Django模型外键过滤器

Question

The problem: I have two django models:

class Driver(models.Model):
   event = models.ForeignKey('Event')
   last_event = ????
   ...
   ...

class Event(models.Model):
   date = models.IntegerField()

Now i need only LAST event for each driver PREFETCHED and CACHED

d = Driver.objects.prefetch_related('last_event???')

How do i achieve it? Is there any way to limit relation as something like:

last_event = models.ForeaignKey('Event', filter = ...??? )

Answer 1

To avoid of making circle dependencies, you can define all the foreign key of driver in the model of Event, and also due to a driver could have many events.

class Event(models.Model):
   driver = models.ForeignKey(Driver, related_name='events')
   date = models.IntegerField()
   created_at = models.DateTimeField(auto_now_add=True)
   objects = EventManager()

and then define a last_event property in Event manager, like this:

class EventManager(models.Manager):
    @property
    def last_event(self, driver):
        return self.filter(driver=driver).lastest('created_at')

Then you can call it by Event.objects.last_event(driver)

If you want to use driver.last_event, then you can define a last_event property in the Driver model:

class Driver(models.Model):
   ...
   @propery
   def last_event(self):
       the_last_event = Event.objects.last_event(self)
       return the_last_event

Answer 2

well i've solved a problem by adding:

    def get_current_tour(self):
       ret = None
       self._my_relations.create_dict_path('current_tour')
       if not self._my_relations['current_tour']:
          try:
            self._my_relations['current_tour'] = \
                DriverTour.objects.filter(start_date__lte=int(time.time()), end_date__gte=int(time.time()), driver=self.id).order_by('-id').all()[0]
            ret = self._my_relations['current_tour']
        except Exception:
            self._my_relations['current_tour'] = None
            ret = None
       else:
          ret = self._my_relations['current_tour']
       return ret

    def set_current_tour(self, args):
       self._my_relations['current_tour'] = args[0]

    current_tour = property(fget=get_current_tour, fset=set_current_tour)

so now it behaves as i planned: like a cacheable property