Malformed query generated by sqlalchemy when using a union with a CTE

I'm trying to use SQLAlchemy to generate a query that involves both CTEs and a union. Unfortunately whenever I apply a union the generated SQL does not include the defined CTE.

Here's the SQL including the WITH, without a UNION:

(Pdb) import sqlalchemy as sa  
(Pdb) print str(sa.select([sa.select([sa.sql.null().label('a1')](sa.select([sa.sql.null().label('a1'))).cte('bb')]))

WITH bb AS
(SELECT NULL AS a1)
 SELECT bb.a1
FROM bb

Now the same query but with a call to union:

(Pdb) print str(sa.select([sa.select([sa.sql.null().label('a1')](sa.select([sa.sql.null().label('a1'))).cte('bb')]).union_all(sa.select([sa.sql.null().label('b1')](sa.sql.null().label('b1')))))

SELECT bb.a1
FROM bb UNION ALL SELECT NULL AS b1

I'd expect the query with the union call to still include the WITH, but it does not.

Here is a patch that, while needs refactoring, generates the CTE as expected:

--- sqlalchemy.orig/sql/compiler.py 2012-05-20 17:39:41.000000000 +0000
+++ sqlalchemy/sql/compiler.py  2012-05-20 17:39:00.000000000 +0000
@@ -562,6 +562,15 @@
         text += (cs._limit is not None or cs._offset is not None) and \
                         self.limit_clause(cs) or ""

+        if self.ctes and \
+            compound_index==1 and not entry:
+            cte_text = self.get_cte_preamble(self.ctes_recursive) + " "
+            cte_text += ", \n".join(
+                [for txt in self.ctes.values()](txt)
+            )
+            cte_text += "\n "
+            text = cte_text + text
+
         self.stack.pop(-1)
         if asfrom and parens:
             return "(" + text + ")"

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