Where in Django can I run startup code that requires models?

Question

On Django startup I need to run some code that requires access to the database. I prefer to do this via models.

Here's what I currently have in apps.py:

from django.apps import AppConfig
from .models import KnowledgeBase

class Pqawv1Config(AppConfig):
    name = 'pqawV1'

    def ready(self):
        to_load = KnowledgeBase.objects.order_by('-timestamp').first()
        # Here should go the file loading code

However, this gives the following exception:

django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet.

So is there a place in Django to run some startup code after the models are initialized?

Move the from .models import KnowledgeBase inside the def ready method. Here importing it in the header results in the fact that the models are loaded too early. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Jan 5, 2019 at 20:09
@WillemVanOnsem, this works, thanks. But how good is this solution? What are the side effects? I'm quite new to Python... — Serge Rogatch
– Serge Rogatch, Commented Jan 5, 2019 at 20:10
the difference is that you simply only import .models when the config calls ready, you thus postpone importing. It was due to "eager" importing that this resulted in trouble. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Jan 5, 2019 at 20:12

willeM_ Van Onsem · Accepted Answer · 2019-01-05 20:16:49Z

18

The problem is that you import .models at the top of your file. This means that, when the file app.py file is loaded, Python will load the models.py file when it evalutes that line. But that is too early. You should let Django do the loading properly.

You can move the import in the def ready(self) method, such that the models.py file is imported when ready() is called by the Django framework, like:

from django.apps import AppConfig

class Pqawv1Config(AppConfig):
    name = 'pqawV1'

    def ready(self):
        from .models import KnowledgeBase
        to_load = KnowledgeBase.objects.order_by('-timestamp').first()
        # Here should go the file loading code

answered Jan 5, 2019 at 20:16

willeM_ Van Onsem

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5 Comments

avocado Over a year ago

Once the data or resources is loaded in ready(), how could we use them in views? Import appconfig?

willeM_ Van Onsem Over a year ago

@advocado: you can obtain the appconfig for a given app with from django.apps import apps; apps.get_app_config('app_name'). If you store your resorce that is loaded on the self (so self.to_load = ...), you thus can access it with apps.get_app_config('app_name').to_load. But you should do that in a function, since the models are loaded before the ready function is triggered.

avocado Over a year ago

Great (and also concise) answer!

There Over a year ago

As an aside, from .models import * will fail ("SyntaxError: import * only allowed at module level") while from .models import KnowledgeBase will succeed

Yatharth Agarwal Over a year ago

This is explicitly discouraged by the docs. The reason is if you run tests, the ready() code will still run against your production database, not the test database. See discussion here.

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