Custom type with transformation upon read

[OoDeLally]

Prerequisites

• I have written a descriptive issue title

Mongoose version

6.8.2

Node.js version

6.17.1

MongoDB version

4.13.0

Operating system

Linux

Issue

I would like to manipulate a type X in my codebase, store it as Y format in the DB, and then get back a X when I fetch.

(codebase) X ==saving==> (db) Y ==fetching==> (codebase) X

X => Y can be done with a custom schema type's cast().
However, I cannot find the way to do the opposite, i.e. deserialize Y into X upon fetching. The document I get from .find() uses Y.

A few workaround I've tried:

• use transform() with .toObject(). But .toObject is only available in hydrated docs everytime, and I would prefer getting lean documents.
• use post-query hooks. This forces me to manually set the hook to every single schema that uses the type X. This is hardly scalable.
• Create my own mongoose.Type and mongoose.SchemaType. I tried to mimic what https://github.com/mongoosejs/mongoose-function is doing. But it did not work, and I see this library is not maintained since 2013, so I'm not sure it would work with current mongoose.

This topic seems similar, but there are no solution proposed: #10423

[IslandRhythms]

Lean may not be the solution here https://mongoosejs.com/docs/api.html#query_Query-lean
You want to transform the data coming out of the db and lean is designed for read only cases.
A cursor may be what you are looking for: https://mongoosejs.com/docs/api/query.html#query_Query-cursor
After a find call, you can iterate through each document converting Y back into X.
Although the easiest solution would be to save it as x to the db if you're having to convert it back when fetching. Is there something preventing you from doing so that you can share?

[OoDeLally]

We want to store large integers.
Our use case would be to use bigint in the codebase, and store them as Long in the mongodb.
Classic number cannot safely store numbers above 9,007,199,254,740,991 (Number.MAX_SAFE_VALUE). So we would like to use bigint, which are arbitrary long.
MongoDB Long can store up to 9,223,372,036,854,775,807 (int64 => 2^63), so it is slightly more than Number.MAX_SAFE_VALUE.

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