使Django模型中的瞬态(非数据库)属性可用于模板

Question

我的一个模型具有未存储在数据库中的属性.视图和模型级别的一切都很好,但我似乎无法在模板中显示这些"非数据库"属性.

下面是一些示例代码,一个反映实际问题域的人工示例,用于演示不需要的行为.

风景:

def odometer(request):
    cars = Car.objects.all()
    for car in cars:
        car.read_meters()
    context = {'cars': cars}
    return render_to_response('odometer.html', context)

型号:

class Car(models.Model):
    name = models.CharField(_('name'), max_length=100, unique=True)

    def read_meters(self):
        for meter in self.meter_set.all():
            meter.read()

    def __unicode__(self):
        return '%s' % self.name

class Meter(models.Model):
    name = models.CharField(_('name'), max_length=100)
    car = models.ForeignKey(Car)

    difference = 0
    previous = 0
    changed = False

    def read(self):
        # this is completely artificial. in the real application we would interface with the hardware
        # meter to get data
        try:
            previous_count = MeterReading.objects.filter(meter__id=self.id).order_by('-stamp')[0].count
        except:
            previous_count = 0
        self.previous = previous_count
        current_count = previous_count

        if (random.randrange(0, 2) == 0):
            self.difference = int(random.random() * 100)
            if self.name == 'Odometer' or (random.randrange(0, 2) == 0):
                current_count += self.difference
            else:
                current_count -= self.difference
                if current_count < 0:
                    current_count = 0
        if current_count > previous_count:
            self.changed = True
        new_reading = MeterReading()
        new_reading.count = current_count
        new_reading.meter = self
        new_reading.save()

    def __unicode__(self):
        return '%s' % self.name

class MeterReading(models.Model):
    count = models.IntegerField(_('count'))
    stamp = models.DateTimeField(editable=False, auto_now_add=True)
    meter = models.ForeignKey(Meter)
    def __unicode__(self):
        return '%s' % self.count

和模板:

{% for car in cars %}
  <h2>{{ car }}</h2>
  {% for meter in car.meter_set.all %}
    <h3>{{ meter }}</h3>
    <p>Difference: {{ meter.difference }}</p>
    <p>Changed: {{ meter.changed }}</p>
    <ul>
      {% for reading in meter.meterreading_set.all %}
      <li>{{ reading }}</li>
      {% endfor %}
    </ul>
  {% endfor %}
{% endfor %}

问题是'meter.difference'和'meter.changed'不输出正确的更新值.我究竟做错了什么？任何建议表示赞赏.

谢谢.

更新:根据Daniel的回答更新了代码:

汽车型号:

class Car(models.Model):
    name = models.CharField(_('name'), max_length=100, unique=True)

    def read_meters(self):
        for meter in self.meters:
            meter.read()

    def __unicode__(self):
        return '%s' % self.name

    @property
    def meters(self):
        if not hasattr(self, '_meters'):
            self._meters = self.meter_set.all()
        return self._meters

和模板:

{% for car in cars %}
  <h2>{{ car }}</h2>
  {% for meter in car.meters %}
    <h3>{{ meter }}</h3>
    <p>{{ meter.name }} difference: {{ meter.difference }}</p>
    <p>Changed: {{ meter.changed }}</p>
    <ul>
      {% for reading in meter.meterreading_set.all %}
      <li>{{ reading }}</li>
      {% endfor %}
    </ul>
  {% endfor %}
{% endfor %}

Answer 1

您没有在模板中看到这些值的原因是每次调用car.meter_set.all()时都会直接从数据库中获取一个全新的查询集.

Django模型实例没有标识,因此即使Meter一个查询集中的对象与另一个查询集中的对象具有相同的数据库值,它们也不共享任何动态属性.

这样做的一个方法是将每节车厢内缓存表的对象,如我这里最近的一个问题.然后,不是car.meter_set.all()在视图,模型和模板中引用,而是执行car.get_meters()或者其他任何操作,并且每次都会获得相同的对象集以及动态属性.