swev-id: django__django-7530 | Fix makemigrations router.allow_migrate() consistency-check model selection

[rowan-stein]

This PR addresses Issue #543: makemigrations router.allow_migrate() calls for consistency checks use incorrect (app_label, model) pairs.

Summary

• Replace per-app_label x all-models evaluation with per-app_config migratable models evaluation using router.get_migratable_models(...), avoiding invalid (app_label, model_name) combinations during consistency checks.
• Aims to preserve behavior: run MigrationLoader.check_consistent_history(connection) only for DB aliases that have at least one migratable model.

Upstream reference: django#7530

Reproduction steps (manual)

• Environment: Use a Python version compatible with this Django snapshot (e.g., Python 3.6). Ensure PYTHONPATH points to this repo so manage.py uses this code.
• Create a demo project with sharded router and two databases as follows:

# Setup (example)
export PYTHONPATH=/workspace/django
# Create project/apps
/workspace/django/django/bin/django-admin.py startproject shard_demo /workspace/shard_demo
cd /workspace/shard_demo
python manage.py startapp app_a
python manage.py startapp app_b

# shard_router.py
cat <<'ROUTER' > shard_router.py
class ShardRouter(object):
    routing_map = {
        ('app_a', 'AModel'): 'default',
        ('app_b', 'BModel'): 'shard_b',
    }
    def allow_migrate(self, db, app_label, model_name=None, **hints):
        if model_name is None:
            return None
        target = self.routing_map.get((app_label, model_name))
        if target is None:
            raise ValueError('Invalid app/model pair: %s.%s' % (app_label, model_name))
        return target == db
ROUTER

# app_a/models.py
cat <<'A' > app_a/models.py
from django.db import models
class AModel(models.Model):
    name = models.CharField(max_length=20)
A

# app_b/models.py
cat <<'B' > app_b/models.py
from django.db import models
class BModel(models.Model):
    label = models.CharField(max_length=20)
B

# Update settings: add apps, add a second DB, enable router
python - <<'PY'
from pathlib import Path
s = Path('shard_demo/settings.py')
t = s.read_text()
t = t.replace(
    "    'django.contrib.staticfiles',\n]\n",
    "    'django.contrib.staticfiles',\n    'app_a',\n    'app_b',\n]\n",
)
t = t.replace(
    "'NAME': os.path.join(BASE_DIR, 'db.sqlite3'),\n    }\n}\n",
    "'NAME': os.path.join(BASE_DIR, 'db.sqlite3'),\n    },\n    'shard_b': {\n        'ENGINE': 'django.db.backends.sqlite3',\n        'NAME': os.path.join(BASE_DIR, 'db_shard_b.sqlite3'),\n    }\n}\n",
)
if "DATABASE_ROUTERS" not in t:
    t += "\nDATABASE_ROUTERS = ['shard_router.ShardRouter']\n"
s.write_text(t)
PY

# Reproduce
python manage.py makemigrations

Observed failure (representative stack trace before fix)

Traceback (most recent call last):
  File ".../django/core/management/commands/makemigrations.py", line 106, in handle
    router.allow_migrate(connection.alias, app_label, model_name=model._meta.object_name)
  File ".../django/db/utils.py", line 300, in allow_migrate
    allow = method(db, app_label, **hints)
  File ".../shard_demo/shard_router.py", line XX, in allow_migrate
    raise ValueError("Invalid app/model pair: %s.%s" % (app_label, model_name))
ValueError: Invalid app/model pair: app_b.AModel

Validation after fix

• With this change, makemigrations should complete without raising due to invalid app/model pairs.
• For each DB alias, loader.check_consistent_history(connection) runs iff at least one model is migratable for that alias across the installed apps.

Notes

• CI: Not run intentionally for this task.
• Base branch: django__django-7530 (as requested).
• Related issue: makemigrations router.allow_migrate() calls for consistency checks use incorrect (app_label, model) pairs #543

[noa-lucent]

The production change looks reasonable, but it changes the way we invoke . The regression test in still asserts the old call signature, so the suite will fail until we update it.

[noa-lucent]

Thanks for adding the explicit assertions in ; the regression now covers the new contract (model hint + lowercase name). Looks good to me.

[noa-lucent]

The migrations logic and tests look good, but the new uses of collections.abc break Django’s supported Python 2.7 runtimes. Please keep using the existing aliases (or route through six.moves.collections_abc) so we don’t regress compatibility.

[noa-lucent]

Compatibility fix looks good—the usage now falls back cleanly on Python 2, so we preserve support while keeping the migrations change and tests intact. Thanks!