django-constance: Unable to run migrations from scratch

The problem

I have a project with 50+ migrations in it and recently decided I want to use Constance for a couple of settings.

Everything was OK while developing/adding dynamic settings. I guess it’s due to the fact that I setup the Constance config first, did the migration and only then start to use dynamic settings values.

Although when I want to do a test deploy (i.e. when starting up a new Vagrant VM) and run all the migrations from scratch it fails. I am unable to run python manage.py migrate database, python manage.py migrate or even python manage.py showmigrations.

  File "/vagrant/my_app/views.py", line 138, in MyViewSet
    my_variable = constance_config.MY_VARIABLE
  File "/home/ubuntu/.virtualenvs/venv/lib/python3.5/site-packages/django/utils/functional.py", line 239, in inner
    return func(self._wrapped, *args)
  File "/home/ubuntu/.virtualenvs/venv/lib/python3.5/site-packages/constance/base.py", line 19, in __getattr__
    result = self._backend.get(key)
  File "/home/ubuntu/.virtualenvs/venv/lib/python3.5/site-packages/constance/backends/database/__init__.py", line 74, in get
    value = self._model._default_manager.get(key=key).value
  File "/home/ubuntu/.virtualenvs/venv/lib/python3.5/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/home/ubuntu/.virtualenvs/venv/lib/python3.5/site-packages/django/db/models/query.py", line 374, in get
    num = len(clone)
  File "/home/ubuntu/.virtualenvs/venv/lib/python3.5/site-packages/django/db/models/query.py", line 232, in __len__
    self._fetch_all()
  File "/home/ubuntu/.virtualenvs/venv/lib/python3.5/site-packages/django/db/models/query.py", line 1118, in _fetch_all
    self._result_cache = list(self._iterable_class(self))
  File "/home/ubuntu/.virtualenvs/venv/lib/python3.5/site-packages/django/db/models/query.py", line 53, in __iter__
    results = compiler.execute_sql(chunked_fetch=self.chunked_fetch)
  File "/home/ubuntu/.virtualenvs/venv/lib/python3.5/site-packages/django/db/models/sql/compiler.py", line 894, in execute_sql
    raise original_exception
  File "/home/ubuntu/.virtualenvs/venv/lib/python3.5/site-packages/django/db/models/sql/compiler.py", line 884, in execute_sql
    cursor.execute(sql, params)
  File "/home/ubuntu/.virtualenvs/venv/lib/python3.5/site-packages/django/db/backends/utils.py", line 80, in execute
    return super(CursorDebugWrapper, self).execute(sql, params)
  File "/home/ubuntu/.virtualenvs/venv/lib/python3.5/site-packages/django/db/backends/utils.py", line 65, in execute
    return self.cursor.execute(sql, params)
  File "/home/ubuntu/.virtualenvs/venv/lib/python3.5/site-packages/django/db/utils.py", line 94, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/home/ubuntu/.virtualenvs/venv/lib/python3.5/site-packages/django/utils/six.py", line 685, in reraise
    raise value.with_traceback(tb)
  File "/home/ubuntu/.virtualenvs/venv/lib/python3.5/site-packages/django/db/backends/utils.py", line 65, in execute
    return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: relation "constance_config" does not exist
LINE 1: ...ce_config"."key", "constance_config"."value" FROM "constance...

So a table that is about to be created is referenced already.

System configuration

Django version: 1.11.3
Python version: 3.5.2,

About this issue

Original URL
State: closed
Created 7 years ago
Comments: 24 (11 by maintainers)

Commits related to this issue

database: Ignore operation errors (#229) — committed to whs/django-constance by whs 7 years ago
Merge pull request #230 from whs/master database: Ignore operation errors (#229) — committed to jazzband/django-constance by camilonova 7 years ago
Merge pull request #18 from jazzband/master database: Ignore operation errors (#229) — committed to khchine5/django-constance by khchine5 7 years ago

Most upvoted comments

Yes, same as #227.

I was able to get around it doing this:

from constance import config as constance_config
...
def get_constance_setting(setting):
    """
    A dirty workaround to enable Constance do the inital migration and not
    fail because the table that is about to be created doesn't exist yet.

    @setting (str) setting to retrieve

    @returns given setting value
    """
    try:
        return getattr(constance_config, setting)
    except ProgrammingError as exc:
        if str(exc).startswith('relation "constance_config" does not exist'):
            # TODO logging a waring message wouldn't hurt.
            pass
        else:
            raise exc

This way you can leave your code (imports, …) as is, but you have to access Constance settings through get_constance_setting('MY_SETTING') and not directly as constance_config.MY_SETTING.

DusanMadar on Jan 9, 2018