Re: register_converter doesn't work with ASGI

Well, one workaround that may serve if your dataset isn't too large is to use a cache (or dict) and pre-load it.

Unlike the db, caches tend to be more async friendly, unless of course your cache is in your db....  :-)

This just needs to be pre-loaded on startup (<your app>.apps.:<your app AppConfig>.ready() is a good spot for this), and maybe add a signal on the relevant model to reload if it changes.

HTH,

David

On Mon, Jun 21, 2021 at 5:22 PM Konstantin Kuchkov <konstantin.kuchkov@gmail.com> wrote:

Thanks for the explanation!

To get it working, I will replace my model converters with basic converters and do corresponding lookups in the views.

On Sunday, 20 June 2021 at 16:16:58 UTC-7 Andrew Godwin wrote:

Ah yes, that's because the URL parsing/conversion all happens in the main (async, in ASGI mode) thread - and because to_python is a a synchronous function interface, there is literally nothing you can do to make it work.

The only real way of fixing this would be for Django to patch the URL converters and run them all in a synchronous mode unless they were decorated as "safe" somehow, much like we do for middleware. It's not a terribly hard patch to make, but it does mean you don't have an immediate solution. There are ways of trying to work around it, but they will all result in you blocking the async thread while the ORM query runs, which would be disastrous in production.

My apologies for not getting this in for the original async view patch - I had forgotten converters ran in the URL resolver.

Andrew

On Sat, Jun 19, 2021, at 12:31 AM, konstanti...@gmail.com wrote:

I'm trying to switch from WSGI to ASGI and I'm having issues with my model id to model converters.

I define my converters like this

def to_python(self, primary_key):

     try:

          return self.model.objects.get(pk=primary_key)

     except self.model.DoesNotExist:

          raise ValueError

When I go to a url where a converter is used I get a SynchronousOnlyOperation.

Seems like converters are executed in an async context.

If I use sync_to_async, I get a coroutine in my view instead of a model instance. Awaiting it here would defeat the purpose of having a converter.

Any ideas how to fix this?

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