[1.5.x] Fixed #19362 -- Detected invalid use of @python_2_unicode_compatible.

Thanks m3wolf for the report and akaariai for reproducing the problem.

Backport of 2ea80b9.

django/db/models/base.py

@@ -416,6 +416,11 @@ class Model(six.with_metaclass(ModelBase, object)):
 
     def __str__(self):
         if not six.PY3 and hasattr(self, '__unicode__'):
+            if type(self).__unicode__ == Model.__str__:
+                klass_name = type(self).__name__
+                raise RuntimeError("%s.__unicode__ is aliased to __str__. Did"
+                                   " you apply @python_2_unicode_compatible"
+                                   " without defining __str__?" % klass_name)
             return force_text(self).encode('utf-8')
         return '%s object' % self.__class__.__name__
 

tests/modeltests/str/models.py

@@ -27,6 +27,14 @@ class Article(models.Model):
         # in ASCII.
         return self.headline
 
+@python_2_unicode_compatible
+class BrokenArticle(models.Model):
+    headline = models.CharField(max_length=100)
+    pub_date = models.DateTimeField()
+
+    def __unicode__(self):      # instead of __str__
+        return self.headline
+
 @python_2_unicode_compatible
 class InternationalArticle(models.Model):
     headline = models.CharField(max_length=100)

tests/modeltests/str/tests.py

@@ -7,7 +7,7 @@ from django.test import TestCase
 from django.utils import six
 from django.utils.unittest import skipIf
 
-from .models import Article, InternationalArticle
+from .models import Article, BrokenArticle, InternationalArticle
 
 
 class SimpleTests(TestCase):
@@ -21,6 +21,16 @@ class SimpleTests(TestCase):
         self.assertEqual(str(a), str('Area man programs in Python'))
         self.assertEqual(repr(a), str('<Article: Area man programs in Python>'))
 
+    @skipIf(six.PY3, "tests Model's default __str__ method under Python 2")
+    def test_broken(self):
+        # Regression test for #19362.
+        a = BrokenArticle.objects.create(
+            headline='Girl wins €12.500 in lottery',
+            pub_date=datetime.datetime(2005, 7, 28)
+        )
+        self.assertRaisesRegexp(RuntimeError, "Did you apply "
+            "@python_2_unicode_compatible without defining __str__\?", str, a)
+
     def test_international(self):
         a = InternationalArticle.objects.create(
             headline='Girl wins €12.500 in lottery',