i am trying to add a new compute field that add a mounth from an existing date field 

the code is working on odoo13 but i don't know why it isn't on the odoo 8

from datetime import datetime, timedelta
from dateutil.relativedelta import relativedelta
from openerp.osv import expression

class contract(models.Model):
 _inherit = 'res.contract' 

mounth = fields.Datetime(string="Date plus mois", compute='_add_mth') 
def _add_mth(self):
  for order in self:
  d = 1 
  if self.date_start:
     date_st = fields.Datetime.from_string(self.date_start)
     dt = date_start_dt + relativedelta(months=d) 
     mounth = fields.Datetime.to_string(dt) 
  order.update({'mounth': mounth}) 

i have this error message Expected singleton: res.contract(137, 138)