Django Rest框架序列化多个生成序列化关系的NoneType实例

Question

我对Django Rest框架非常陌生，我一直试图为我的一个模型编写一个序列化程序。对于我的项目，我打算根据json标准输出JSON结果，为此，我使用了SerializerMethodField，其中我按如下方式调用了get_data()方法：

models.py

class Level(MPTTModel):
    name = models.CharField(max_length=100)
    parent = TreeForeignKey('self', null=True, blank=True, related_name='children', db_index=True)

    class MPTTMeta:
        order_insertion_by = ['name']


class Group(models.Model):
    name = models.CharField(max_length=100)
    level_set = models.ManyToManyField(Level, blank=True)

serializers.py

class LevelSerializer(serializers.ModelSerializer):
    data = serializers.SerializerMethodField()

    class Meta:
        model = Level
        fields = ('data',)

    def get_data(self, instance):
        return {
            "type": "Level",
            "uuid": instance.id_key(),
            "attributes": {
                "name": instance.name,
            },
            "relationships": {
                "children": self.get_children_recursive(),
            },
        }

    def get_children_recursive(self, child=None):
        """
        Generates a tree of levels structured according to JSON API standards.
        """
        if not child:
            children = self.instance.get_children()
            level = self.instance
        else:
            children = child.get_children()
            level = child
        tree = {
            'data': {
                'type': 'Level',
                'uuid': level.id_key(),
                'attributes': {
                    'name': level.name,
                },
                'relationships': {
                    'children': [],
                    'parents': [],
                }
            }
        }
        for child in children:
            tree['data']['relationships']['children'].append(self.get_children_recursive(child))
        return tree

class GroupSerializer(serializers.ModelSerializer):
    root_level_set = LevelSerializer(many=True)

    class Meta:
        model = Group
        fields = ('id_key', 'name', 'root_level_set')

奇怪的是，如果我转到shell并尝试序列化一个级别的实例，它会很好地工作，但是尝试序列化一个组会在get_children_recursive()上给我一个错误，这与if not child statement所说的'NoneType' object has no attribute 'get_children'是一致的。产出如下：

运行：

from core.serializers import LevelSerializer
from core.models import Level
lvl = Level.objects.all()[0]
serializer = LevelSerializer(lvl)
print(serializer.data)

根据我按照JSON标准设计的JSON结构，输出嵌套级别及其子级别。

不过，如果我跑：

from core.serializers import GroupSerializer
from core.models import Group
grp = Group.objects.all()[0]
serializer = GroupSerializer(grp)
print(serializer.data)

产出：

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/home/maumau/.virtualenvs/olist/lib/python3.5/site-packages/rest_framework/serializers.py", line 503, in data
    ret = super(Serializer, self).data
  File "/home/maumau/.virtualenvs/olist/lib/python3.5/site-packages/rest_framework/serializers.py", line 239, in data
    self._data = self.to_representation(self.instance)
  File "/home/maumau/.virtualenvs/olist/lib/python3.5/site-packages/rest_framework/serializers.py", line 472, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
  File "/home/maumau/.virtualenvs/olist/lib/python3.5/site-packages/rest_framework/serializers.py", line 614, in to_representation
    self.child.to_representation(item) for item in iterable
  File "/home/maumau/.virtualenvs/olist/lib/python3.5/site-packages/rest_framework/serializers.py", line 614, in <listcomp>
    self.child.to_representation(item) for item in iterable
  File "/home/maumau/.virtualenvs/olist/lib/python3.5/site-packages/rest_framework/serializers.py", line 472, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
  File "/home/maumau/.virtualenvs/olist/lib/python3.5/site-packages/rest_framework/fields.py", line 1653, in to_representation
    return method(value)
  File "/mnt/SHARED-DRIVE/Workspace/interview-tests/work-at-olist/core/serializers.py", line 20, in get_data
    "children": self.get_children_recursive(),
  File "/mnt/SHARED-DRIVE/Workspace/interview-tests/work-at-olist/core/serializers.py", line 29, in get_children_recursive
    children = self.instance.get_children()
AttributeError: 'NoneType' object has no attribute 'get_children'

对我来说，自我的级别序列化拥有属性实例集，而调用级别序列化的Group序列化也不具有属性实例集，这似乎没有任何意义。有线索吗？

Answer 1

为什么要自己设计所有的东西？让djrf做一些魔术:)

class LevelSerializer(serializers.ModelSerializer):
    type = serializers.SerializerMethodField()
    attributes = serializers.SerializerMethodField()
    relationships = serializers.SerializerMethodField()

    def get_type(self, instance):
        return "Level"

    def get_attributes(self, instance):
        return {
            "name": instance.name
        }

    def get_relationships(self, instance):
        return {
            "children": self.__class__(instance.get_children(), many=True).data,
            "parents": []
        }

    class Meta:
        model = Level
        fields = ('type', 'attributes', 'relationships')


class GroupSerializer(serializers.ModelSerializer):
    level_set = LevelSerializer(many=True)

    class Meta:
        model = Group
        fields = ('name', 'level_set')

为什么您的代码不工作？因为错误的代码：

1. 看看在这2种情况下调用LevelSerializer有什么不同？？一个参数- many=True；当many=True -序列化程序没有instance属性时。
2. 在(1)旁边，get_children_recursive的第一次运行将复制第一个元素作为第一个子元素
3. 使用您的方法，您根本不需要ModelSerializer，因为所有操作都是手动完成的--在这种情况下，简单的Serializer会更好。
4. 通过创建虚拟data字段--使代码变得更加复杂，为什么不直接使用数据来避免使用一个data键的字典？

我的示例代码缺少id_key字段--您没有在这里记录它，但是添加它应该是一次狂风。

使用额外的模块rest_framework_recursive，可以简化代码：

from rest_framework_recursive.fields import RecursiveField


class LevelSerializer(serializers.ModelSerializer):
    type = serializers.SerializerMethodField()
    children = serializers.ListField(child=RecursiveField(), source='get_children')

    def get_type(self, instance):
        return "Level"

    class Meta:
        model = Level
        fields = ('type', 'name', 'children')

Answer 2

序列化的属性名：root_level_set。模型：level_set.

DRF试图在组中找到属性root_level_set，但没有找到它，并将其替换为None。这就是为什么你会犯这样的错误。

修正：

1. 在序列化程序中重命名root_level_set -> level_set。
2. 或将source添加到字段：root_level_set = LevelSerializer(source='level_set', many=True)