Made negative indexing on QuerySet instances raise an assertion error…

… (previously

it just returned incorrect results).


git-svn-id: http://code.djangoproject.com/svn/django/trunk@2992 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/query.py

@@ -95,6 +95,9 @@ def __iter__(self):
 
     def __getitem__(self, k):
         "Retrieve an item or slice from the set of results."
+        assert (not isinstance(k, slice) and (k >= 0)) \
+            or (isinstance(k, slice) and (k.start is None or k.start >= 0) and (k.stop is None or k.stop >= 0)), \
+            "Negative indexing is not supported."
         if self._result_cache is None:
             if isinstance(k, slice):
                 # Offset:

tests/modeltests/basic/models.py

@@ -283,6 +283,16 @@ def __repr__(self):
     ...
 AssertionError: Cannot combine queries once a slice has been taken.
 
+# Negative slices are not supported, due to database constraints.
+# (hint: inverting your ordering might do what you need).
+>>> Article.objects.all()[-1]
+Traceback (most recent call last):
+    ...
+AssertionError: Negative indexing is not supported.
+>>> Article.objects.all()[0:-5]
+Traceback (most recent call last):
+    ...
+AssertionError: Negative indexing is not supported.
 
 # An Article instance doesn't have access to the "objects" attribute.
 # That's only available on the class.