django db ProgrammingError

Question

When i try to reset database (i created a new empty database)

i got this error when i run this command python manage.py makemigrations (or migrate)

    for pattern in self.url_patterns:
  File "django\utils\functional.py", line 48, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "django\urls\resolvers.py", line 589, in url_patterns
    patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
  File "django\utils\functional.py", line 48, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "django\urls\resolvers.py", line 582, in urlconf_module
    return import_module(self.urlconf_name)
  File \python\python39\lib\importlib\__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 790, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "project\urls.py", line 18, in <module>
    path('api/v2/', include('project.api.urls'), name='api'),
  File "django\urls\conf.py", line 34, in include
    urlconf_module = import_module(urlconf_module)
  File "python\python39\lib\importlib\__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 986, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 680, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 790, in exec_module
  File "<frozen importlib._bootstrap>", line 228, in _call_with_frames_removed
  File "project\api\urls.py", line 3, in <module>
    from .views import *
  File "project\api\views.py", line 146, in <module>
    class SampleViewSet(viewsets.ModelViewSet):
  File "project\api\views.py", line 148, in SampleViewSet
    general = General.objects.get(is_active=True)
  File "django\db\models\manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "django\db\models\query.py", line 425, in get
    num = len(clone)
  File "django\db\models\query.py", line 269, in __len__
    self._fetch_all()
  File "django\db\models\query.py", line 1308, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "django\db\models\query.py", line 53, in __iter__
    results = compiler.execute_sql(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size)
  File "django\db\models\sql\compiler.py", line 1156, in execute_sql
    cursor.execute(sql, params)
  File "django\db\backends\utils.py", line 98, in execute
    return super().execute(sql, params)
  File "django\db\backends\utils.py", line 66, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "django\db\backends\utils.py", line 75, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "django\db\backends\utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "env\lib\site-packages\django\db\utils.py", line 90, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "env\lib\site-packages\django\db\backends\utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
        django.db.utils.ProgrammingError: HATA:  "app_general" object doesnt exist
    LINE 1: ...", "app_general"."field1" FROM "field_2...

views.py

class SampleViewSet(viewsets.ModelViewSet):
    general = General.objects.get(is_active=True)
    queryset = model.objects.filter(field=site_general.field)

i deleted all migration files in all apps (without __init__ ) and also .pyc files

is there any idea for this problem?

(edit i added Traceback)

@AbdulAzizBarkat class SampleViewSet(viewsets.ModelViewSet): general = General.objects.get(is_active=True) queryset = model.objects.filter(field=site_general.field) — Jim
– Jim, Commented May 28, 2021 at 13:15
What is site_general in model.objects.filter(field=site_general.field)? — Abdul Aziz Barkat
– Abdul Aziz Barkat, Commented May 28, 2021 at 13:21
@AbdulAzizBarkat I do not think views.py will cause problems because I changed the database many times but now there is a problem and I did not change views.py and that view. — Jim
– Jim, Commented May 28, 2021 at 13:26

Abdul Aziz Barkat · Accepted Answer · 2021-05-28 13:31:37Z

3

All the non-method attributes of a class get evaluated /executed during the declaration or definition of the class. Hence when the interpreter reaches your class SampleViewSet it tries to execute the line general = General.objects.get(is_active=True) but of course your database doesn't exist yet and since calling .get implies a database query you get an error.

In general any statement that might cause a database query should be either run by the app configs ready method, by some middleware / context processor, or by the view in a request. Looking at the code it doesn't look like you use general anywhere, so you can simply remove it:

class SampleViewSet(viewsets.ModelViewSet):
    queryset = model.objects.filter(field=site_general.field)

You might say model.objects.filter(...) would also cause a query, but .filter returns a QuerySet and queryset's are lazy and hence evaluated only when needed, so that will not cause any error.

answered May 28, 2021 at 13:31

Abdul Aziz Barkat

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1 Comment

Jim Over a year ago

I got it on the comment line and it worked, but it was a strange experience. Thanks for help

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