Django 尽早评估多对多关系的查询集以在异步函数中使用

Question

I'm using a Django-Channels consumer for websocket async communication.

I have something like this below

class Command(UUIDModel):
    owner = models.ForeignKey(AUTH_USER_MODEL, default=None, on_delete=models.SET_DEFAULT, null=True, blank=True, related_name='commands')
    name = models.CharField('Name', default='New Command', max_length=128, blank=True, null=True)


class Secret(UUIDModel):
    owner = models.ForeignKey(AUTH_USER_MODEL, default=None, on_delete=models.SET_DEFAULT, null=True, blank=True, related_name='secrets')
    command = models.ManyToManyField(Command, blank=True, related_name='secrets')


@sync_to_async
def get_command(pk):
    command = Command.objects.get(id=pk)
    return command


class CommandConsumer(AsyncWebsocketConsumer):

    @log_exceptions
    async def command(self, event):
        log.debug(event)

        command = await get_command(event.get('command').get('id'))
        log.debug(command)
        log.debug(command.secrets)
        log.debug(command.secrets.all())  # Fails here

        return

I get a SynchronousOnlyOperation error when running this, right when it evaluates the queryset for Secrets in the ManyToMany field.

Is there a way to force the queryset to evaluate ahead of time in the synchronous get_command function, instead of in the async websocket? That way I can easily access the secrets via command.secrets .

Currently, my workaround is to just handle the secrets as a separate variable

@sync_to_async
def get_command(pk):
    command = Command.objects.get(id=pk)
    secrets = list(command.secrets.all())
    return command, secrets

Answer 1

Use prefetch_related https://docs.djangoproject.com/en/3.2/ref/models/querysets/#prefetch-related

@sync_to_async
def get_command(pk):
    command = Command.objects.prefetch_related("secrets").get(id=pk)
    return command


class CommandConsumer(AsyncWebsocketConsumer):

    @log_exceptions
    async def command(self, event):
        log.debug(event)

        command = await get_command(event.get('command').get('id'))
        log.debug(command)
        log.debug(command.secrets)
        log.debug(command.secrets.all())  # No longer fails

        return