Django

Question

我有三个模型：Trip、City 和 GoogleApiCityCache。

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

我想减少模板中的查询数量：

{% 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。

所以属性formatted_address必须从这个对象中找到一个GoogleApiCityCache并得到formatted_address。

尝试使用 prefetch_related 但没有用。

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，city 和 language_code 的 GoogleApiCityCache 可以是 none。在这种情况下，请求 Google Api，创建一个新的 GoogleApiCityCache 对象并返回。

Answer 1

当您 prefetch_related('trips__city__city_caches') 时，您正在预取 city_caches。这意味着下面的循环不会引起任何额外的查询。

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

但是，在您的代码中，formatted_address 调用 get_cache_by_lang，后者调用 get_or_request，后者调用 GoogleApiCityCache.objects.get()，这将导致额外的查询。

为了利用 prefetch_related，您需要重写 get_cache_by_lang，使其循环遍历 self.city_caches.all() 以找到正确的对象。

Django - prefetch_related 对于特定情况不起作用

Django - prefetch_related for specific situation doesn't work

python

sql

orm

django-orm