django的prefetch（）

prefetch()类方法：

prefetch_related, on the other hand, does a separate lookup for each relationship, and does the 'joining' in Python. This allows it to prefetch many-to-many and many-to-one objects, which cannot be done using
For example, suppose you have these models

from django.db import models

class Topping(models.Model):
    name = models.CharField(max_length=30)

class Pizza(models.Model):
    name = models.CharField(max_length=50)
    toppings = models.ManyToManyField(Topping)

    def __str__(self):
        return "%s (%s)" % (
            self.name,
            ", ".join(topping.name for topping in self.toppings.all()),
        )
and run:

class Restaurant(models.Model):
    pizzas = models.ManyToManyField(Pizza, related_name='restaurants')
    best_pizza = models.ForeignKey(Pizza, related_name='championed_by', on_delete=models.CASCADE)

>>> Pizza.objects.all() ["Hawaiian (ham, pineapple)", "Seafood (prawns, smoked salmon)"...

The problem with this is that every time Pizza.__str__() asks for self.toppings.all() it has to query the database, so Pizza.objects.all() will run a query on the Toppings table for every item in the Pizza QuerySet.　　

也就是说，上面的代码中每一次调用__str__(),又要多一次查询，而不是一次all()查询。

prefetch()可以只用两次查询，不是通过外键关联一次查询的方式。

>>> Pizza.objects.all().prefetch_related('toppings')　　

原理是，prefetch（）将其查到的结果存到了缓存区，并做好了关联。

也可以多次连表

 Restaurant.objects.prefetch_related('pizzas__toppings')

另外，prefetch可以通过Prefetch对象，实现自定义的查询。

>>> vegetarian_pizzas = Pizza.objects.filter(vegetarian=True)
>>> Restaurant.objects.prefetch_related(
...     Prefetch('pizzas', to_attr='menu'),
...     Prefetch('pizzas', queryset=vegetarian_pizzas, to_attr='vegetarian_menu'))

　需要注意的是。有一些函数会清空prefetch（）存下的数据

Remember that, as always with QuerySets, any subsequent chained methods which imply a different database query will ignore previously cached results, and retrieve data using a fresh database query. So, if you write the following:

>>> pizzas = Pizza.objects.prefetch_related('toppings')
>>> [list(pizza.toppings.filter(spicy=True)) for pizza in pizzas]

...then the fact that pizza.toppings.all() has been prefetched will not help you. The prefetch_related('toppings') implied pizza.toppings.all(), but pizza.toppings.filter() is a new and different query. The prefetched cache can't help here; in fact it hurts performance, since you have done a database query that you haven't used. So use this feature with caution!

Also, if you call the database-altering methods add(), remove(), clear() or set(), onrelated managers, any prefetched cache for the relation will be cleared.