diff -r 9fec62e076a1 purchase.py
--- a/purchase.py       Mon Apr 22 12:18:26 2013 +0200
+++ b/purchase.py       Thu May 23 18:24:08 2013 +0200
@@ -1,12 +1,14 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of this
 # repository contains the full copyright notices and license terms.
+from trytond.model import Model
 from trytond.pool import PoolMeta

 __all__ = ['PurchaseLine']
 __metaclass__ = PoolMeta


-class PurchaseLine:
+class PurchaseLine(Model):
+    'PurchaseLine'
     __name__ = 'purchase.line'

     @classmethod