Improved warning message when reloading models. Refs #23621.

Thanks dfunckt and Tim Graham.

django/apps/registry.py

@@ -211,7 +211,9 @@ class Apps(object):
             if (model.__name__ == app_models[model_name].__name__ and
                     model.__module__ == app_models[model_name].__module__):
                 warnings.warn(
-                    "Model '%s.%s' was already registered." % (model_name, app_label),
+                    "Model '%s.%s' was already registered. "
+                    "Reloading models is not advised as it can lead to inconsistencies, "
+                    "most notably with related models." % (model_name, app_label),
                     RuntimeWarning, stacklevel=2)
             else:
                 raise RuntimeError(

tests/apps/tests.py

@@ -234,7 +234,10 @@ class AppsTests(TestCase):
             type(str("SouthPonies"), (models.Model,), body)
             self.assertEqual(len(w), 1)
             self.assertTrue(issubclass(w[-1].category, RuntimeWarning))
-            self.assertEqual(str(w[-1].message), "Model 'southponies.apps' was already registered.")
+            self.assertEqual(str(w[-1].message),
+                 "Model 'southponies.apps' was already registered. "
+                 "Reloading models is not advised as it can lead to inconsistencies, "
+                 "most notably with related models.")
 
         # If it doesn't appear to be a reloaded module then we expect
         # a RuntimeError.