如何访问DRF中ListSerializer父类上的serializer.data？

Question

在返回到Response(serializer.data, status=something)之前，尝试访问serializer.data时出现错误：

Getting KeyError when attempting to get a value for field <field> on serializer <serializer>.

这发生在所有字段上（因为我试图访问父字段而不是子字段上的.data，请参见下文）

类定义如下：

class BulkProductSerializer(serializers.ModelSerializer):

    list_serializer_class = CustomProductListSerializer

    user = serializers.CharField(source='fk_user.username', read_only=False)

    class Meta:
        model = Product
        fields = (
            'user',
            'uuid',
            'product_code',
            ...,
        )

CustomProductListSerializer是一个serializers.ListSerializer，并且有一个重写的save()方法，允许它正确地处理批量创建和更新。

以下是批量产品ViewSet的示例视图：

def partial_update(self, request):

    serializer = self.get_serializer(data=request.data,
                        many=isinstance(request.data, list),
                        partial=True)
    if not serializer.is_valid():
        return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
    serializer.save()
    pdb.set_trace()
    return Response(serializer.data, status=status.HTTP_200_OK)

试图访问跟踪（或者后面的行）处的serializer.data会导致错误。下面是完整的跟踪（tl；dr skip，下面是我用调试器诊断的地方）：

 Traceback (most recent call last):
  File "/lib/python3.5/site-packages/django/core/handlers/exception.py", line 41, in inner
    response = get_response(request)
  File "/lib/python3.5/site-packages/django/core/handlers/base.py", line 249, in _legacy_get_response
    response = self._get_response(request)
  File "/lib/python3.5/site-packages/django/core/handlers/base.py", line 187, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/lib/python3.5/site-packages/django/core/handlers/base.py", line 185, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/lib/python3.5/site-packages/django/views/decorators/csrf.py", line 58, in wrapped_view
    return view_func(*args, **kwargs)
  File "/lib/python3.5/site-packages/rest_framework/viewsets.py", line 86, in view
    return self.dispatch(request, *args, **kwargs)
  File "/lib/python3.5/site-packages/rest_framework/views.py", line 489, in dispatch
    response = self.handle_exception(exc)
  File "/lib/python3.5/site-packages/rest_framework/views.py", line 449, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/lib/python3.5/site-packages/rest_framework/views.py", line 486, in dispatch
    response = handler(request, *args, **kwargs)
  File "/application/siop/views/API/product.py", line 184, in partial_update
    return Response(serializer.data, status=status.HTTP_200_OK)
  File "/lib/python3.5/site-packages/rest_framework/serializers.py", line 739, in data
    ret = super(ListSerializer, self).data
  File "/lib/python3.5/site-packages/rest_framework/serializers.py", line 265, in data
    self._data = self.to_representation(self.validated_data)
  File "/lib/python3.5/site-packages/rest_framework/serializers.py", line 657, in to_representation
    self.child.to_representation(item) for item in iterable
  File "/lib/python3.5/site-packages/rest_framework/serializers.py", line 657, in <listcomp>
    self.child.to_representation(item) for item in iterable
  File "/lib/python3.5/site-packages/rest_framework/serializers.py", line 488, in to_representation
    attribute = field.get_attribute(instance)
  File "/lib/python3.5/site-packages/rest_framework/fields.py", line 464, in get_attribute
    raise type(exc)(msg)
KeyError: "Got KeyError when attempting to get a value for field `user` on serializer `BulkProductSerializer`.\nThe serializer field might be named incorrectly and not match any attribute or key on the `OrderedDict` instance.\nOriginal exception text was: 'fk_user'."

