MongoDB - How to convert every date-field in an array of objects? - node.js

Question

I Have a bunch of sensordata stored in mongoDB. They are stored like this:
{
"data": [
{
"date": ISODate("2020-02-08T18:06:25.507+00:00"),
"temperature": 20.3,
"humidity": 53.7
},
{
"date": ISODate("2020-02-08T18:07:25.507+00:00"),
"temperature": 21,
"humidity": 54
}
]
}
The day-field is generated by new Date() with JavaScript.
Now i just want the get all the data and convert the "date"-field to a time-field. The result should look like this:
{
"data": [
{
"date": "18:06:25",
"temperature": 20.3,
"humidity": 53.7
},
{
"date": "18:07:25",
"temperature": 21,
"humidity": 54
}
]
}
So is there a way to convert every "date"-field in the array to a "time"-field by using db.collection.aggregate?
I tried using this:
db.collection.aggregate([
{},
{
"$project": {
"data.date": { $dateToString: { format: "%H:%M:%S",date: "$date" } },
"daydata.temperature": 1,
"daydata.humidity": 1
}
}
])
I know it doesnt work, because i dont have any "date"-field outside of "data". But i dont know how to reach the date-field of every data-object and convert it.

Answer 1

You can use $unwind then $project then $group in aggregate.
db.data.aggregate([
{ $unwind: "$data" },
{
$project: {
"data.date": {
$dateToString: { format: "%H:%M:%S", date: "$data.date" },
},
"data.temperature": 1,
"data.humidity": 1,
},
},
{
$group: {
_id: "$_id",
data: { $push: "$data" },
},
},
]);
Also, date should be ISODate
{
"data": [
{
"date": ISODate("2020-02-08T18:06:25.507+00:00"),
"temperature": 20.3,
"humidity": 53.7
},
{
"date": ISODate("2020-02-08T18:07:25.507+00:00"),
"temperature": 21,
"humidity": 54
}
]
}

Answer 2

Since the data.date field is a string, use the sub-string operator to extract the time part of the date field.
db.test.aggregate( [
{
$unwind: "$data"
},
{
$addFields: {
"data.time": { $substrCP: [ "$data.date", 11, 8 ] }
}
},
{
$project: { "data.date": 0 }
},
{
$group: {
_id: "$_id",
data: { $push: "$data" },
// other_fld: { $first: "$other_fld" }
}
}
] ).pretty()