ORM User does not save shopping cart values

When using ORM instead of automodeler on the Model_Vendo_Core_User_ORM class, the $_shopping_cart value is never set. I tried doing it like its done on the automodeler code, setting it in the construct, $this->_shopping_cart = new Model_Order; But ORM does not save the property when serializing for session save, hence all the values are lost. Any ideas on how to overcome this?
