Fixed #23615 -- Validate that a Model instance's "check" attribute is a method.

The "check" name is a reserved word used by Django's check framework,
and cannot be redefined as something else other than a method, or the check
framework will raise an error.

This change amends the django.core.checks.model_check.check_all_models()
function, so that it verifies that a model instance's attribute "check"
is actually a method. This new check is assigned the id "models.E020".

tests/check_framework/tests.py

@@ -8,11 +8,13 @@ from django.apps import apps
 from django.conf import settings
 from django.core import checks
 from django.core.checks import Error, Warning
+from django.core.checks.model_checks import check_all_models
 from django.core.checks.registry import CheckRegistry
 from django.core.checks.compatibility.django_1_6_0 import check_1_6_compatibility
 from django.core.checks.compatibility.django_1_7_0 import check_1_7_compatibility
 from django.core.management.base import CommandError
 from django.core.management import call_command
+from django.db import models
 from django.db.models.fields import NOT_PROVIDED
 from django.test import TestCase
 from django.test.utils import override_settings, override_system_checks
@@ -327,3 +329,56 @@ class SilencingCheckTests(TestCase):
 
         self.assertEqual(out.getvalue(), 'System check identified no issues (1 silenced).\n')
         self.assertEqual(err.getvalue(), '')
+
+
+class CheckFrameworkReservedNamesTests(TestCase):
+
+    def setUp(self):
+        self.current_models = apps.all_models[__package__]
+        self.saved_models = set(self.current_models)
+
+    def tearDown(self):
+        for model in (set(self.current_models) - self.saved_models):
+            del self.current_models[model]
+        apps.clear_cache()
+
+    @override_settings(SILENCED_SYSTEM_CHECKS=['models.E020'])
+    def test_model_check_method_not_shadowed(self):
+        class ModelWithAttributeCalledCheck(models.Model):
+            check = 42
+
+        class ModelWithFieldCalledCheck(models.Model):
+            check = models.IntegerField()
+
+        class ModelWithRelatedManagerCalledCheck(models.Model):
+            pass
+
+        class ModelWithDescriptorCalledCheck(models.Model):
+            check = models.ForeignKey(ModelWithRelatedManagerCalledCheck)
+            article = models.ForeignKey(ModelWithRelatedManagerCalledCheck, related_name='check')
+
+        expected = [
+            Error(
+                "The 'ModelWithAttributeCalledCheck.check()' class method is "
+                "currently overridden by 42.",
+                hint=None,
+                obj=ModelWithAttributeCalledCheck,
+                id='models.E020'
+            ),
+            Error(
+                "The 'ModelWithRelatedManagerCalledCheck.check()' class method is "
+                "currently overridden by %r." % ModelWithRelatedManagerCalledCheck.check,
+                hint=None,
+                obj=ModelWithRelatedManagerCalledCheck,
+                id='models.E020'
+            ),
+            Error(
+                "The 'ModelWithDescriptorCalledCheck.check()' class method is "
+                "currently overridden by %r." % ModelWithDescriptorCalledCheck.check,
+                hint=None,
+                obj=ModelWithDescriptorCalledCheck,
+                id='models.E020'
+            ),
+        ]
+
+        self.assertEqual(check_all_models(), expected)