Question

我有三个模型：Trip，City和GoogleApiCityCache。

如果我想知道这个城市的名称，我会在@property对象上使用City，该对象试图获取当前语言的GoogleApiCityCache。

我想减少模板中的一些查询：

{% for trip in trips %}
    {{ trip.city.formatted_address }}
{% endfor %}

现在，它执行新的数据库查询以在每次迭代中查找GoogleApiCityCache。我想预取这些缓存。

因此，属性formatted_address必须从此对象中找到GoogleApiCityCache并获取formatted_address。

尝试使用prefetch_related，但它没有帮助。

def profile(request, slug):
    owner = User.objects.filter(userprofile__slug=slug).prefetch_related('trips__city__city_caches').first()
    trips = Trip.objects.filter(user=owner).select_related('city').prefetch_related('city__city_caches')
    return render(request, 'profiles/profile/profile.html', {'owner': owner,'trips':trips})

我的模特：

class Trip(models.Model):
    user = models.ForeignKey('auth.User', related_name='trips')
    city = models.ForeignKey('locations.City', related_name='trips')
    date_from = models.DateField()
    date_to = models.DateField()
    detail = models.TextField(null=True, blank=True)
    # participants = models.ManyToManyField('auth.User', related_name='participating_on_trips', blank=True)
    objects = TripManager()

class City(models.Model):
    ...
    def get_cache_by_lang(self, lang=None, force_refresh=False):
        if not lang:
            lang = get_language() or 'en'
        cache, requested = GoogleApiCityCache.objects.get_or_request(city=self, lang=lang)
        return cache

    @property
    def formatted_address(self):
        return self.get_cache_by_lang().formatted_address

class GoogleApiCityCacheManager(models.Manager):
    def _request_create(self, city, lang):
        """ Create cache (wasn't found) """
        ....
        return cache

    def get_or_request(self, city, lang):
        created = False
        try:
            cache = GoogleApiCityCache.objects.get(city=city, language_code=lang)
        except GoogleApiCityCache.DoesNotExist:
            cache = self._request_create(city, lang)
            created = True
        return cache,created

class GoogleApiCityCache(models.Model):
    language_code = models.CharField(max_length=5)
    api_json_response = JSONField(null=True, blank=True)
    city = models.ForeignKey('locations.City', on_delete=models.CASCADE, related_name='city_caches')
    objects = GoogleApiCityCacheManager()

    class Meta:
        unique_together = ('city', 'language_code')


    @property
    def formatted_address(self):
        try:
            return self.api_json_response['result']['formatted_address']
        except:
            return None

如您所见，GoogleApiCityCache和city的{{1}}可以是无。在这种情况下，请求Google Api，创建一个新的language_code对象并返回。

Answer 1

当您prefetch_related('trips__city__city_caches')时，您正在预取city_caches。这意味着以下循环不会导致任何额外的查询。

for cache in city.city_caches.all()
    print(cache)

但是，在您的代码formatted_address调用get_cache_by_lang调用调用get_or_request的{{1}}，这将导致额外的查询。

为了利用GoogleApiCityCache.objects.get()，您需要重写prefetch_related，以便循环遍历get_cache_by_lang以找到正确的对象。

Django - 针对特定情况的prefetch_related不起作用

1 个答案: