Fixed #11863: added a Model.objects.raw() method for executing raw SQL queries and yield models.

See `docs/topics/db/raw.txt` for details.

Thanks to seanoc for getting the ball rolling, and to Russ for wrapping things up.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11921 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/query_utils.py

@@ -20,6 +20,13 @@ class CyclicDependency(Exception):
     """
     pass
 
+class InvalidQuery(Exception):
+    """
+    The query passed to raw isn't a safe query to use with raw.
+    """
+    pass
+
+
 class CollectedObjects(object):
     """
     A container that stores keys and lists of values along with remembering the