectId("54f2394580b72b00079d1a54"), "outT" : 10.88, "inT3" : 22.3, "light" : 338, "humidity" : 41.4, "pressure" : 990.43, "inT1" : 22.75, "logtime" : ISODate("2015-02-28T21:55:17.690Z"), "inT2" : 311.72 }
...
As you can see there is a single time element and multiple readings logged. I want to aggregate across all of the readings to provide a max min and average for each variable grouped by hour of day. I have managed to do this for a single variable using the following aggregation script:
db.logs.aggregate(
[
{
$match: {
logtime: {
$gte: ISODate("2015-03-01T00:00:00.000Z"),
$lt: ISODate("2015-03-03T00:00:00.000Z")
}
}
},
{
$project: {_id: 0, logtime: 1, outT: 1}
},
{
$group: {
_id: {
day: {$dayOfYear: "$logtime"},
hour: {$hour: "$logtime"}
},
average: {$avg: "$outT"},
max: {$max: "$outT"},
min:{$min: "$outT"}
}
}
]
)
which produces:
{ "_id" : { "day" : 61, "hour" : 22 }, "average" : 3.1878750000000116, "max" : 3.44, "min" : 3 }
{ "_id" : { "day" : 61, "hour" : 14 }, "average" : 13.979541666666638, "max" : 17.81, "min" : 8.81 }
...
I would like to produce output which looks like:
{"outT": { output from working aggregation above },
"inT1": { ... },
...
}
Everything I try seems to throw an error in the mongo console. Can anyone help?
Thanks

Answer 1

You can do this by including each statistic in your $group with a different name and then following that with a $project stage to reshape it into your desired format:
db.logs.aggregate([
{
$match: {
logtime: {
$gte: ISODate("2015-02-28T00:00:00.000Z"),
$lt: ISODate("2015-03-03T00:00:00.000Z")
}
}
},
{
$project: {_id: 0, logtime: 1, outT: 1, inT1: 1}
},
{
$group: {
_id: {
day: {$dayOfYear: "$logtime"},
hour: {$hour: "$logtime"}
},
outT_average: {$avg: "$outT"},
outT_max: {$max: "$outT"},
outT_min:{$min: "$outT"},
inT1_average: {$avg: "$inT1"},
inT1_max: {$max: "$inT1"},
inT1_min:{$min: "$inT1"}
}
},
{
$project: {
outT: {
average: '$outT_average',
max: '$outT_max',
min: '$outT_min'
},
inT1: {
average: '$inT1_average',
max: '$inT1_max',
min: '$inT1_min'
}
}
}
])
This gives you output that looks like:
{
"_id" : {
"day" : 59,
"hour" : 21
},
"outT" : {
"average" : 10.88,
"max" : 10.88,
"min" : 10.88
},
"inT1" : {
"average" : 22.78,
"max" : 22.81,
"min" : 22.75
}
}

Answer 2

$max in Mongodb gets the maximum of the corresponding values from all documents in the collection. $min gets the minimum values from all documents in the collection. $avg gets the average value from the collection.
you must go through the Mongodb link for sample examples.