title: Way to override field function in v8
body: <p>I want to override a original field function in Odoo.</p>
  
  <p>According to the answer here: <a href="https://stackoverflow.com/questions/32859723/odoo-how-to-override-original-function">Odoo: How to override original function</a> 
  i just have to define exactly the same method as in the original model. So here is my code:</p>
  
  <pre><code>class paiement_client_difference_montant(models.Model):
      _inherit="account.voucher"
  
      #writeoff_amount=fields.Float(compute='_get_writeoff_amount')
  
  def _get_writeoff_amount(self, cr, uid, ids, name, args, context=None):
      print '_get_writeoff_amount _inherit'
      if not ids: return {}
      currency_obj = self.pool.get('res.currency')
      res = {}
      for voucher in self.browse(cr, uid, ids, context=context):
          debit = credit = 0.0
          sign = voucher.type == 'payment' and -1 or 1
          for l in voucher.line_dr_ids:
              debit += l.amount
          for l in voucher.line_cr_ids:
              credit += l.amount
          currency = voucher.currency_id or voucher.company_id.currency_id
          res[voucher.id] =  currency_obj.round(cr, uid, currency, voucher.amount - sign * (credit - debit))
      return res
  </code></pre>
  
  <p>But that code is never reached.
  Any help please. Thank you.</p>