Select_related（和 prefetch_related）未按预期工作？

Question

我的模特：

class Anything(models.Model):
    first_owner = models.ForeignKey(Owner, related_name='first_owner')
    second_owner = models.ForeignKey(Owner, related_name='second_owner')

class Something(Anything):
    one_related = models.ForeignKey(One, related_name='one_related', null=True)
    many_related = models.ManyToManyField(One, related_name='many_related')

class One(models.Model):
    first = models.IntegerField(null=True)
    second = models.IntegerField(null=True)

在我的代码中，我想用这样的代码对我的数据库做一个小总结：

all_owners = Owner.objects.all()
first_selection = []
second_selection = []

objects = {}

for owner in all_owners:
<<0>>    
items = Something.objects.filter(Q(first_owner=owner)|Q(second_owner=owner)).order_by('date').all() 

    #Find owners, who have at least 100 "Something" elements related
    if(items.count() > 100):
        first_selection.append(owner)
        objects[owner] = items

    #Find owners, who have at least 80 "Something" with at least one many_related elements related,
    if(items.filter(many_related__isnull=False).distinct().count() > 80):
        second_selection.append(owner)
        objects[owner] = items

# Now i pass first_selection and second_selection and objects to functions, but following loops will produce the same problem im getting:

<<1>>
for owner in first_selection:
    for something in objects[owner]:
        rel = something.one_related
        print(str(rel.first) + "blablabla" + str(rel.second))
<<2>>
for owner in first_selection:
    for something in objects[owner]:
        rel = something.one_related
        print(str(rel.first) + "blablabla" + str(rel.second))

<<3>>
for owner in second_selection:
    for something in objects[owner]:
        rel = something.many_related.first()
        if rel != None""
            print(str(rel.first) + "blablabla" + str(rel.second))
<<4>>
for owner in second_selection:
    for something in objects[owner]:
        rel = something.many_related.first()
        if rel != None:
            print(str(rel.first) + "blablabla" + str(rel.second))

问题是： <<1>> 循环执行需要 30 分钟，<<2>> 循环需要 2 秒执行，尽管它们使用相同的数据。
我知道为什么会这样——因为第一个循环获取所有 one_related 字段并将其存储在缓存中。所以我将 <<0>> 中的代码更改为：

        items = Something.objects.filter(Q(first_owner=owner)|Q(second_owner=owner)).order_by('date').select_related('one_related').all()

当我查看生成的查询时，它看起来会在表上执行连接。
但问题仍然存在（第一个循环需要几分钟，第二个循环需要几秒钟），事实上我使用 mysqltuner 来显示执行的查询数量 - 它在第一个循环中增长，尽管它不应该......

我想这同样适用于第 3 和第 4 个循环以及 prefetch_related，尽管我没有足够的内存来测试它。

Answer 1

所以，我知道不带参数调用的 select_related() 不会预取可为 null 的对象。不过我不知道，在调用 select_related('one_related') 之后，如果相关对象的字段可为空，它只会 select 相关对象的 ID。

总而言之，我的问题的答案是替换：

Something.objects.select_related('one_related')

和

Something.objects.select_related('one_related', 'one_related__first', 'one_related__second')

Select_related（和 prefetch_related）未按预期工作？

Select_related (and prefetch_related) not working as intended?

python

django

django-orm