What is the right way to handle a hierarchy of same-class objects?

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I have a class called (MyTest::)Category.

The table of Category has a field ("father") containing the id of the 
father category of each category.

Objects that don't have a father, have 0 in that field.

The problem is that if I check for the truth value of $obj->father, I 
get a true value regardless of whether that record has a father or not.

e.g.>  if ($obj->father) { print "has father" }

The reason for this is that $obj->father returns an object reference (a 
reference of a non-existant object nevertheless).

My class definition is as follows, and my executable is run as a 
stand-alone program:

package MyTest::Category;
use base 'MyTest::DBI';
__PACKAGE__->set_up_table('tblCategories');
__PACKAGE__->has_a(father => 'MyTest::Category');

Yet, when I call "retrieve" with a non-existant key, I get null instead 
of an object reference.

My questions:
1) Is this how CDBI is supposed to work or is there a contradiction 
between "retrieve" and "father"?
2) Should I maybe be using might_have instead of has_a for this case?

Thanks a lot and sorry for bothering you once again,
- Karjala