Problem : if the foreign key is null with ODMG

[蔡博至]

Hi!
    If the foreign key is null, the error occur.
    In my case , the departments of a company. The department object there is a 
super-department attribute.
It referance to another department. But the top department's super-department is none, 
it FK is null. It will raise error when retrieving the top department.
    How to solve this problem?
  
Thanks.
Dogie Tsai