如何遍历 django 中的反向查询集？

Question

我试图在一个反向的 object 上遍历两次，最初 for 循环有效，但在第二个循环上无效。

x = Subscription.objects.filter(customer_id=customer_id).order_by('-id')[:count]
tmp = reversed(x)
y = call_function(subs=tmp) # inside this function as well object is of type reversed and i am able to loop over it inside the call_function.

for j in tmp: # this loop is not running at all. here as well tmp is a reversed object
    print(j)

# call_function(subs=s)
def call_function(s):
    for i in s:
       print(i)

Answer 1

reversed(…) [Python-doc]是一个迭代器，而不是一个迭代器，所以第二次循环时，迭代器已经“用尽”了。 因此，它只是一个“浅对象”，以相反的顺序“遍历” QuerySet 。

因此，您使用reversed(…)两次。 这不会进行第二次查询，因为QuerySet会缓存结果，所以：

x = Subscription.objects.filter(customer_id=customer_id).order_by('-id')[:count]
tmp = reversed(x)
y = call_function(subs=tmp)

for j in reversed(x):
    print(j)