[SQLObject] Saving a rol with a derivative atribute

Hello all.

I would like to know if it is possible to create a model class with a
default attribute derived from another.

An example:

class Post(SQLObject):
*title = StringCol()*
content = StringCol()
create_time = DateTimeCol(default=datetime.now)
tags = RelatedJoin('Tag')
author = ForeignKey('Author')
*slug = StringCol(default=title.replace(" ", "-"))*


Thank you very much.