Hi guys,I'm relatively new to Odoo (2 months) and currently programming a custom module which inherits "hr.expense.line" in order to show the country and the start and end of the journey on the expense lines. Based on these informations i want to calculate the amount of expenses. I need a proper method to override the "_amount" method in "hr.expense.lines". This method should calculate the difference between the 2 datetime fields and regard the different countries.  I'd also like to get the allowable expenses dynamically from a website. How is this possible in odoo(e.g. screen scraper or foreign db)? I'm thankful for any kind of help, below is my start code.

# -*- coding: utf-8 -*-
from openerp import models, fields,api 
from datetime import datetime, timedelta
from openerp.tools import DEFAULT_SERVER_DATETIME_FORMAT
import openerp.addons.decimal_precision as dp
class spesen_model(models.Model):
    _inherit = 'hr.expense.line'
    x_country = fields.Selection( [('d','Deutschland'), ('s','Schweiz'),('au','Österreich')],
                                 'Land')                           
    x_time_begin = fields.Datetime('Anfangsdatum')
    x_time_end = fields.Datetime('Enddatum')
    @api.v8
    @api.multi
    @api.depends('x_country', 'x_time_begin', 'x_time_end')
    def _amount(self):
        for expense in self:
            var = 0.0
            if expense.x_country == 'd':
                var += 100.0
            elif expense.x_country == 's':
                var += 170.0
            elif expense.x_country == 'au':
                var += 230.0
            expense.total_amount = var

total_amount = fields.Float(compute='_amount', string='Total', digits_compute=dp.get_precision('Account'))