class coef(models.Model):

    _inherit='sale.order.line'

    coefficient=fields.Float(string="Coefficient UB (%)", require = True)

    # fonction permettant de transmettre les valeurs du champs coefficient des devis vers les factures

    def _prepare_order_line_invoice_line(self, cr, uid, line, account_id=False, context=None):

        ret = super(coef, self)._prepare_order_line_invoice_line(cr, uid, line, account_id=False, context=context)

        if line.product_id:

            if line.coefficient:

                ret['coefficient'] = line.coefficient

        return ret

    def product_id_change(self, cr, uid, ids, pricelist, product, qty=0,

            uom=False, qty_uos=0, uos=False, name='', partner_id=False,

            lang=False, update_tax=True, date_order=False, packaging=False, fiscal_position=False, flag=False, context=None):

    product_obj = self.pool.get('product.product').browse(cr, uid, product)

    return super(coef, self).product_id_change(cr, uid, ids, pricelist, product, qty,

    uom, qty_uos, uos, name, partner_id,

    lang, update_tax, date_order, packaging, fiscal_position, flag, context)

Sorry for the quality

could you please paste the code here?