Refs #26351 -- Added check hook to support database-related checks

Thanks Tim Graham and Shai Berger for the reviews.

django/core/checks/__init__.py

@@ -10,6 +10,7 @@ from .registry import Tags, register, run_checks, tag_exists
 # Import these to force registration of checks
 import django.core.checks.caches  # NOQA isort:skip
 import django.core.checks.compatibility.django_1_8_0  # NOQA isort:skip
+import django.core.checks.database  # NOQA isort:skip
 import django.core.checks.model_checks  # NOQA isort:skip
 import django.core.checks.security.base  # NOQA isort:skip
 import django.core.checks.security.csrf  # NOQA isort:skip

django/core/checks/database.py

@@ -0,0 +1,11 @@
+from django.db import connections
+
+from . import Tags, register
+
+
+@register(Tags.database)
+def check_database_backends(*args, **kwargs):
+    issues = []
+    for conn in connections.all():
+        issues.extend(conn.validation.check(**kwargs))
+    return issues

django/core/checks/registry.py

@@ -13,6 +13,7 @@ class Tags(object):
     admin = 'admin'
     caches = 'caches'
     compatibility = 'compatibility'
+    database = 'database'
     models = 'models'
     security = 'security'
     signals = 'signals'
@@ -70,6 +71,11 @@ class CheckRegistry(object):
         if tags is not None:
             checks = [check for check in checks
                       if hasattr(check, 'tags') and set(check.tags) & set(tags)]
+        else:
+            # By default, 'database'-tagged checks are not run as they do more
+            # than mere static code analysis.
+            checks = [check for check in checks
+                      if not hasattr(check, 'tags') or Tags.database not in check.tags]
 
         for check in checks:
             new_errors = check(app_configs=app_configs)

django/core/management/base.py

@@ -368,6 +368,9 @@ class BaseCommand(object):
                 translation.activate(saved_locale)
         return output
 
+    def _run_checks(self, **kwargs):
+        return checks.run_checks(**kwargs)
+
     def check(self, app_configs=None, tags=None, display_num_errors=False,
               include_deployment_checks=False, fail_level=checks.ERROR):
         """
@@ -376,7 +379,7 @@ class BaseCommand(object):
         If there are only light messages (like warnings), they are printed to
         stderr and no exception is raised.
         """
-        all_issues = checks.run_checks(
+        all_issues = self._run_checks(
             app_configs=app_configs,
             tags=tags,
             include_deployment_checks=include_deployment_checks,

django/core/management/commands/migrate.py

@@ -6,6 +6,7 @@ from collections import OrderedDict
 from importlib import import_module
 
 from django.apps import apps
+from django.core.checks import Tags, run_checks
 from django.core.management.base import BaseCommand, CommandError
 from django.core.management.sql import (
     emit_post_migrate_signal, emit_pre_migrate_signal,
@@ -56,6 +57,11 @@ class Command(BaseCommand):
             help='Creates tables for apps without migrations.',
         )
 
+    def _run_checks(self, **kwargs):
+        issues = run_checks(tags=[Tags.database])
+        issues.extend(super(Command, self).check(**kwargs))
+        return issues
+
     def handle(self, *args, **options):
 
         self.verbosity = options['verbosity']

django/db/backends/base/validation.py

@@ -1,9 +1,12 @@
 class BaseDatabaseValidation(object):
     """
-    This class encapsulates all backend-specific model validation.
+    This class encapsulates all backend-specific validation.
     """
     def __init__(self, connection):
         self.connection = connection
 
+    def check(self, **kwargs):
+        return []
+
     def check_field(self, field, **kwargs):
         return []