在回溯的L657处（source here)）我得到：

iterable = data.all() if isinstance(data, models.Manager) else data
return [
    self.child.to_representation(item) for item in iterable
]

这使我想知道（进一步深入跟踪）为什么serializer.fields不可用。我怀疑这是因为序列化程序是一个CustomProductListSerializer父对象，而不是BulkProductSerializer子对象，我是对的。在返回Response(serializer.data)之前的pdb跟踪中：

(Pdb) serializer.fields
*** AttributeError: 'CustomProductListSerializer' object has no attribute 'fields'
(Pdb) serializer.child.fields
{'uuid': UUIDField(read_only=False, required=False, validators=[]) ...(etc)}
(Pdb) 'user' in serializer.child.fields
True
(Pdb) serializer.data
*** KeyError: "Got KeyError when attempting to get a value for field `user` on serializer `BulkProductSerializer`.\nThe serializer field might be named incorrectly and not match any attribute or key on the `OrderedDict` instance.\nOriginal exception text was: 'fk_user'."
(Pdb) serializer.child.data
{'uuid': '08ec13c0-ab6c-45d4-89ab-400019874c63', ...(etc)}

好的，那么在我的ViewSet中描述的情况下，正确的方法是获取完整的serializer.data，并在父序列化程序类的resopnse中返回它？

编辑：

class CustomProductListSerializer(serializers.ListSerializer):

    def save(self):
        instances = []
        result = []
        pdb.set_trace()
        for obj in self.validated_data:
            uuid = obj.get('uuid', None)
            if uuid:
                instance = get_object_or_404(Product, uuid=uuid)
                # Specify which fields to update, otherwise save() tries to SQL SET all fields.
                # Gotcha: remove the primary key, because update_fields will throw exception.
                # see https://stackoverflow.com/a/45494046
                update_fields = [k for k,v in obj.items() if k != 'uuid']
                for k, v in obj.items():
                    if k != 'uuid':
                        setattr(instance, k, v)
                instance.save(update_fields=update_fields)
                result.append(instance)
            else:
                instances.append(Product(**obj))

        if len(instances) > 0:
            Product.objects.bulk_create(instances)
            result += instances

        return result

Answer 1

如注释中所述，我仍然认为异常可能是由于BulkProductSerializer类中的user字段造成的，实际上与ListSerializer无关

文档here中提到的序列化程序DRF中可能还有另一个小错误（但很重要）。下面是如何指定list_serializer_class：

class CustomListSerializer(serializers.ListSerializer):
    ...

class CustomSerializer(serializers.Serializer):
    ...
    class Meta:
        list_serializer_class = CustomListSerializer

注意，它是在Meta类内部指定的，而不是外部指定的。所以我认为在您的代码中，使用many=True切换到列表序列化程序是不理解的。这会导致不更新问题。

Update-添加用于更新嵌套列表序列化程序的示例

似乎问题更多的是关于实现嵌套列表序列化器更新的通用方法，而不是实际的错误。因此，我将尝试提供一个示例代码。

一些注释：

• 如果我们使用ModelViewSet，列表路由将不允许PUT或PATCH，因此无论是update还是partial_update都不会被调用（reference）。因此，我直接使用POST，这要简单得多。
• 如果您想使用PUT/补丁，请查看此答案here
• 我们总是可以直接在Post请求中添加一个查询参数，如允许更新或部分来区分Post/PUT/PATCH
• 与其像我将要使用的问题那样使用uuid，它应该是非常相同的

很简单

作为参考，模型如下：

class Product(models.Model):
    name = models.CharField(max_length=200)
    user = models.ForeignKey(User, null=True, blank=True)

    def __unicode__(self):
        return self.name

步骤1：确保序列化程序更改为列表序列化程序

class ProductViewSet(viewsets.ModelViewSet):
    serializer_class = ProductSerializer
    queryset = Product.objects.all()

    def get_serializer(self, *args, **kwargs):
        # checking for post only so that 'get' won't be affected
        if self.request.method.lower() == 'post':
            data = kwargs.get('data')
            kwargs['many'] = isinstance(data, list)
        return super(ProductViewSet, self).get_serializer(*args, **kwargs)

