[1.7.x] Fixed #23621 -- Warn for duplicate models when a module is reloaded.

Previously a RuntimeError was raised every time two models clashed
in the app registry. This prevented reloading a module in a REPL;
while it's not recommended to do so, we decided not to forbid this
use-case by turning the error into a warning.

Thanks dfunckt and Sergey Pashinin for the initial patches.

Backport of 8c4ca16c65 and b62f72498a from master

1 files changed, 39 insertions, 0 deletions

diff --git a/tests/apps/tests.py b/tests/apps/tests.py
index 513ed32039..1a308efb6a 100644
--- a/tests/apps/tests.py
+++ b/tests/apps/tests.py
@@ -3,6 +3,7 @@ from __future__ import unicode_literals
 import os
 import sys
 from unittest import skipUnless
+import warnings
 
 from django.apps import apps, AppConfig
 from django.apps.registry import Apps
@@ -208,6 +209,44 @@ class AppsTests(TestCase):
             apps.get_model("apps", "SouthPonies")
         self.assertEqual(new_apps.get_model("apps", "SouthPonies"), temp_model)
 
+    def test_model_clash(self):
+        """
+        Test for behavior when two models clash in the app registry.
+        """
+        new_apps = Apps(["apps"])
+        meta_contents = {
+            'app_label': "apps",
+            'apps': new_apps,
+        }
+
+        body = {}
+        body['Meta'] = type(str("Meta"), tuple(), meta_contents)
+        body['__module__'] = TotallyNormal.__module__
+        type(str("SouthPonies"), (models.Model,), body)
+
+        # When __name__ and __module__ match we assume the module
+        # was reloaded and issue a warning. This use-case is
+        # useful for REPL. Refs #23621.
+        body = {}
+        body['Meta'] = type(str("Meta"), tuple(), meta_contents)
+        body['__module__'] = TotallyNormal.__module__
+        with warnings.catch_warnings(record=True) as w:
+            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. "
+                 "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.
+        body = {}
+        body['Meta'] = type(str("Meta"), tuple(), meta_contents)
+        body['__module__'] = TotallyNormal.__module__ + '.whatever'
+        with six.assertRaisesRegex(self, RuntimeError,
+                "Conflicting 'southponies' models in application 'apps':.*"):
+            type(str("SouthPonies"), (models.Model,), body)
 
 class Stub(object):
     def __init__(self, **kwargs):