Migrate command runs system checks regardless of the value of requires_system_checks

[Jacob Walls]

Without --skip-checks=False, the migrate command runs system checks regardless of the value of requires_system_checks, which is [] by ​default (itself a little misleading) but ultimately something I would like to be able to override at the project level. The fact that [] is not respected and not overridable seems like a bug, and AFAIK it's an asymmetry with other commands. (EDIT: turns out, runserver behaves in a similar way, with a ​comment from 17 years ago, predating migrations, explaining there is a separate mechanism from the one that became requires_system_checks that runs the system checks. I think we can improve the story here.)

---

• I wrote a system check following the ​documented example inside Model.check().
• I did this as part of a feature adding a new column to my model, and I queried this column in my check. I made the necessary migration.
• During manage.py migrate my check ran before my column was added, raising ProgrammingError.
• I verified --skip-checks works like a charm, but I don't want to impose cryptic failures on my fellow developers who expect plain migrate to work.
• I attempted to override the migrate command in my project, like this:

/app/management/commands/migrate.py

from django.core.checks.registry import registry
from django.core.management.commands.migrate import Command as MigrateCommand


class Command(MigrateCommand):
    # Silence model checks that may depend on new columns.
    requires_system_checks = list(registry.tags_available() - {"models"})

• Result: no change: ProgrammingError

• Then, I added this patch to Django. (--skip-checks still works; here, I have to avoid it being added again):

  django/core/management/commands/migrate.py

  @@ -19 +19 @@
       help = (
           "Updates database schema. Manages both apps with migrations and those without."
       )
  -    requires_system_checks = []
  +    requires_system_checks = "__all__"
   
       def add_arguments(self, parser):
  -        parser.add_argument(
  -            "--skip-checks",
  -            action="store_true",
  -            help="Skip system checks.",
  -        )
           parser.add_argument(
               "app_label",
               nargs="?",
  @@ -99 +94 @@
       def handle(self, *args, **options):
           database = options["database"]
           if not options["skip_checks"]:
  -            self.check(databases=[database])
  +            self.check(tags=self.requires_system_checks, databases=[database])
   
           self.verbosity = options["verbosity"]
           self.interactive = options["interactive"]

• Result: check skipped as expected via my project-level override of the migrate command.

---

I suppose there's another question here about whether Tags.models checks should run as part of the migrate command, or whether the change I made at the project level should be pulled into core, given that this is a reasonably realistic scenario for development? I can take that to the forum later. But at the moment, I'm just hoping to get the override working. Happy to PR this if welcome :-)