Do any one know about parent and child class

Hi
i just want to know how to develop structure by using parent class and child class
i have following error

  File "/home/maulik/workspace/server/bin/netsvc.py", line 247, in dispatch
    result = LocalService(service_name)(method, *params)
  File "/home/maulik/workspace/server/bin/netsvc.py", line 76, in __call__
    return getattr(self, method)(*params)
  File "/home/maulik/workspace/server/bin/service/web_services.py", line 577, in execute
    res = service.execute(db, uid, object, method, *args)
  File "/home/maulik/workspace/server/bin/osv/osv.py", line 58, in wrapper
    return f(self, dbname, *args, **kwargs)
  File "/home/maulik/workspace/server/bin/osv/osv.py", line 119, in execute
    res = pool.execute_cr(cr, uid, obj, method, *args, **kw)
  File "/home/maulik/workspace/server/bin/osv/osv.py", line 111, in execute_cr
    return getattr(object, method)(cr, uid, *args, **kw)
  File "/home/maulik/workspace/server/bin/osv/orm.py", line 2871, in create
    pleft_old = cr.fetchone()[0]
TypeError: 'NoneType' object is unsubscriptable