Django Rest Framework：TypeError - 禁止直接分配给多对多集合的前端

Question

我有一个自定义 User 模型和一个 Group 模型，它们由 UserGroup 通过模型（多对多关系）链接：

models.py

class User(models.Model):
    username = models.CharField(primary_key=True, max_length=32, unique=True)
    user_email = models.EmailField(max_length=32, unique=False) # Validates an email through predefined regex which checks ‘@’ and a ‘.’
    user_password = models.CharField(max_length=32)
    user_avatar_path = models.CharField(max_length=64)

class Group(models.Model):
    group_id = models.AutoField(primary_key=True)
    group_name = models.CharField(max_length=32, unique=False)
    group_admin = models.ForeignKey(
        User, 
        on_delete=models.CASCADE, 
        related_name='my_groups'
        )
    members = models.ManyToManyField(
        User,
        related_name='groups', # The name to use for the relation from the related object back to this one.
        through='UserGroup' # Attaches a Junction table to the Many to Many relationship.
        )

class UserGroup(models.Model): # Manually specified Junction table for User and Group
    user = models.ForeignKey(
        User, 
        on_delete=models.CASCADE, 
        related_name='user_groups'
        )
    group = models.ForeignKey(
        Group, 
        on_delete=models.CASCADE, 
        related_name='user_groups'
        )

我正在尝试将多个用户与一个组相关联，使用 PATCH 请求更新组的 members 属性。使用以下 GroupSerializer，我可以在创建组时将用户关联为组的成员，方法是覆盖序列化程序的 create 函数：

serializers.py

class GroupSerializer(serializers.ModelSerializer):
    members = MemberSerializer(many=True, required=False)
    group_admin = serializers.SlugRelatedField(slug_field='username', queryset=User.objects.all()) # A Group object is related to a User object by username

    class Meta:
        model = Group
        fields = ['group_id', 'group_name', 'group_admin', 'members']

    def create(self, validated_data): # Overriden so that when a group is created, the group admin is automatically declared as a member.
        group = Group.objects.create(**validated_data)
        group_admin_data = validated_data.pop('group_admin')
        group.members.add(group_admin_data)

        return group
    
    def update(self, instance, validated_data):
       members_data = validated_data.pop('members') # Comes from the request body, gets the members list
        #print('output: ' + str(members_data[0].items()))
        add_remove = self.context['add_remove'] # Comes from the View 
        if members_data is not None:
            if add_remove == 'add':
                for member in members_data:
                    instance.members.add(member['username'])
            elif add_remove == 'remove':
                for member in members_data:
                    instance.members.remove(member['username'])  
            
        return super().update(instance, validated_data)

覆盖序列化程序的 update 函数时，我无法更新与组关联的成员。从以下 GroupUpdate 视图调用序列化程序：

views.py

class GroupUpdate(generics.UpdateAPIView):
    serializer_class = GroupSerializer

    def get_object(self):
        queryset = Group.objects.all()
        group_id = self.kwargs['group_id']
        if group_id is not None:
            queryset = queryset.filter(group_id=group_id).first()
        return queryset

    def get_serializer_context(self): # Passes the URL paramters to the GroupSerializer (serializer doesn't have kwargs).
        context = super().get_serializer_context()
        context['add_remove'] = self.kwargs['add_remove']
        print(self.request.data)
        return context

    def perform_update(self, serializer):
        serializer=GroupSerializer(data=self.request.data, partial=True)
        serializer.is_valid(raise_exception=True)
        return super().perform_update(serializer)

在 GroupUpdate 的 perform_update 函数中，我收到以下内容：TypeError: Direct assignment to the forward side of a many-to-many set is prohibited. Use members.set() instead. 但我不确定为什么会出现此错误，考虑到我能够关联用户在 create 函数中以几乎相同的方式进行分组。

这是 PATCH 请求作为 JSON 正文的内容：

{
    "members": [
        {
            "username": "small_man"
        }
    ]
}

self.request.data的输出是{'members': [{'username': 'small_man'}]}。

Answer 1

您应该在创建序列化程序时指定更新对象的实例，否则序列化程序的保存方法将调用 create 而不是 update:

def perform_update(self, serializer):
    instance = self.get_object() 
    serializer=GroupSerializer(instance, data=self.request.data, partial=True)
    serializer.is_valid(raise_exception=True)
    return super().perform_update(serializer)

顺便说一句，perform_update 看起来是多余的，您可以删除它，因为序列化程序验证无需额外修改即可工作。

Django Rest Framework：TypeError - 禁止直接分配给多对多集合的前端

Django Rest Framework: TypeError - Direct assignment to the forward side of a many-to-many set is prohibited

python

django

serialization

many-to-many

django-rest-framework