Combine arrays in MongoDB $group aggregation - mongodb

Question

I am using Mongo daily bucketing pattern. Each daily document contains an array with value calculated for every hour for that day:
{
meter: 'meterId',
date: 'dailyBucket',
hourlyConsumption: [0,0,1,1,1,2,2,2,4,4,4,4,3,3,3...] // array with 24 values for every hour of a day
}
Now in one of my aggregation queries, I would like to group documents for the same day of multiple meters and get a result like this:
INPUT (consumption of multiple meters in a same day)
{
meter: 'MeterA',
date: '2021-05-01',
hourlyConsumption: [0,0,1,1,1,2,2,2,4,4,4,4,3,3,3...]
},
{
meter: 'MeterB',
date: '2021-05-01',
hourlyConsumption: [10,10,10,10,10,10,10,10,10,10,10,10,10,10,10...]
}
RESULT (combined into single document)
{
date: '2021-05-01',
hourlyConsumption: [10,10,11,11,11,12,12,12,14,14,14,14,13,13,13...]
}
is there a way to achieve this without using $accumulator?

Answer 1

You can use $reduce
db.collection.aggregate([
{
$group: {
_id: "$date",
hourlyConsumption: { $push: "$hourlyConsumption" }
}
},
{
$set: {
hourlyConsumption: {
$reduce: {
input: "$hourlyConsumption",
initialValue: [],
in: { $map: { input: { $range: [ 0, 23 ] },
as: "h",
in: {
$sum: [
{ $arrayElemAt: [ "$$value", "$$h" ] },
{ $arrayElemAt: [ "$$this", "$$h" ] }
]
}
}
}
}
}
}
}
])
Mongo Playground
Or you use $unwind and $group:
db.collection.aggregate([
{
$unwind: {
path: "$hourlyConsumption",
includeArrayIndex: "hour"
}
},
{
$group: {
_id: {
date: "$date",
hour: "$hour"
},
hourlyConsumption: { $sum: "$hourlyConsumption" }
}
},
{ $sort: { "_id.hour": 1 } },
{
$group: {
_id: "$_id.date",
hourlyConsumption: { $push: "$hourlyConsumption" }
}
}
])
Mongo Playground
However, when you use $unwind, then you actually contradict your bucketing design pattern.