sale_data_partner/models/sale.py

from openerp.osv import fields, osv
from openerp import api
import openerp.addons.decimal_precision as dp


class SaleOrder(osv.Model):
    _inherit = 'sale.order'

    _columns = {
        'city' : fields.char('Ciudad', related='partner_id.city',store=True),
    }