def create_accountant_user(self):
        Group = Model.get('res.group')
        accountant = Model.get('res.user')
        accountant.name = 'Accountant'
        accountant.login = 'accountant'
        accountant.password = 'accountant'
        account_group, = Group.find([('name', '=', 'Account')])
        accountant.groups.append(account_group)
        accountant.save()

        self.accountant = accountant

Traceback (most recent call last):
  File "test_stock_invoice.py", line 42, in setUp
    self.create_accountant_user()
  File "test_stock_invoice.py", line 97, in create_accountant_user
    accountant.groups.append(account_group)
  File "/usr/local/lib/python2.7/dist-packages/proteus/__init__.py", line 224, in __get__
    relation = Model.get(self.definition['relation'], instance._config)
AttributeError: 'NoneType' object has no attribute '_config'