merge from branches/clauseelement-nonzero

adds a __nonzero__ to _BinaryExpression to avoid faulty comparisons during hash
collisions (which only occur on Jython)
fixes #1547

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/sqlalchemy/orm/session.py b/lib/sqlalchemy/orm/session.py
index 002ccd509..760dfe81f 100644
--- a/lib/sqlalchemy/orm/session.py
+++ b/lib/sqlalchemy/orm/session.py
@@ -865,7 +865,7 @@ class Session(object):
                     return self.__binds[c_mapper.base_mapper]
                 elif c_mapper.mapped_table in self.__binds:
                     return self.__binds[c_mapper.mapped_table]
-            if clause:
+            if clause is not None:
                 for t in sql_util.find_tables(clause):
                     if t in self.__binds:
                         return self.__binds[t]