聚合（Max（'id'））返回异常'str'对象没有属性'email'

Question

我一直在尝试获取具有最高 id 的用户，但没有成功。 这是我的用户模型：

class User(models.Model):
    email=models.EmailField(unique=True, null=False)
    name=models.TextField(null=True)

它的序列化器：

class UserSerializer(serializers.ModelSerializer):
    class Meta:
        model = User
        fields = ('id', 'email', 'name')

观点：

class GetHighestValue(generics.ListAPIView):
    serializer_class = UserSerializer

    def get_queryset(self):
        return User.objects.aggregate(Max('id'))

尝试获取字段 email 的值时出现 AttributeError on 序列化程序UserSerializer。序列化器字段可能被命名 不正确且不匹配 str 实例上的任何属性或键。 原始异常文本是：“str”对象没有属性“email”。

Traceback (most recent call last):   
File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py", line 149, in get_response
     response = self.process_exception_by_middleware(e, request)   
File "/usr/local/lib/python2.7/dist-packages/django/core/handlers/base.py", line 147, in get_response
     response = wrapped_callback(request, *callback_args, **callback_kwargs)   
File "/usr/local/lib/python2.7/dist-packages/django/views/decorators/csrf.py", line 58, in wrapped_view
     return view_func(*args, **kwargs)   
File "/usr/local/lib/python2.7/dist-packages/django/views/generic/base.py", line 68, in view
     return self.dispatch(request, *args, **kwargs)   
File "/home/user/.local/lib/python2.7/site-packages/rest_framework/views.py", line 466, in dispatch
     response = self.handle_exception(exc)   
File "/home/user/.local/lib/python2.7/site-packages/rest_framework/views.py", line 463, in dispatch
     response = handler(request, *args, **kwargs)   
File "/home/user/.local/lib/python2.7/site-packages/rest_framework/generics.py", line 201, in get
     return self.list(request, *args, **kwargs)   
File "/home/user/.local/lib/python2.7/site-packages/rest_framework/mixins.py", line 48, in list
     return Response(serializer.data)   
File "/home/user/.local/lib/python2.7/site-packages/rest_framework/serializers.py", line 674, in data
     ret = super(ListSerializer, self).data   
File "/home/user/.local/lib/python2.7/site-packages/rest_framework/serializers.py", line 239, in data
     self._data = self.to_representation(self.instance)   
File "/home/user/.local/lib/python2.7/site-packages/rest_framework/serializers.py", line 614, in to_representation
     self.child.to_representation(item) for item in iterable   
File "/home/user/.local/lib/python2.7/site-packages/rest_framework/serializers.py", line 463, in to_representation
     attribute = field.get_attribute(instance)   
File "/home/user/.local/lib/python2.7/site-packages/rest_framework/fields.py", line 422, in get_attribute
     raise type(exc)(msg) 
     AttributeError: Got AttributeError when attempting to get a value for field `email` on serializer `UserSerializer`. 
     The serializer field might be named incorrectly and not match any attribute or key on the `str` instance. 
     Original exception text was: 'str' object has no attribute 'email'.

Answer 1

问题来了

def get_queryset(self):
    return User.objects.aggregate(Max('id'))

预期的返回值是一个查询集。但是聚合不返回查询集。使用 User.objects.get() 也不返回查询集。返回查询集的唯一方法是使用all() 或filter()

def get_queryset(self):
    return User.objects.order_by('-id')[:1]

这里隐含了 all() 并且 [:1] 确保您返回包含对象而不是单个对象的可迭代对象。

Answer 2

问题就在你的眼前，

当您尝试获取查询集时，您使用的是aggregate() 方法。

但aggregate() 不返回查询集，而是返回名称-值对的字典。 详情见https://docs.djangoproject.com/en/1.9/topics/db/aggregation

与aggregate() 不同，annotate() 不是终结子句。 annotate() 子句的输出是一个 QuerySet；可以使用任何其他 QuerySet 操作来修改此 QuerySet，包括filter()、order_by()。

希望有帮助。

Answer 3

我正在扩展 zhaochy 所写的内容。尝试将视图的 get_queryset() 方法更改为以下。我们不是返回聚合的结果（不是查询集，因为它已经被评估了），而是使用该数字来查找与最大 id 关联的实例并返回它（这是一个查询集）。

class GetHighestValue(generics.ListAPIView):
    serializer_class = UserSerializer

    def get_queryset(self):
        max_id = User.objects.aggregate(Max('id')).get('id__max')
        return User.objects.filter(id=max_id)

免责声明：我在手机上写了这篇文章，我无法对其进行测试。在 cmets 中告诉我这是否解决了您的问题，我会根据需要进行编辑。