[1.5.x] Fixed #19895 -- Made second iteration over invalid queryset raise an exception too

When iteration over a queryset raised an exception, the result cache
remained initialized with an empty list, so subsequent iterations returned
an empty list instead of raising an exception

Backport of 2cd0edaa477b327024e4007c8eaf46646dcd0f21 from master.

2 files changed, 19 insertions, 2 deletions

diff --git a/django/db/models/query.py b/django/db/models/query.py
index 952739e939..b46528599e 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -102,7 +102,7 @@ class QuerySet(object):
             len(self)
 
         if self._result_cache is None:
-            self._iter = self.iterator()
+            self._iter = self._safe_iterator(self.iterator())
             self._result_cache = []
         if self._iter:
             return self._result_iter()
@@ -337,6 +337,18 @@ class QuerySet(object):
 
             yield obj
 
+    def _safe_iterator(self, iterator):
+        # ensure result cache is cleared when iterating over a queryset
+        # raises an exception
+        try:
+            for item in iterator:
+                yield item
+        except StopIteration:
+            raise
+        except Exception:
+            self._result_cache = None
+            raise
+
     def aggregate(self, *args, **kwargs):
         """
         Returns a dictionary containing the calculations (aggregation)
diff --git a/tests/modeltests/basic/tests.py b/tests/modeltests/basic/tests.py
index db8c079962..1dd1176d8b 100644
--- a/tests/modeltests/basic/tests.py
+++ b/tests/modeltests/basic/tests.py
@@ -2,7 +2,7 @@ from __future__ import absolute_import, unicode_literals
 
 from datetime import datetime
 
-from django.core.exceptions import ObjectDoesNotExist, MultipleObjectsReturned
+from django.core.exceptions import ObjectDoesNotExist, MultipleObjectsReturned, FieldError
 from django.db.models.fields import Field, FieldDoesNotExist
 from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
 from django.utils import six
@@ -639,3 +639,8 @@ class ModelTest(TestCase):
         Article.objects.bulk_create([Article(headline=lazy, pub_date=datetime.now())])
         article = Article.objects.get()
         self.assertEqual(article.headline, notlazy)
+
+    def test_invalid_qs_list(self):
+        qs = Article.objects.order_by('invalid_column')
+        self.assertRaises(FieldError, list, qs)
+        self.assertRaises(FieldError, list, qs)
\ No newline at end of file