_inherits doesnt see orm objects

Hello.
I'm trying to create my own little addon.

import time
import netsvc
 from osv import fields, osv
 import pooler
 from tools import config
 from tools.translate import _

class account_invoice(osv.osv):
    _name = "account.invoice"
    _inherit = "account.invoice"
     def _get_order(self, cr, uid, ids, field_name, args, context={}):
             res={}
             sids = self.pool.get('sale.order').search(cr, uid, [('invoice_ids','=',ids)],context=context)
             if sids:
                 for id in ids:
                    res[id]=[x for x in sids]
                 return res
            else:
                return False
   _columns = {
         'order_id': fields.function(_get_order , method=True,type='many2one' , obj='sale.order',string='Order'),
         }
     _defalts = {}
account_invoice()

And when installing it, i get:
 "File "/usr/lib/python2.6/site-packages/openerp-server/osv/osv.py", line 232, in createInstance
    assert pool.get(parent_name), "parent class %s does not exist in module %s !" % (parent_name, module)
AssertionError: parent class account.invoice does not exist in module rez !"

Please help, i have no idea what have i missed to initialize