[5.0] Add db_default= parameter to models Field classes

[Skorpyon]

Django 5 feature update.

Database-computed default values - db_default arg and attr on model Field.

Tests not written, looks like it haven't things to test.

Full Django 5 updates list.

[intgr]

Thanks! Let's avoid using Any if possible.

[intgr]

Tests not written, looks like it haven't things to test.

Yeah, don't worry about tests for .pyi files. Unless it's a particularly complicated case of decorators or generics.

[Skorpyon]

@intgr may you advice, please?

Python 3.12.0 (main, Nov 17 2023, 17:15:54) [Clang 15.0.0 (clang-1500.0.40.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> print("\n".join(sys.path))

/Users/anton/.pyenv/versions/3.12.0/lib/python312.zip
/Users/anton/.pyenv/versions/3.12.0/lib/python3.12
/Users/anton/.pyenv/versions/3.12.0/lib/python3.12/lib-dynload
/Users/anton/Documents/projects/github/django-stubs/.venv/lib/python3.12/site-packages
/Users/anton/Documents/projects/github/django-stubs
/Users/anton/Documents/projects/github/django-stubs/ext

But bash ./scripts/stubtest.sh gives:

error: django failed to find stubs
error: django.apps failed to find stubs
error: django.apps.config failed to find stubs
...
note: unused allowlist entry django.views.static.DEFAULT_DIRECTORY_INDEX_TEMPLATE
note: unused allowlist entry django.views.static.builtin_template_path
Found 3010 errors (checked 888 modules)

I stuck, django-stubs installed:

> pip freeze
...
distlib==0.3.8
Django==5.0
-e git+ssh://git@github.com/Skorpyon/django-stubs.git@2cf7a510b57c47d2f56ece659cf4796e0f5fb8f3#egg=django_stubs
-e git+ssh://git@github.com/Skorpyon/django-stubs.git@2cf7a510b57c47d2f56ece659cf4796e0f5fb8f3#egg=django_stubs_ext&subdirectory=ext
filelock==3.13.1
...

[intgr]

But bash ./scripts/stubtest.sh gives:
Found 3010 errors (checked 888 modules)

Try removing .mypy_cache before running stubtest. There seems to be a bug, but I haven't bothered to debug/report it yet.

[Skorpyon]

Here we go...
@intgr may you check files.pyi, please, not sure that it is correct. FileField haven't _ST generic in declaration.

[Skorpyon]

I remove db_default from ManyToManyField.__init__() args at all.

Technically it may be passed to init, but latest Django codebase written so ugly, without any forward thinking.
That feature should be realized using some thing like DbDefaultMixin, adding db_default behaviour to specific fields only, but in Django it realized on top level, added to top parent class Field, so db_default available for all field subclasses, no matter have it sense or no.

If db_default have been set in ManyToManyField Django producing pretty fun SQL.

class DBDefaultModel(models.Model):
    created = models.DateField(db_default=Now())
    circumference = models.FloatField(db_default=2 * Pi())
    default_value = models.CharField(max_length=10, db_default=None)
    manyfield = models.ManyToManyField(BlogPost, db_default=123)

>>> from server.apps.main.models import DBDefaultModel
>>> mm = DBDefaultModel()
>>> mm.save()

INSERT INTO "main_dbdefaultmodel" ("created",  "circumference",  "default_value")
VALUES (DEFAULT, DEFAULT, DEFAULT)
RETURNING 
    "main_dbdefaultmodel"."id",
    "main_dbdefaultmodel"."created",
    "main_dbdefaultmodel"."circumference",
    "main_dbdefaultmodel"."default_value",
    "main_dbdefaultmodel"."manyfield";

django.db.utils.ProgrammingError: column main_dbdefaultmodel.manyfield does not exist
СТРОКА 1: ...mference", "main_dbdefaultmodel"."default_value", "main_dbde...

So Django trying retrieve .manyfield column value, thats not exists for sure, because it is many-to-many descriptor with intermediate table.

[intgr]

I'll leave this open for a few days in case another maintainer wants to chime in. If not, we can merge this.

[intgr]

Sorry for sitting on this for so long.

When I was thinking about this later, I realized that _ST typevar is usually unbound in model declarations (e.g. field = CharField(db_default=...) rather than field = CharField[str, str](db_default=...); the typevars are resolved later by mypy plugin.

In mypy, this doesn't cause errors at usage location because unbound typevars default to Any in mypy. But I wasn't sure about pyright.

I tested this now, seems that pyright handles it the same way: https://pyright-play.net/?pyrightVersion=1.1.353&pythonVersion=3.12&pythonPlatform=Linux&strict=true&locale=en&code=GYJw9gtgBALgngBwJYDsDmUkQWEMoDiApikSEgMYA0UAKokQGoCGIAUG7VALx0MsgAFACJawgJQcKAG2YBnOVABCsgBYAxJEWkATQcVLkKAbVoBdcQC42UW1B1FgUAPrPUSGK8FztwGjoAjZwdgZgBXaRhLOigAHygAOTBSHigAOgzxKABaAD5E5KJojLSONmAwMFSVZg0tXUFA4MdwyO4ARgAmAGZJAGI7QaG7AD0x8YmJqBQqsnB2IA

I think we can improve this later with typevar defaults on Field classes. There are other reasons as well why that's useful. So I think it's OK to merge this as is.