Functions inside MongoDB query - mongodb

Question

I was wondering if was possible to utilize functions inside of MongoDB query by including a function inside of options block or something.
Suppose my database collection has a document:
{"year": "2320"}
And I have function to format this document's year-field:
const reformat = function(input){
return input.year.substring(2,4)+input.year.substring(0,2)
}
And I'll be calling a fetch function like this:
const test = {"year": "2023"}
fetchSomething(test, {}, reformat)
My fetch function looks like this:
async function fetchSomething(query, projection, options){
const dat = await mongoCli.db("testDB").collection("testCollection").findOne(query, projection, options);
return dat;
}
So, reformat-function should modify the database's document to match year-attribute of test-variable, not the other way around.
I'm not sure if MongoDB even supports it. Looking at the MongoDB documentation...

Answer 1

All you could do is like this:
const reformat = function(){
return {allowDiskUse: true}
}
mongoCli.db("testDB").collection("testCollection").findOne(query, projection, reformat());
The option is an input attribute of findOne, you cannot re-define it.
Or you can use $function in aggregation pipeline:
db.mongoCli.aggregate([
{
$set: {
year: {
$function: {
body: function (input) {
return input.year.substring(2, 4) + input.year.substring(0, 2)
},
args: ["$$ROOT"],
lang: "js"
}
}
}
}
])
Please be aware, executing JavaScript inside an aggregation expression may decrease performance. Only use the $function operator if the provided pipeline operators cannot fulfill your application's needs. $substrCP and $concat are also available as pipeline operator.

Answer 2

I now realize is that I can accomplish it with a cursor:
const reformat = function(input){
return input.year.substring(2,4)+input.year.substring(0,2)
}
async function fetchSomething(query, options){
const cursor = await mongoCli.db("testDB").collection("testCollection").find()
let doc;
while (await cursor.hasNext()) {
doc = await cursor.next();
if (options(doc) == query.year){ break; }
else { doc = null; }
}
cursor.close();
return doc;
}
const test = {"year": "2023"}
fetchSomething(test, reformat)
Though there might be a better way to do it.