def update_session(self, user_id, session_id, patch_doc):
valid_patch = SessionDoc.create_patch(patch_doc)
# check that document exists
assert (self.session_manager.get(user_id, session_id))
version = self.session_manager.update(session_id, valid_patch)
logging.info(_i18n._LI('Session %(id)s updated to version %(version)s' %
{'id': session_id, 'version': version}))
return version
