Problem with Vitest's --singleThread/--no-threads flag when running mongoose and mongodb-memory-server

[revosw]

I'm running tests with mongodb-memory-server so that I can test locally.

The following error is printed when running pnpm exec vitest --no-threads run

OverwriteModelError: Cannot overwrite `user` model once compiled.

From the documentation:

singleThread

Run all tests with the same environment inside a single worker thread. This will disable built-in module isolation (your source code or inlined code will still be reevaluated for each test), but can improve test performance. Before Vitest 0.29.0 this was equivalent to using --no-threads.

Since all tests share the same environment, I'm guessing that's why models are created multiple times.

Reproduction repository:
https://github.com/revosw/vitest-mms-repro

The exact reason I'm using --singleThread is because I've found it problematic to run tests in AWS CodeBuild with a constrained environment of 0.25vCPU and 0.5GB RAM with multiple workers. Prisma-based vitest runs work flawlessly with --singleThread, but the ones based on mongoose are struggling with the aforementioned error

[revosw]

I have solved the problem. I also wrongly stated that I ran --singleThread because of the constrained environment in CodeBuild. The real reason is because a dependency I have installed, sharp, was problematic without --singleThread. They even have a section in the documentation about worker threads, and how calling require("sharp") or import "sharp" sometime before the worker threads are created should solve the problem. And it did!

I followed what was commented in this thread about globalSetup. Inside the globalSetup file, I did import "sharp", and suddenly running without --singleThread isn't problematic anymore.
vitest-dev/vitest#740 (comment)

And as for mongoose, since I'm not using --singleThread anymore, I have no issues with "Cannot overwrite user model once compiled"