How do I do calculation in sort in aggreation mongoose - mongodb

Question

I am trying to do this calculation but I am having some trouble. Anyone have any ideas how I can do calculation inside my aggregation? I wanna do something like this:
const test = await Test.aggregate([
{
$sort: {
$divide: [
'value',
Math.pow(1.1, new Date() - 'date'),
],
},
},
]);
For example here, I wanna do 1.1^number of days has passed. The Test schema has a "value" of type Float and a date of type date.

Answer 1

You have two issues here:
You're trying to use javascript functions within the pipeline, while this is possible by using $function it is not recommended, especially if you can execute the same logic using Mongodb operators.
$sort stage has this following structure:
{ $sort: { <field1>: <sort order>, <field2>: <sort order> ... } }
As you can tell it's not being followed in your example as you're trying to use an expression.
So how can we solve these?
Well you can use $pow instead of Math.pow, $$NOW instead of new Date() and $subtract instead of the - javascript operator.
You will also need to add a "sortField" to sort by to match the $sort stage structure, all of this would look like this:
db.collection.aggregate([
{
"$addFields": {
"sortField": {
$divide: [
"$value",
{
$pow: [
1.1,
{
$subtract: [
"$$NOW",
"$date"
]
}
]
}
]
}
}
},
{
$sort: {
sortField: 1
}
}
])
Mongo Playground
Mind you subtracting dates will give you result in miliseconds, you will have to divide it by the required number ( 60 * 1000 * 60 * 24 for a day ) to get the right mesaurement.