How can I implement Auto calculated field in mongodb? - node.js

Question

I have a collection of workorders where it have time_started and time_completed values. I want to have auto-calculated field called duration that automatically calculated time_completed - time_started. What is the best way?
Essentially what I want is, when App post requests with a completed time, my duration is auto calculated.
Example data
router.post('/completed', function (req, res) {
const time_completed = req.body.time_completed
const workorder_id = req.body.workorder_id
db.collection(workorder).updateOne(
{ _id: ObjectId(workorder_id) },
{
$set: {
time_completed: time_completed,
}
},
function (err, result) {
if (err) throw err
res.send('Updated')
}
)
});

Answer 1

Query
pipeline update requires >= MongoDB 4.2
add the time_completed
add the duration also
*replace the 6 with the javascript variable that holds the time_completed Date
*duration will be in milliseconds
Test code here
db.collection.update(
{"_id": 1},
[
{
"$set": {
"time_completed": 6,
"duration": {
"$subtract": [
6,
"$time_started"
]
}
}
}
])
Edit
You have strings on your database, i thought it was dates, best thing to do is to convert all those string-dates to Date with $dataFromString like the code bellow, and use the first query.
To get the string if you needed from Date you can do $stringFromDate when you need it.
Query
same like above but it converts string dates to Date to do the substraction (keeps the dates in strings inside the database)
Test code here
db.collection.update({
"_id": 1
},
[
{
"$set": {
"time_completed": "2021-11-21T00:00:00.000Z",
"duration": {
"$subtract": [
ISODate("2021-11-21T00:00:00Z"),
{
"$dateFromString": {
"dateString": "$time_started"
}
}
]
}
}
}
])