https://github.com/openmrs/openmrs-core/blob/1.9.x/api/src/main/resources/org/openmrs/api/db/hibernate/Order.hbm.xml#L57 

https://github.com/openmrs/openmrs-core/blob/1.9.x/api/src/main/resources/org/openmrs/api/db/hibernate/Order.hbm.xml#L77

Only order_type_id has not-null=true