模板中的django select_related [英] django select_related in template

问题描述

有时在django模板中使用select_related是有意义的。例如，假设我有一个类扩展DetailView类b

 类DemoCarView（DetailView）：
 model = Car 
  

基于以下设计模型

 ＃汽车
类汽车（models.Model）：
 name = models.CharField（max_length = 32）
 
＃制造商
类制造商（models.Model）：
 name = models.CharField（max_length = 32）
 
＃Parts 
 class Part（models.Model）：
 name = models。 CharField（max_length = 32）
 car = models.ForeignKey（Car）
 manufacturer = models.ForeignKey（Manufacturer）
  

然后是html模板

  {{car.name}} 
 < ; UL> 
 {％for car.part_set.all％中的部分} 
< li> {{part.name}}  -  {{part.manufacturer.name}}< / li> 
 {％endfor％} 
< / ul> 
  

这对于获得汽车，组成它的零件以及这些零件的制造商来说非常合适。但是，这将使用2 + number_of_parts SQL查询来执行此操作。轻松修复如下：

  {{car.name}} 
< ul> 
 {％for car.part_set.select_related.all％中的部分} 
< li> {{part.name}}  -  {{part.manufacturer.name}}< / li> 
 {％endfor％} 
< / ul> 
  

现在运行最佳的2个查询。但是， select_related 正在将部件与每个外键相连接。有没有办法限制这只是所需的相关表。在Python中，它只是：

$ p $ Part.objects.select_related（'manufacturer'）。filter（car = car）


这可以在模板中完成吗？

我知道我可以很容易地在视图中做到这一点，通过在过滤器上返回'car'和一个用于'code> select_related（'manufacturer'）的零件'的上下文，但是与上面使用的DetailView子类相比，它的代码更多一些。就像这样：

$ p $ 类DemoCarViewPreload（TemplateView）：
template_name ='demo / car_detail_preload.html'
def get_context_data（self，** kwargs）：
context = super（DemoCarViewPreload，self）.get_context_data（** kwargs）
car = Car.objects.get（pk = kwargs.get（'pk '））
context ['car'] = car
context ['parts'] = Part.objects.select_related（'manufacturer'）。filter（car = car）
return context $然而，这需要模板更加具体到这个视图，因为它现在需要使用'b




$ b

零件'上下文而不是 car.part_set.all 。

解决方案

如何在<$ c上使用简单的方法$ c $>车模型？

  class Car（models.Model）：
。 .. 
 def parts_with_manufacturers（self）：
 return self.part_set.select_related（'manufacturer'）
  

，然后是

  {％for car.parts_with_manufacturers％} 
< li> {{part.name}}  -  {{part.manufacturer.name}}< / li> 
 {％endfor％} 
  

Sometimes it makes sense to use select_related in a django template. For example, say I have a class extending DetailView

class DemoCarView(DetailView):
    model = Car

Based on the following contrived model

# Cars
class Car(models.Model):
    name = models.CharField(max_length=32)

# Manufacturers
class Manufacturer(models.Model):
    name = models.CharField(max_length=32)

# Parts
class Part(models.Model):
    name = models.CharField(max_length=32)
    car = models.ForeignKey(Car)
    manufacturer = models.ForeignKey(Manufacturer)

The html template is then

{{ car.name }}
<ul>
{% for part in car.part_set.all %}
    <li>{{ part.name }} - {{ part.manufacturer.name }} </li>
{% endfor %}
</ul>

This works perfectly to get the car, the parts that comprise it, and the manufacturers of those parts. However, this will use 2+number_of_parts SQL queries to do so. Easily fixed like so:

{{ car.name }}
<ul>
{% for part in car.part_set.select_related.all %}
    <li>{{ part.name }} - {{ part.manufacturer.name }} </li>
{% endfor %}
</ul>

Now the optimum of 2 queries is run. However, select_related is joining the Parts with every Foreign Key that it has. Is there a way to restrict this to just the desired related tables. In Python it's just:

Part.objects.select_related('manufacturer').filter(car=car)

Can this be done in the template?

Note: I know I can do this in the view very easily by returning a context for 'car' and one for 'parts' with select_related('manufacturer') on the filter, but it's quite a bit more code compared to the DetailView sub-class I've used above. Something like this:

class DemoCarViewPreload(TemplateView):
    template_name = 'demo/car_detail_preload.html'
    def get_context_data(self, **kwargs):
        context = super(DemoCarViewPreload, self).get_context_data(**kwargs)
        car = Car.objects.get(pk=kwargs.get('pk'))
        context['car'] = car
        context['parts'] = Part.objects.select_related('manufacturer').filter(car=car)
        return context

However, this requires the template to be more specific to this view as it will now need to use the 'parts' context rather than car.part_set.all. Also, it's simply more work to make this view in the first place.

解决方案

How about a simple method on the Car model?

class Car(models.Model):
    ...
    def parts_with_manufacturers(self):
        return self.part_set.select_related('manufacturer')

and then

{% for part in car.parts_with_manufacturers %}
    <li>{{ part.name }} - {{ part.manufacturer.name }} </li>
{% endfor %}

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