django - "Cannot filter a query once a slice has been taken"

Question

我收到这个错误

Caught AssertionError while rendering: Cannot filter a query once a slice has been taken.

在这条线上

{% if form.non_field_errors %} 

当我尝试这样做时

copy_pickup_address = ModelChoiceField(required=False, queryset=Address.objects.filter(shipment_pickup__user=user).order_by('-shipment_pickup__created')[:5])

但是我需要对它进行切片，因为我只想要最后 5 个地址。它呈现良好，直到我选择一个地址并提交表格。为什么不喜欢这个？我怎样才能绕过它？

Answer 1

解释在 https://docs.djangoproject.com/en/1.8/ref/models/querysets/ :

even though slicing an unevaluated QuerySet returns another unevaluated QuerySet, modifying it further (e.g., adding more filters, or modifying ordering) is not allowed, since that does not translate well into SQL and it would not have a clear meaning either.