Fixed #7298: prevent update() on sliced QuerySet since UPDATE doesn't…

… reliably support LIMIT/OFFSET. Thanks, George Vilches.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7601 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/query.py

@@ -292,6 +292,8 @@ def update(self, **kwargs):
         Updates all elements in the current QuerySet, setting all the given
         fields to the appropriate values.
         """
+        assert self.query.can_filter(), \
+                "Cannot update a query once a slice has been taken."
         query = self.query.clone(sql.UpdateQuery)
         query.add_update_values(kwargs)
         query.execute_sql(None)
@@ -306,6 +308,8 @@ def _update(self, values):
         code (it requires too much poking around at model internals to be
         useful at that level).
         """
+        assert self.query.can_filter(), \
+                "Cannot update a query once a slice has been taken."
         query = self.query.clone(sql.UpdateQuery)
         query.add_update_fields(values)
         query.execute_sql(None)

tests/modeltests/update/models.py

@@ -63,5 +63,12 @@ def __unicode__(self):
 >>> DataPoint.objects.values('value').distinct()
 [{'value': u'thing'}]
 
+We do not support update on already sliced query sets.
+
+>>> DataPoint.objects.all()[:2].update(another_value='another thing')
+Traceback (most recent call last):
+    ...
+AssertionError: Cannot update a query once a slice has been taken.
+
 """
 }