Fill data with NULL value if it is not present in the timeperiod using mongodb aggregation pipeline - mongodb

Question

I have to write an aggreagtion pipeline in which I will pass:
Timestamps of start date and end data for a day
I have to divide the data into 30min buckets and find data in between that buckets like:
2023-01-16T00:30:00.000+00:00 , 2023-01-16T01:00:00.000+00:00, 2023-01-16T01:30:00.000+00:00 and so on.
If data is not present in any particular bucket fill the values of that bucketa with zero but give the timestamp like:
2023-01-16T01:00:00.000+00:00 ther is no data give {timestamp:2023-01-16T01:00:00.000+00:00,a:0,b:0,c:0}
I have done the following:
[{
$match: {
$and: [
{
timestamp: {
$gte: ISODate('2023-01-16T00:00:00.000Z'),
$lt: ISODate('2023-01-16T23:59:59.000Z')
}
}
]
}
}, {
$group: {
_id: {
$toDate: {
$subtract: [
{
$toLong: '$timestamp'
},
{
$mod: [
{
$toLong: '$timestamp'
},
1800000
]
}
]
}
},
in: {
$sum: '$a'
},
out: {
$sum: '$b'
},
Count: {
$sum: 1
}
}
}, {
$addFields: {
totalIn: {
$add: [
'$in',
'$out'
]
},{
$sort: {
_id: 1
}
}]
Result is:
[{
"_id": {
"2023-01-16T12:00:00.000+00:00"
}
},
"totalIn": 397,
"count":22
},
{
"_id": {
"2023-01-16T01:30:00.000+00:00"
}
},
"totalIn": 222,
"count":2
}
...]
expected result:
[{
"_id": {
"2023-01-16T12:00:00.000+00:00"
}
},
"totalIn": 397,
"count":22
},
{
"_id": {
"2023-01-16T12:30:00.000+00:00"
}
},
"totalIn": 0,
"count":0
},
{
"_id": {
"2023-01-16T01:00:00.000+00:00"
}
},
"totalIn": 0,
"count":0
},
{
"_id": {
"2023-01-16T12:00:00.000+00:00"
}
},
"totalIn": 222,
"count":2
}
...]

Answer 1

One option is to use $range with $dateAdd:
db.collection.aggregate([
{$match: {timestamp: {
$gte: startDate,
$lt: endDate
}}},
{$group: {
_id: {$dateTrunc: {date: "$timestamp", unit: "minute", binSize: 30}},
in: {$sum: "$a"},
out: {$sum: "$b"},
count: {$sum: 1}
}},
{$group: {
_id: 0,
data: {$push: {
timestamp: "$_id",
totalIn: {$add: ["$in", "$out"]},
count: "$count"
}}
}},
{$project: {
_id: 0, data: 1,
bins: {$map: {
input: {$range: [
0,
{$multiply: [
{$dateDiff: {
startDate: startDate,
endDate: endDate,
unit: "hour"
}},
2
]}
]},
in: {$dateAdd: {
startDate: startDate,
unit: "minute",
amount: {$multiply: ["$$this", 30]}
}}
}}
}},
{$unwind: "$bins"},
{$set: {data: {$filter: {
input: "$data",
cond: {$eq: ["$bins", "$$this.timestamp"]}
}}}},
{$project: {
_id: "$bins",
count: {$ifNull: [{$first: "$data.count"}, 0]},
totalIn: {$ifNull: [{$first: "$data.totalIn"}, 0]}
}}
])
See how it works on the playground example