we create a custom module which inherit from calendar module ..
to read an event , odoo show me this message

Odoo Server Error

Traceback (most recent call last):
  File "/home/nebil/Odoo8/odoo/openerp/http.py", line 544, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/home/nebil/Odoo8/odoo/openerp/http.py", line 581, in dispatch
    result = self._call_function(**self.params)
  File "/home/nebil/Odoo8/odoo/openerp/http.py", line 317, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "/home/nebil/Odoo8/odoo/openerp/service/model.py", line 118, in wrapper
    return f(dbname, *args, **kwargs)
  File "/home/nebil/Odoo8/odoo/openerp/http.py", line 314, in checked_call
    return self.endpoint(*a, **kw)
  File "/home/nebil/Odoo8/odoo/openerp/http.py", line 810, in __call__
    return self.method(*args, **kw)
  File "/home/nebil/Odoo8/odoo/openerp/http.py", line 410, in response_wrap
    response = f(*args, **kw)
  File "/home/nebil/Odoo8/odoo/addons/web/controllers/main.py", line 944, in call_kw
    return self._call_kw(model, method, args, kwargs)
  File "/home/nebil/Odoo8/odoo/addons/web/controllers/main.py", line 936, in _call_kw
    return getattr(request.registry.get(model), method)(request.cr, request.uid, *args, **kwargs)
  File "/home/nebil/Odoo8/odoo/openerp/api.py", line 268, in wrapper
    return old_api(self, *args, **kwargs)
  File "/home/nebil/Odoo8/tss/tss/objects/calendar_rdv.py", line 106, in get_attendee_detail
    for attendee in meeting.attendee_ids:
  File "/home/nebil/Odoo8/odoo/openerp/fields.py", line 835, in __get__
    self.determine_value(record)
  File "/home/nebil/Odoo8/odoo/openerp/fields.py", line 928, in determine_value
    record._prefetch_field(self)
  File "/home/nebil/Odoo8/odoo/openerp/api.py", line 266, in wrapper
    return new_api(self, *args, **kwargs)
  File "/home/nebil/Odoo8/odoo/openerp/models.py", line 3246, in _prefetch_field
    result = records.read(list(fnames), load='_classic_write')
  File "/home/nebil/Odoo8/odoo/openerp/api.py", line 266, in wrapper
    return new_api(self, *args, **kwargs)
  File "/home/nebil/Odoo8/odoo/openerp/api.py", line 592, in new_api
    result = method(self._model, cr, uid, self.ids, *args, **old_kwargs)
  File "/home/nebil/Odoo8/tss/tss/objects/calendar_rdv.py", line 186, in read
    real_data = super(optim_calendar_event, self).read(cr, uid, [real_id for calendar_id, real_id in select], fields=fields2, context=context, load=load)
  File "/home/nebil/Odoo8/odoo/openerp/api.py", line 268, in wrapper
    return old_api(self, *args, **kwargs)
  File "/home/nebil/Odoo8/odoo/addons/calendar/calendar.py", line 1687, in read
    res = real_data[real_id].copy()
KeyError: 4