Django-ORM 而不是 python for 循环

Question

大家好！

这么简单的问题，有时我曾经用 python for 循环遍历我的模型，这对网站的性能不利。

我有3个3型号：

class A(models.Model):
    Bs = ManyToManyField(B)

class B(models.Model):
    Cs = ManyToManyField(C)

class C(models.Model):
    name = CharField(max_length=100)

如果我想让 C 模型的所有实例与 A 的实例相关，我将如何进行而不是这个python for 循环？

all_c = []
for b in a_instance.Bs.all():
    for c in b.Cs.all():
        all_c.append(c)

Answer 1

你可以使用 prefetch_related https://docs.djangoproject.com/en/2.0/ref/models/querysets/#prefetch-related

all_c = []
for b in a_instance.Bs.all().prefetch_related('Cs'):
    for c in b.Cs.all():
        all_c.append(c)

但更好的方法是只过滤 C 模型

all_c = C.objects.filter(b_set__a_set__in=[a_instance])
# or if you need it to be list and not queryset
all_c = list(C.objects.filter(b_set__a_set__in=[a_instance]))

Django-ORM 而不是 python for 循环

Django-ORM instead of python for loop

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