Add a system check to prohibit models that start with an underscore

[Victor Porton]

Running manage.py test for the attached Django project causes a meaningless exception.

It is probably because _UsersGroup class name starts with underscore.

$ python3 manage.py test 
Creating test database for alias 'default'...
E
======================================================================
ERROR: test_users (auth_app.tests.UsersGroupTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/porton/Projects/gnosis-error/auth_app/tests.py", line 13, in setUp
    self.A.subgroups.add(self.B)
  File "/usr/lib/python3/dist-packages/django/db/models/fields/related_descriptors.py", line 881, in add
    self._add_items(self.source_field_name, self.target_field_name, *objs)
  File "/usr/lib/python3/dist-packages/django/db/models/fields/related_descriptors.py", line 1025, in _add_items
    .values_list(target_field_name, flat=True)
  File "/usr/lib/python3/dist-packages/django/db/models/query.py", line 731, in values_list
    clone = self._values(*fields)
  File "/usr/lib/python3/dist-packages/django/db/models/query.py", line 714, in _values
    query.add_fields(field_names, True)
  File "/usr/lib/python3/dist-packages/django/db/models/sql/query.py", line 1630, in add_fields
    name.split(LOOKUP_SEP), opts, alias, allow_many=allow_m2m)
  File "/usr/lib/python3/dist-packages/django/db/models/sql/query.py", line 1402, in setup_joins
    names, opts, allow_many, fail_on_missing=True)
  File "/usr/lib/python3/dist-packages/django/db/models/sql/query.py", line 1327, in names_to_path
    "Choices are: %s" % (name, ", ".join(available)))
django.core.exceptions.FieldError: Cannot resolve keyword 'to' into field. Choices are: from__usersgroup, from__usersgroup_id, id, to__usersgroup, to__usersgroup_id

----------------------------------------------------------------------
Ran 1 test in 0.002s

FAILED (errors=1)
Destroying test database for alias 'default'...

[Victor Porton]

A project which triggers a bug

[Tim Graham]

My first instinct is to add a system check to prohibit models with underscores in the name. Maybe we should first ask if anyone is successfully using such models.

[Tim Graham]

I ​posted on django-developers to ask if anyone is using underscores in model names.

[Tim Graham]

The mailing list indicates that underscores are used in model names, so it's probably enough to prevent underscores at the start of a name.

[Quentin Fulsher]

If no one minds, I would like to work on this ticket. It seems like this project would just require adding a system check for the underscore and then adding tests to check the code is validating the names correctly.

[Tim Graham]

Yes, you may claim the ticket, no need to ask.

As Michal suggested on the mailing list, 'I think it makes sense to also check for and flag as errors model names starting or ending with underscores, or containing double underscores, since those can lead to invalid field names."

[Quentin Fulsher]

I submitted a pull request for this ticket here:

​https://github.com/django/django/pull/7370

I am having some issues getting the test cases to run correctly. Its probably just a misunderstanding of how the tests actually work. I go over it in more detail in the pull request.

[Tim Graham <timograham@…>]

In f62abfc:

Fixed #27295 -- Added a system check to prohibit model names that start or end with an underscore or contain double underscores.