Choices prevent from making migrations

Question

Django 3.0.4

    def get_choices():
        blank_choice = (None, '----')
        choices = []
        # choices = [blank_choice] + [(str(item.id), str(item)) for item in YandexTextAdTemplates.objects.all()]
        return choices
    
    
    class CopyForm(forms.Form):
        ad_template = forms.ChoiceField(choices=get_choices())

I need choices in my form. Now the necessary string is commented out. If I return choices back, it will work. But it will not make migrations.

(venv) michael@michael:~/PycharmProjects/ads3/ads_manager$ python manage.py makemigrations
Traceback (most recent call last):
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/db/backends/utils.py", line 86, in _execute
    return self.cursor.execute(sql, params)
psycopg2.errors.UndefinedTable: relation "yandex_ads_yandextextadtemplates" does not exist
LINE 1: ...ndextextadtemplates"."audience_yandex_pixel" FROM "yandex_ad...
                                                             ^


The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "manage.py", line 21, in <module>
    main()
  File "manage.py", line 17, in main
    execute_from_command_line(sys.argv)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/core/management/__init__.py", line 401, in execute_from_command_line
    utility.execute()
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/core/management/__init__.py", line 395, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/core/management/base.py", line 328, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/core/management/base.py", line 366, in execute
    self.check()
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/core/management/base.py", line 392, in check
    all_issues = self._run_checks(
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/core/management/base.py", line 382, in _run_checks
    return checks.run_checks(**kwargs)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/core/checks/registry.py", line 72, in run_checks
    new_errors = check(app_configs=app_configs)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/core/checks/urls.py", line 13, in check_url_config
    return check_resolver(resolver)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/core/checks/urls.py", line 23, in check_resolver
    return check_method()
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/urls/resolvers.py", line 407, in check
    for pattern in self.url_patterns:
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/utils/functional.py", line 48, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/urls/resolvers.py", line 588, in url_patterns
    patterns = getattr(self.urlconf_module, "urlpatterns", self.urlconf_module)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/utils/functional.py", line 48, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/urls/resolvers.py", line 581, in urlconf_module
    return import_module(self.urlconf_name)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 783, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/home/michael/PycharmProjects/ads3/ads_manager/ads_manager/urls.py", line 35, in <module>
    path('yandex_ads/', include("yandex_ads.urls", namespace="yandex_ads"), name="yandex_ads"),
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/urls/conf.py", line 34, in include
    urlconf_module = import_module(urlconf_module)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 783, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/home/michael/PycharmProjects/ads3/ads_manager/yandex_ads/urls.py", line 3, in <module>
    from .views import RequestYandexAds, ShowYandexAds, CreateYandexAds, CopyAdTemplate
  File "/home/michael/PycharmProjects/ads3/ads_manager/yandex_ads/views.py", line 6, in <module>
    from .forms import CopyForm
  File "/home/michael/PycharmProjects/ads3/ads_manager/yandex_ads/forms.py", line 12, in <module>
    class CopyForm(forms.Form):
  File "/home/michael/PycharmProjects/ads3/ads_manager/yandex_ads/forms.py", line 13, in CopyForm
    ad_template = forms.ChoiceField(choices=get_choices())
  File "/home/michael/PycharmProjects/ads3/ads_manager/yandex_ads/forms.py", line 8, in get_choices
    choices = [blank_choice] + [(str(item.id), str(item)) for item in YandexTextAdTemplates.objects.all()]
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/db/models/query.py", line 276, in __iter__
    self._fetch_all()
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/db/models/query.py", line 1261, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/db/models/query.py", line 57, in __iter__
    results = compiler.execute_sql(chunked_fetch=self.chunked_fetch, chunk_size=self.chunk_size)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/db/models/sql/compiler.py", line 1151, in execute_sql
    cursor.execute(sql, params)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/db/backends/utils.py", line 100, in execute
    return super().execute(sql, params)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/db/backends/utils.py", line 68, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/db/backends/utils.py", line 77, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/db/backends/utils.py", line 86, in _execute
    return self.cursor.execute(sql, params)
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/db/utils.py", line 90, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/home/michael/PycharmProjects/ads3/venv/lib/python3.8/site-packages/django/db/backends/utils.py", line 86, in _execute
    return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: relation "yandex_ads_yandextextadtemplates" does not exist
LINE 1: ...ndextextadtemplates"."audience_yandex_pixel" FROM "yandex_ad...

So, I commented the line out, organized empty list for choices. Migrations have been made.

But it I need the correct functionality and possibility to make migrations. What can I try next?

Answer 1

You cannot (at the very least should not) evaluate Querysets at module level which is what happens in your case as you call the get_choices function in your field definition:

ad_template = forms.ChoiceField(choices=get_choices())

Pass it a callable instead:

ad_template = forms.ChoiceField(choices=get_choices)

This way, the choices are built dynamically every time the form is instantiated which is what you actually want. See the docs on the choices parameter.