Question

employee = Employee.objects.filter('age' = 99)

我们假设此查询集为空。

如果我使用employee[0]，那将返回索引超出范围错误，那么是否可以在此处使用None作为默认值？

`employee[0] or None`? # This won't work, but this is what I mean.

Answer 1

如果您只想获得一个实例，请使用get，而不是filter：

employee = Employee.objects.get(age = 99)

如果不存在，您将获得Employee.DoesNotExist异常，您需要捕获该异常。如果有一个以上的99岁员工，您将获得一个您可能想要捕获的Employee.MultipleObjectsReturned异常。

如果你感到懒惰，总会有django-annoying的get_object_or_None！

from annoying.functions import get_object_or_None

obj = get_object_or_None(Employee, age=99)

如果您不想使用所有django-annoying，您可以随时在某处添加get_object_or_None，它看起来像这样：

def get_object_or_None(klass, *args, **kwargs):
    """
    Uses get() to return an object or None if the object does not exist.

    klass may be a Model, Manager, or QuerySet object. All other passed
    arguments and keyword arguments are used in the get() query.

    Note: Like with get(), a MultipleObjectsReturned will be raised if
    more than one object is found.
    """
    queryset = _get_queryset(klass)
    try:
        return queryset.get(*args, **kwargs)
    except queryset.model.DoesNotExist:
        return None

Answer 2

在Django＆gt; = 1.6上有first方法，所以你可以这样做：

employee = Employee.objects.filter(age=99).first()

但请注意，只有在知道QuerySet保证返回1或0结果时才有意义。

Answer 3

这应该有效：

employee[0] if employee else None

Answer 4

first_employee = employee[0] if employee.count() > 0 else None

Django的;是否可以为queryset设置默认值

4 个答案: