Question

如果我在没有自定义管理器的情况下执行以下所有操作，则一切都按预期工作：

class Content(models.Model):
   name = models.CharField(max_length=200)
   def __str__(self):
      return self.name
   class Meta:
      app_label = 'game'

class Requirement(models.Model):
   content = models.ForeignKey(Content, on_delete=models.CASCADE, related_name = 'requirements')
   value = models.IntegerField(default=0)
   def __str__(self):
      return "{} requires value {}".format(self.content,self.value)
   class Meta:
      app_label = 'game'

def testPrefetchOrig():
   contents = Content.objects.filter(name__startswith = 'a').prefetch_related('requirements')
   for c in contents:
      for r in c.requirements.all():
         print r
         logging.warning(r)
   logging.warning("Done with query")

这一次预取数据，再也不会：

DEBUG:django.db.backends:(0.001) SELECT "game_content"."id", "game_content"."name", "game_content"."deleted" FROM "game_content" WHERE "game_content"."name"::text LIKE 'a%'; args=(u'a%',)
DEBUG:django.db.backends:(0.001) SELECT "game_requirement"."id", "game_requirement"."content_id", "game_requirement"."value", "game_requirement"."deleted" FROM "game_requirement" WHERE "game_requirement"."content_id" IN (5, 6); args=(5, 6)
alphabet requires value 5
WARNING:root:alphabet requires value 5
alphabet requires value 3
WARNING:root:alphabet requires value 3
albatross requires value 1
WARNING:root:albatross requires value 1
albatross requires value 0
WARNING:root:albatross requires value 0
WARNING:root:Done with query

但是，我想使用自定义管理器来处理已删除的条目的过滤＆＃39;通过设置已删除的标志。

class DeletedItemsQuerySet(models.query.QuerySet):
   def get(self, *args, **kwargs):
      kwargs['deleted']=False
      return models.query.QuerySet.get(self, *args, **kwargs)
   def all(self):
      return self.filterNoDeleted()
   def filterNoDeleted(self, *args, **kwargs):
      kwargs['deleted']=False
      return models.query.QuerySet.filter(self, *args, **kwargs)
   def getDeleted(self, *args, **kwargs):
      return models.query.QuerySet.get(self, *args, **kwargs)
   def filterDeleted(self, *args, **kwargs):
      return models.query.QuerySet.filter(self, *args, **kwargs)

class DeletedItemsManager(models.Manager.from_queryset(DeletedItemsQuerySet)):
   def all(self):
      return super(models.Manager,self).all().filterNoDeleted()

然后我们修改我们的模型来使用它：

class Content(models.Model):
   name = models.CharField(max_length=200)
   objects = DeletedItemsManager()
   deleted = models.BooleanField(default=False)
   def __str__(self):
      return self.name
   class Meta:
      app_label = 'game'

class Requirement(models.Model):
   content = models.ForeignKey(Content, on_delete=models.CASCADE, related_name = 'requirements')
   value = models.IntegerField(default=0)
   deleted = models.BooleanField(default=False)
   objects = DeletedItemsManager()
   def __str__(self):
      return "{} requires value {}".format(self.content,self.value)
   class Meta:
      app_label = 'game'

def testPrefetchOrig():
   contents = Content.objects.filter(name__startswith = 'a').prefetch_related('requirements')
   for c in contents:
      for r in c.requirements.all():
         print r
         logging.warning(r)
   logging.warning("Done with query")

这会预取数据，但仍会查询数据：

DEBUG:django.db.backends:(0.001) SELECT "game_content"."id", "game_content"."name", "game_content"."deleted" FROM "game_content" WHERE "game_content"."name"::text LIKE 'a%'; args=(u'a%',)
DEBUG:django.db.backends:(0.001) SELECT "game_requirement"."id", "game_requirement"."content_id", "game_requirement"."value", "game_requirement"."deleted" FROM "game_requirement" WHERE "game_requirement"."content_id" IN (5, 6); args=(5, 6)
DEBUG:django.db.backends:(0.000) SELECT "game_requirement"."id", "game_requirement"."content_id", "game_requirement"."value", "game_requirement"."deleted" FROM "game_requirement" WHERE ("game_requirement"."content_id" = 5 AND "game_requirement"."deleted" = false); args=(5, False)
alphabet requires value 5
WARNING:root:alphabet requires value 5
alphabet requires value 3
WARNING:root:alphabet requires value 3
DEBUG:django.db.backends:(0.001) SELECT "game_requirement"."id", "game_requirement"."content_id", "game_requirement"."value", "game_requirement"."deleted" FROM "game_requirement" WHERE ("game_requirement"."content_id" = 6 AND "game_requirement"."deleted" = false); args=(6, False)
albatross requires value 1
WARNING:root:albatross requires value 1
albatross requires value 0
WARNING:root:albatross requires value 0
WARNING:root:Done with query

如何使用自定义管理器并仍然具有prefetch_related工作？

Answer 1

好。 DeletedItemsManager在每个all（）方法调用上添加一个额外的filterNoDeleted（）方法调用。根据{{3}}

中的注释，该过滤器调用会破坏缓存

但是，我确实需要过滤掉已删除的相关对象。

我应该能够在Requirement表中选择所有必需的行，但我不确定如何在Queryset中设置正确的字段以反映这些结果。但是，如果我只想阅读结果，我可以把它变成字典。

有没有人有更优雅的解决方案？

注意：有一个对Django的filtered_relation添加的pull请求，但是这对prefetch_related还没有用。也许将来Django会支持这个用例。

Answer 2

def testPrefetchOrig():
contents = Content.objects.filter(name__startswith = 'a').prefetch_related(Prefetch('requirements', queryset=Requirement.objects.filterNoDeleted(),to_attr='undeletedRequirements'))
for c in contents:
    for r in c.undeletedRequirements:
        print r
print "Done with query"

Django中的Custom Manager破坏了prefetch_related的缓存

2 个答案: