[1.7.x] 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".

Conflicts:
	docs/ref/checks.txt

Backport of a5c77417a6 from master

1 files changed, 24 insertions, 9 deletions

diff --git a/django/core/checks/model_checks.py b/django/core/checks/model_checks.py
index 3f5a36b41b..3d0d56af58 100644
--- a/django/core/checks/model_checks.py
+++ b/django/core/checks/model_checks.py
@@ -1,28 +1,43 @@
 # -*- coding: utf-8 -*-
 from __future__ import unicode_literals
 
-from itertools import chain
+import inspect
 import types
 
 from django.apps import apps
-
-from . import Error, Tags, register
+from django.core.checks import Error, Tags, register
 
 
 @register(Tags.models)
 def check_all_models(app_configs=None, **kwargs):
-    errors = [model.check(**kwargs)
-        for model in apps.get_models()
-        if app_configs is None or model._meta.app_config in app_configs]
-    return list(chain(*errors))
+    errors = []
+    for model in apps.get_models():
+        if app_configs is None or model._meta.app_config in app_configs:
+            if not inspect.ismethod(model.check):
+                errors.append(
+                    Error(
+                        "The '%s.check()' class method is "
+                        "currently overridden by %r." % (
+                            model.__name__, model.check),
+                        hint=None,
+                        obj=model,
+                        id='models.E020'
+                    )
+                )
+            else:
+                errors.extend(model.check(**kwargs))
+    return errors
 
 
 @register(Tags.models, Tags.signals)
 def check_model_signals(app_configs=None, **kwargs):
-    """Ensure lazily referenced model signals senders are installed."""
+    """
+    Ensure lazily referenced model signals senders are installed.
+    """
+    # Avoid circular import
     from django.db import models
-    errors = []
 
+    errors = []
     for name in dir(models.signals):
         obj = getattr(models.signals, name)
         if isinstance(obj, models.signals.ModelSignal):