MongoDB conditional query depending on possible dates - mongodb

Question

I have a scenario where I want to pull documents that have a lastAlertSentDate field that's over 30 days old. This will run in a daily cron job. Upon querying, this field will then be reset to NOW. So it's meant to act as a "rotating 30 day window" if you will.
The complication here is that the field won't exist if it hasn't been set yet. In this edge case, we'll then have to use a createdDate field of the document to do the 30-day comparison against.
So effectively, I want something like, "If lastAlertSentDate exists, then get all docs where it's older than 30days from now. ---Otherwise, get all docs where createdDate is older than 30days from now"
So the logic between both fields are the same, it's just the field itself that can be different. Because of this, I was thinking to first USE addFields a dateToUseField and then do a match on the second stage based on this.
[
{
'$addFields': {
'dateToUse': {
'$cond': {
'if': {
'$ne': [
'$lastAlertSentDate', undefined
]
},
'then': '$lastAlertSentDate',
'else': '$createdDate'
}
}
}
}, {
'$match': {
'dateToUse': {
'$lte': '30_DAYS_PRIOR'
}
}
}
]
So the else part doesn't seem to work. It doesn't assign $createdDate to dateToUse.
What am I missing? Also, how can I condense this? I'm sure I don't need the addFields first and I can do everything within the $match

Answer 1

You have two options here:
Use a $or query with two predicates, where each of them is a $and predicate:
Either lastAlertSentDate does not exists and createdDate > n
Or lastAlertSentDate exists and it is > n
Playground Link
db.collection.find({
$or: [
{
$and: [
{
"lastAlertSentDate": {
"$exists": false
}
},
{
"createdDate": {
$gt: 5
}
}
]
},
{
$and: [
{
"lastAlertSentDate": {
"$exists": true
}
},
{
"lastAlertSentDate": {
$gt: 5
}
}
]
}
]
})
Use an aggregation using the $ifNull
Playground Link
db.collection.aggregate([
{
$match: {
$expr: {
$gt: [
{
"$ifNull": [
"$lastAlertSentDate",
"$createdDate"
]
},
5
]
}
}
}
])