Django，在 prefetch_related 之后更新对象

Question

我有以下型号：

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


class Book(models.Model):
    title = models.CharField(max_length=100)
    publisher = models.ForeignKey(Publisher)

在我的views.py中，当我想展示出版商的页面时，我也想展示他们的书，所以我通常会这样做：

publisher = Publisher.objects.prefetch_related('book_set').filter(pk=id).first()

然后，经过一些处理，我也对书籍进行了一些处理

for book in publisher.book_set.all():
    foo()

效果很好，但我有一个问题。如果在查询和 for 循环之间添加了一本书，publisher.book_set.all() 将不会有新添加的书，因为它已被预取。

有没有办法更新发布者对象？

Answer 1

您可以删除实例上的整个预取缓存：

if hasattr(publisher, '_prefetched_objects_cache'):
    del publisher._prefetched_objects_cache

如果您只想删除特定的预取关系：

if hasattr(publisher, '_prefetched_objects_cache'):
    publisher._prefetched_objects_cache.pop('book_set', None)

Answer 2

也有可能从 Django doc 中删除所有 prefetch_related:

To clear any prefetch_related behavior, pass None as a parameter::

non_prefetched = qs.prefetch_related(None)

Answer 3

有一个更好的方法来清除预取属性，仅使用 public Django API，即 refresh_from_db() method:

# Reloads all fields from the object as well as clearing prefetched attributes
publisher.refresh_from_db()
# Clears one prefetched attribute, will be re-fetched on next access
publisher.refresh_from_db(fields=['book_set'])

for book in publisher.book_set.all():

Django，在 prefetch_related 之后更新对象

Django, update the object after a prefetch_related

python

django

django-orm