Calculate amount of minutes between multiple date ranges, but don't calculate the overlapping dates in MongoDB - mongodb

Question

I am creating a way to generate reports of the amount of time equipment was down for, during a given time frame. I will potentially have 100s to thousands of documents to work with. Every document will have a start date and end date, both in BSON format and will generally be within minutes of each other. For simplicity sake I am also zeroing out the seconds.
The actual aggregation I need to do, is I need to calculate the amount of minutes between each given date, but there may be other documents with overlapping dates. Any overlapping time should not be calculated if it's been calculated already. There are various other aggregations I'll need to do, but this is the only one that I'm unsure of, if it's even possible at all.
{
"StartTime": "2020-07-07T18:10:00.000Z",
"StopTime": "2020-07-07T18:13:00.000Z",
"TotalMinutesDown": 3,
"CreatedAt": "2020-07-07T18:13:57.675Z"
}
{
"StartTime": "2020-07-07T18:12:00.000Z",
"StopTime": "2020-07-07T18:14:00.000Z",
"TotalMinutesDown": 2,
"CreatedAt": "2020-07-07T18:13:57.675Z"
}
The two documents above are examples of what I'm working with. Every document gets the total amount of minutes between the two dates stored in the document (This field serves another purpose, unrelated). If I were to aggregate this to get total minutes down, the output of total minutes should be 4, as I'm not wanting to calculate the overlapping minutes.

Answer 1

Finding overlap of time ranges sounds to me a bit abstract. Let's try to convert it to a concept that databases are usually used for: discrete values.
If we convert the times to discrete value, we will be able to find the duplicate values, i.e. the "overlapping values" and eliminate them.
I'll illustrate the steps using your sample data. Since you have zeroed out the seconds, for simplicity sake, we can start from there.
Since we care about minute increments we are going to convert times to "minutes" elapsed since the Unix epoch.
{
"StartMinutes": 26569090,
"StopMinutes": 26569092,
}
{
"StartMinutes": 26569092,
"StopMinutes": 26569092
}
We convert them to discrete values
{
"minutes": [26569090, 26569091, 26569092]
}
{
"minutes": [26569092, 26569093]
}
Then we can do a set union on all the arrays
{
"allMinutes": [26569090, 26569091, 26569092, 26569093]
}
This is how we can get to the solution using aggregation. I have simplified the queries and grouped some operations together
db.collection.aggregate({
$project: {
minutes: {
$range: [
{
$divide: [{ $toLong: "$StartTime" }, 60000] // convert to minutes timestamp
},
{
$divide: [{ $toLong: "$StopTime" }, 60000]
}
]
},
}
},
{
$group: { // combine to one document
_id: null,
_temp: { $push: "$minutes" }
}
},
{
$project: {
totalMinutes: {
$size: { // get the size of the union set
$reduce: {
input: "$_temp",
initialValue: [],
in: {
$setUnion: ["$$value", "$$this"] // combine the values using set union
}
}
}
}
}
})
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