[web2py] database foreign key question

[bussiere adrien]

Thanks to you my project make nice progress :
http://img26.imageshack.us/img26/1271/loginscreena.png

But i need to know some things :
here is my database :


db.define_table('Skill',
 Field('Nom', unique=True),
 format = '%(Nom)s')

db.define_table('SkillLevel',
 Field('Level', 'integer'),
 Field('Skill', db.Skill),
 Field('Parent', db.Skill),
 format = '%(Skill)s')


and :
if db(db.SkillLevel.Skill.Nom == ''toto").count() == 0:

Don't work i says to me skill have no nom attribute


then an other question :
db.define_table('Demande',
 Field('Topic', unique=True),
 Field('Texte'),
 Field('Skills','list:reference SkillDemande'),
 Field('Items','list:reference Item'),
 Field('auth_user',db.auth_user),
 format = '%(Topic)s')


db.define_table('Item',
 Field('Nom', unique=True),
 Field('SkillUse', db.SkillLevel),
 format = '%(Nom)s')


How to select an item by Nom ?
How to add an item in demande ?

regards
Bussiere