I´m trying to extend calendar.event as prototype inheritance.  It looks like it is trying to access the relationship with wrong fields. I gess I´m making some mistake with this type of inheritance. 

My code is this.

# -*- coding: utf-8 -*-
'''
Created on 17 de dic. de 2015
@author: seba
'''
from openerp import models, fields, api
from openerp.exceptions import ValidationError
from datetime import date
import logging
_logger = logging.getLogger(__name__)
class turns_turn(models.Model):
    _name = 'turns.turn'
    _inherit = 'calendar.event'
    
    court = fields.Many2one("turns.court","Cancha",required="True",help="La chancha que se vaya a ocupar en este turno")
    cash_advance = fields.Float("Seña",required="True",default=0,help="Seña para reserva de cancha")
    client = fields.Many2one("turns.client",string="Cliente",required = "True",help="El cliente al cual se le asigna el turno")
    total_cost = fields.Float("Costo total",readonly=True,help="Costo total calculado como precio de cancha unitario x duración del turno")
    state = fields.Selection([("reserva","Reserva de turnos"),
                            ("canchatomada","Cancha Tomada"),
                            ("turnocerrado","Turno Cerrado"),
                            ],string="Estado")
    
    _defaults = {
        'state': 'reserva',
        'name': 'turno',
        'start': '2016-01-19 09:00:00',
        'stop': '2016-01-19 09:00:00'
    }
    
    @api.model
    @api.returns('self', lambda value:value.id)
    def create(self, vals):
        court_selected = self.env['turns.court'].search([('id','=',vals['court'])])
        vals['total_cost'] = vals['duration'] * court_selected.court_unity_price
        return models.Model.create(self, vals)
    
    def name_get(self, cr, uid, ids, context=None):
        if context is None:
            context = {}
        if isinstance(ids, (int, long)):
            ids = [ids]
    
        res = []
        for record in self.browse(cr, uid, ids, context=context):
            name = record.client
            res.append((record.id, name))
        
        return res
    
    def take_court(self):
        self.state = "canchatomada"