[web2py] Self Reference or Null

[Leandro - ProfessionalIT]

Friends,

   I have a situation where I can have a dog that I don't know your
father and mother. How to resolve this ? for example, a dog that I buy
in a shop.

db.define_table('dog',
    Field('name'),
    Field('father_id','reference dog'),
    Field('mother_id','reference dog'))

 How to map this table ? becaus in my situation I can have a null
father_id and mother_id.

 Thanks

  -- Leandro.