MongoDB: Creating a calculated field measuring the difference between date fields - mongodb

Question

Context
This question uses the same collection/ document schema construct from my other question: MongoDB: Creating calculated fields using the switch function (column aliasing)
Data Schema
Note: cal_date1 is the calculated value shown in my other post linked above. When performing these steps they are done in a pipeline so that (I hope) the calculated columns can be thought of as part of the general document schema as shown below.
{
_id:ObjectId("619756f12c115f24df503c26"),
uniqueid:"12345678",
date1:"2021-11-02 20:04:50.253",
date2:"2021-11-03 18:10:57.520",
date3:"2021-11-08 07:08:00.000",
date4:"2021-11-08 14:40:00.000",
date5:"2021-11-08 08:34:00.000",
cal_date1: "2021-11-03 18:10:57.520"
}
Questions
How can I calculate the difference in date values between two particular dates (with some conditional logic behind it).
Extension
Is there a way of calculate the working days between two dates where "working" is defined as dates that are inclusive of Mon-Fri (no Sat, Sun)?
Tried
I have been playing with the $dateDiff operator inside a switch function as shown below however come across the error unknown operator: $cal_date1
db.collection.aggregate([
{
$project:
{
"uniqueid": 1,
"date1": 1,
"date2": 1,
"date3": 1,
"date4":1,
"date5": 1,
"cal_date1": {
$switch: {
branches: [
{ case: {$ne:["$date2",null]}, then: "$date2"},
{ case: {$ne:["$date3",null]}, then: "$date3"},
{ case: {$ne:["$date4",null]}, then: "$date4"},
{ case: {$ne:["$date5",null]}, then: "$date5"}
],
default: "blank"
}
},
"cal_date2": {
$switch: {
branches: [
{ case: {$ne:["$date4",null]}, then: "$date4"},
{ case: {$ne:["$date4",null]}, then: "$date5"}
],
default: "blank"
}
},
"cal_date3": {
$switch: {
branches: [
{ case: {$ne:["$date5",null]}, then: "$date5"}
],
default: "blank"
}
}
}
}
])
--updated code
"cal_days_between_date1_caldate1": {
$switch:
{
branches: [
{case: { $eq: ["$date1", null]}, then: "blank"},
{case: { $eq: ["$cal_date1", "blank"]}, then: "blank"}
],
default: {
$dateDiff: {
startDate: {
$dateFromString: {
dateString: "$date1"
}
},
endDate: {
$dateFromString: {
dateString: "$cal_date1"
}
},
unit: "day"
}
}
}
}
Update: 2021-11-24: T3:08pm UTC
Have altered the case expression slightly to try and get a boolean output and now have a new error of unknown operator: $dateDiff
Update: 2021-11-30: T8:36pm UTC
Have added in the code which contains the referenced fields (top of code chunk) along with the additions of the $dateFromString operator. The calculated date code is meant to act as the next step in the pipeline which I output along with the other fields.

Answer 1

The problem is that you are trying to use Date operators on fields that are strings.
You should transforms your strings to dates before with $dateFromString. Check this playground and docs.
Also there was some mistakes in the $eq arrays
db.collection.aggregate({
"$project": {
"cal_days_between_date1_caldate1": {
$switch: {
branches: [
{
case: {
$eq: [
"$date1",
null
]
},
then: "blank"
},
{
case: {
$eq: [
"$cal_date1",
"blank"
]
},
then: "blank"
}
],
default: {
$dateDiff: {
startDate: {
$dateFromString: {
dateString: "$date1"
}
},
endDate: {
$dateFromString: {
dateString: "$cal_date1"
}
},
unit: "day"
}
},
}
}
}
})