Mongo Aggregation Get Difference between values of array objects - arrays

Question

Good day, I am having difficulty using aggregation in mongodb to get the date difference between two values in an array, my document structure looks as follows:
{
"_id" : ObjectId("5c591420a890362dec70fcf4"),
"tracetag" : "T00005",
"amounts" : [
{
"_id" : ObjectId("5c5915bea890362dec71015a"),
"startdate" : ISODate("2019-02-15T04:49:02.000+0000"),
"enddate" : ISODate("2019-02-15T04:50:05.000+0000"),
"amount" : 18.975946800811833,
},
{
"_id" : ObjectId("5c5915bea890362dec71015b"),
"startdate" : ISODate("2019-02-16T04:49:02.000+0000"),
"enddate" : ISODate("2019-02-16T04:51:52.000+0000"),
"amount" : 3.5755730555836203,
},
{
"_id" : ObjectId("5c5915bea890362dec71015c"),
"startdate" : ISODate("2019-02-17T04:49:02.000+0000"),
"enddate" : ISODate("2019-02-17T04:50:04.000+0000"),
"amount" : 2.3937573313380383,
}
],
}
the number of "amounts" entries are not fixed, so it could be 1 or 50 etc.
for each document, I would like to get the difference between the 'enddate' and 'startdate' (aka the duration) and I would like to sum the duration (lets call this totalduration). I can do the total 'amount' without issues, but getting the duration gives me error ""cant $subtract aarray from a array", my aggregation looks like this:
{
"$project" : {
"tracetag" : 1.0,
"totalamount" : {
"$sum" : "$amounts.amount"
},
"totalduration" : {
"$sum" : {
"$subtract" : [
"$amounts.enddate",
"$amounts.startdate"
]
}
}
}
},

Answer 1

OK so I managed to answer my own question: the 'amounts' need to be split using the $unwind operator, then a field can be added to each record (duration), in my case I wanted the duration in minutes. Afterwards I grouped everything again
{
"$unwind" : { "path" : "$amounts" }
},
{
"$addFields" : {
"duration" : { "$divide" : [ { "$subtract" : [
"$enddate", "$startdate" ]}, 60000.0 ] }
}
},