I have 3 objects : students, score and subject

class Student(osv.osv):
     _name= "astudent"
     _columns={
     'name': fields.char(string='Name',size=32),
     'subject' : fields.many2many('asubject', 'table', 'stu', 'subj', 'Subject'),
     'score' : fields.one2many('ascore','student','Score'),
}

class subject(osv.osv):
     _name = "asubject"
     _columns={
     'name': fields.char('Subject Name'),
    'score' : fields.one2many('ascore','subject','Score'),
}

class score(osv.osv):
     _name = "ascore"
     _columns={
     'score': fields.float('Diem', required=True),
     'student' : fields.many2one('astudent', string='Student', delegate=True, required=True),
     'subject' : fields.many2one('asubject', string='Subject', delegate=True, required=True,),
}

In xml file of "astudent", I chose the subject, and after, I will set the "score". I want when I set the "score", I just can chose the "object" (inside "ascore") between the "object" are chosed.

I think I must use "related" or "domain" but I can't do that

Sorry for my bad english, thanks for any help.