Mongo aggregation: partitioning values into groups (by partition) - mongodb

Question

Using this approach, I can group a set of event documents by time. This solution returns the same input documents but with a partition number added.
How can I do the same thing but return the partitions instead? An example output document would be:
{
partition: 0,
startDate: ISODate("1900-04-12T18:30:00.000Z"),
endDate: ISODate("2019-04-12T18:30:00.000Z"),
numEvents: 27
}

Answer 1

Let's say this is the current output of your aggregation:
{
"_id" : null,
"datesWithPartitions" : [
{
"date" : ISODate("2019-04-12T18:30:00Z"),
"partition" : 0
},
{
"date" : ISODate("2019-04-12T20:00:00Z"),
"partition" : 1
},
{
"date" : ISODate("2019-04-12T20:10:00Z"),
"partition" : 1
},
{
"date" : ISODate("2019-04-12T21:00:00Z"),
"partition" : 2
},
{
"date" : ISODate("2019-04-12T21:15:00Z"),
"partition" : 2
},
{
"date" : ISODate("2019-04-12T21:45:00Z"),
"partition" : 3
},
{
"date" : ISODate("2019-04-12T23:00:00Z"),
"partition" : 4
}
]
}
To get the data in a format you need you need to append following aggregation steps:
db.col.aggregate([
{
$unwind: "$datesWithPartitions"
},
{
$group: {
_id: "$datesWithPartitions.partition",
numEvents: { $sum: 1 },
startDate: { $min: "$datesWithPartitions.date" },
endDate: { $max: "$datesWithPartitions.date" }
}
},
{
$project: {
_id: 0,
partition: "$_id",
startDate: 1,
endDate: 1,
numEvents: 1
}
}
])
$unwind will return single date per document and then you can apply $group with $min and $max to get partition boundaries and $sum to count partition elements