Django: how to use a DB parameter in a default attribute of model field?

Question

I have created a (kind of) singleton to put all the app parameters in my database:

class SingletonModel(models.Model):

    def save(self, *args, **kwargs):
        self.pk = 1
        super(SingletonModel, self).save(*args, **kwargs)

    @classmethod
    def load(cls):
        return cls.objects.all().get()

    class Meta:
        abstract = True


class AppParameters(SingletonModel, models.Model):

    DEFAULT_BALANCE_ALERT_THRESHOLD = models.PositiveIntegerField(default=5)
    # other parameters...

It worked pretty well, until I tried to use one of these parameters in a default attribute of a model field:

class Convive(models.Model):

    balance_alert_threshold = models.IntegerField(
        default=AppParameters.load().DEFAULT_BALANCE_ALERT_THRESHOLD,
        blank=True,
        null=True)

This seemed to work too, but when I use a script to reinitialise local data, the first manage.py migrate produce a DoesNotExist since my Singleton does not exist yet. It happens because of a file importing Convive model.

How would you solve this? Is there a way to "delay" the evaluation of the default field?

Thanks.

EDIT After posting this, I think that if my code processes db queries at import time, something may be wrong with it...

Alasdair · Accepted Answer · 2018-01-17 09:57:01Z

1

Create a method that returns the default value,

def get_default_balance_alert_threshold():
    return AppParameters.load().DEFAULT_BALANCE_ALERT_THRESHOLD

then use that method as your default.

class Convive(models.Model):
    balance_alert_threshold = models.IntegerField(
        default=get_default_balance_alert_threshold,
        blank=True,
        null=True,
    )

answered Jan 17, 2018 at 9:57

Alasdair

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1 Comment

David Dahan Over a year ago

Simple and clever, thanks! That solved both problem (DoesNotExist error and potential DB queries run at import time)

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