text: from django.db.models.signals import pre_save
  
  from models import Event, Calendar
  
  def optionnal_calendar(sender, **kwargs):
      event = kwargs.pop('instance')
          
      if not isinstance(event, Event):
          return True
      if not event.calendar:
          try:
              calendar = Calendar._default_manager.get(name='default')
          except Calendar.DoesNotExist:
              calendar = Calendar(name='default', slug='default')
              calendar.save()
  
          event.calendar = calendar
      return True
  
  pre_save.connect(optionnal_calendar)
  