Fixed another instance like [12099]. Thanks again, Alex Gaynor.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12100 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/base.py

@@ -745,9 +745,7 @@ class Model(object):
             if not getattr(self, '_adding', False) and self.pk is not None:
                 qs = qs.exclude(pk=self.pk)
 
-            # This cute trick with extra/values is the most efficient way to
-            # tell if a particular query returns any results.
-            if qs.extra(select={'a': 1}).values('a').order_by():
+            if qs.exists():
                 errors.setdefault(field, []).append(
                     self.date_error_message(lookup_type, field, unique_for)
                 )