Fixed #7210 -- Added F() expressions to query language. See the documentation for details on usage.

Many thanks to:
    * Nicolas Lara, who worked on this feature during the 2008 Google Summer of Code.
    * Alex Gaynor for his help debugging and fixing a number of issues.
    * Malcolm Tredinnick for his invaluable review notes.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9792 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/query_utils.py

@@ -17,6 +17,9 @@ class QueryWrapper(object):
     def __init__(self, sql, params):
         self.data = sql, params
 
+    def as_sql(self, qn=None):
+        return self.data
+
 class Q(tree.Node):
     """
     Encapsulates filters as objects that can then be combined logically (using