步骤2：通过重写create函数来实现ListSerializer

class ProductListSerializer(serializers.ListSerializer):
    def create(self, validated_data):
        new_products = [Product(**p) for p in validated_data if not p.get('id')]
        updating_data = {p.get('id'): p for p in validated_data if p.get('id')}
        # query old products
        old_products = Product.objects.filter(id__in=updating_data.keys())
        with transaction.atomic():
            # create new products
            all_products = Product.objects.bulk_create(new_products)
            # update old products
            for p in old_products:
                data = updating_data.get(p.id, {})
                # pop id to remove
                data.pop('id')
                updated_p = Product(id=p.id, **data)
                updated_p.save()
                all_products.append(updated_p)
        return all_products


class ProductSerializer(serializers.ModelSerializer):
    user = serializers.SlugRelatedField(slug_field='username', queryset=User.objects.all())
    id = serializers.IntegerField(required=False)

    class Meta:
        model = Product
        fields = '__all__'
        list_serializer_class = ProductListSerializer

Answer 2

您的错误与ListSerializer无关，但获取字段user时出现问题：

KeyError: "Got KeyError when attempting to get a value for field user on serializer BulkProductSerializer.

The serializer field might be named incorrectly and not match any attribute or key on the OrderedDict instance.

Original exception text was: 'fk_user'."

确保您的Product模型有一个fk_user字段。

您还将BulkProductSerializer上的user字段定义为可写，但尚未告诉序列化程序如何处理它。。。

最简单的方法是使用SlugRelatedField：

class BulkProductSerializer(serializers.ModelSerializer):

    list_serializer_class = CustomProductListSerializer

    user = serializers.SlugRelatedField(
                            slug_field='username',
                            queryset=UserModel.objects.all(),
                            source='fk_user'
    )

    class Meta:
        model = Product
        fields = (
            'user',
            'uuid',
            'product_code',
            ...,
        )

这应该能很好地处理错误，例如当username不存在时。。。

Answer 3

在我试图访问serializer.data并获取KeyError的跟踪中，我注意到serializer.data只包含initial_data中的key/vaule对，而不包含实例数据（因此，我假设KeyError；某些模型字段的key不存在，因为它是一个partial_update请求）。但是，serializer.child.data包含列表中最后一个子项的所有实例数据。

所以，我转到定义data的^{} source：

249    @property
250    def data(self):
251        if hasattr(self, 'initial_data') and not hasattr(self, '_validated_data'):
252            msg = (
253                'When a serializer is passed a `data` keyword argument you '
254                'must call `.is_valid()` before attempting to access the '
255                'serialized `.data` representation.\n'
256                'You should either call `.is_valid()` first, '
257                'or access `.initial_data` instead.'
258            )
259            raise AssertionError(msg)
260
261        if not hasattr(self, '_data'):
262            if self.instance is not None and not getattr(self, '_errors', None):
263                self._data = self.to_representation(self.instance)
264            elif hasattr(self, '_validated_data') and not getattr(self, '_errors', None):
265                self._data = self.to_representation(self.validated_data)
266            else:
267                self._data = self.get_initial()
268        return self._data

第265行有问题。我可以通过在断点处调用serializer.child.to_representation({'uuid': '87956604-fbcb-4244-bda3-9e39075d510a', 'product_code': 'foobar'})来复制错误。

调用partial_update()在单个实例上工作正常（因为self.instance已设置，self.to_representation(self.instance)工作正常）。但是，对于批量部分更新（）实现，self.validated_data缺少模型字段，to_representation()将无法工作，因此我将无法访问.data属性。

一种选择是维护某种类型的产品实例列表，并在第265行重写data的定义：

self._data = self.to_representation(self.instances)

不过，我真的更喜欢有经验的人回答这类问题，因为我不确定这是否是一个明智的解决方案，因此我将悬赏金留空，希望有人能建议一些更明智的做法。