from openerp import models,fields,api

class account_invoice(models.Model):
    _name = 'account.invoice'
    _inherit = 'account.invoice'

    @api.one
    @api.depends('partner_id')
    def _partner_data(self):
        self.partner_ruc = self.partner_id.ruc

    partner_ruc = fields.Char('R.U.C.',compute='_partner_data',store=True, update=True)